What type of cell do you start with in Meiosis?

What type of cell do you start with in Meiosis?

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Okay, I was learning about mitosis and meiosis in school and had a question. I know in Mitosis you first start off with aDiploid (2N)cell and then end up with two daughter cells that are alsoDiploid. However, in Meiosis, I know you end up with fourHaploid Cells (N), but what exactly do you start off with? Is it like a single egg makes four more eggs or something? Or does it start off with aDiploid Celland then end up with theHaploids? If so, what exactly is the starting cell called?

Meiosis starts with a diploid cell and produces four haploid cells. In animals, the starting diploid cell is usually called a germ cell and the surviving haploid cells become gametes (sperm and ova). (In animals, the female mitotic sequence produces only one ovum; the other three haploid cells become "polar bodies".)

In other organisms such as plants, the starting diploid cell is typically not called a "germ cell" as it is not distinguished early in the organism's life. Instead the starting cell can be any undifferentiated diploid cell that finds itself in the appropriate location (e.g. in a flower) at the appropriate time.

During mitosis a diploid cell (2n = two copies of each chromosome, one from each parent) replicates its DNA so that it now has four copies of each chromosome. Then it divides, each daughter cell receives two copies of each chromosome and is again 2n.

In meiosis a diploid cell (2n) replicates its DNA so that it now has four copies of each chromosome. Then it divides, each daughter cell receives two copies of each chromosome and is again 2n. Then each of these divides once more without replicating DNA so that there are now four cells each with one copy of each chromosome (1n).

You might be tempted to think of a diploid cell which has replicated its DNA as tetraploid, but this word is not normally used in this context, since this is a transient 4n state.

This is a very broad overview. Have a look at the Wikipedia entry for meiosis to get a more detailed view and extended terminology.

@mgkrebbs (in comments):

If we are considering the meiotic divisions that create gametes, then in spermatogenesis the cell which undergoes meiosis is a primary spermatocyte, and in oogenesis it is a primary oocyte. Primary spermatocytes and primary oocytes are both diploid cells which undergo DNA replication before entering meiosis I.

Meiosis starts with a somatic (diploid) cell. To make a long story very, very short, this one cell undergoes cytokinesis twice. You end up with four haploid cells, called gametes or sex cells.

What type of cell do you start with in Meiosis? - Biology

Meiosis is the nuclear division of diploid cells into haploid cells, which is a necessary step in sexual reproduction.

Learning Objectives

Describe the importance of meiosis in sexual reproduction

Key Takeaways

Key Points

  • Sexual reproduction is the production of haploid cells and the fusion of two of those cells to form a diploid cell.
  • Before sexual reproduction can occur, the number of chromosomes in a diploid cell must decrease by half.
  • Meiosis produces cells with half the number of chromosomes as the original cell.
  • Haploid cells used in sexual reproduction, gametes, are formed during meiosis, which consists of one round of chromosome replication and two rounds of nuclear division.
  • Meiosis I is the first round of meiotic division, while meiosis II is the second round.

Key Terms

  • haploid: of a cell having a single set of unpaired chromosomes
  • gamete: a reproductive cell, male (sperm) or female (egg), that has only half the usual number of chromosomes
  • diploid: of a cell, having a pair of each type of chromosome, one of the pair being derived from the ovum and the other from the spermatozoon

Introduction: Meiosis and Sexual Reproduction

The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resemble their parent or parents. Sexual reproduction requires fertilization: the union of two cells from two individual organisms. Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid. The number of sets of chromosomes in a cell is called its ploidy level. If the reproductive cycle is to continue, then the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again or there will be a continual doubling in the number of chromosome sets in every generation. Therefore, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.

Offspring Closely Resemble Their Parents: In kind means that the offspring of any organism closely resemble their parent or parents. The hippopotamus gives birth to hippopotamus calves (a). Joshua trees produce seeds from which Joshua tree seedlings emerge (b). Adult flamingos lay eggs that hatch into flamingo chicks (c).

Sexual reproduction is the production of haploid cells (gametes) and the fusion (fertilization) of two gametes to form a single, unique diploid cell called a zygote. All animals and most plants produce these gametes, or eggs and sperm. In most plants and animals, through tens of rounds of mitotic cell division, this diploid cell will develop into an adult organism.

Haploid cells that are part of the sexual reproductive cycle are produced by a type of cell division called meiosis. Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid, so the resulting cells have half the chromosomes as the original. To achieve this reduction in chromosomes, meiosis consists of one round of chromosome duplication and two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis II, the second round of meiotic division, includes prophase II, prometaphase II, and so on.

What happens to chromosomes during reproduction?

Reproduction depends on specialized sex cells, called gametes. These cells only contain 23 chromosomes, or half the genetic material of other cells. Gametes are commonly known as sperm cells in males, and egg cells in females.
During reproduction, the 23 chromosomes from the egg cell and those from the sperm cell combine to make a full set of 46 chromosomes. This is called a zygote. This zygote can then develop into a child.

Anaphase I

In anaphase I of meiosis, the following events occur:

  • Chromosomes move to the opposite cell poles. Similar to mitosis, microtubules such as the kinetochore fibers interact to pull the chromosomes to the cell poles.
  • Unlike in mitosis, sister chromatids remain together after the homologous chromosomes move to opposite poles.

At the end of anaphase I of meiosis, the cell enters into telophase I.

In cell division, the cell that is dividing is called the "parent" cell. The parent cell divides into two "daughter" cells. The process then repeats in what is called the cell cycle.

Cell division of cancerous lung cell (Image from NIH)

Cells regulate their division by communicating with each other using chemical signals from special proteins called cyclins. These signals act like switches to tell cells when to start dividing and later when to stop dividing. It is important for cells to divide so you can grow and so your cuts heal. It is also important for cells to stop dividing at the right time. If a cell can not stop dividing when it is supposed to stop, this can lead to a disease called cancer.

Some cells, like skin cells, are constantly dividing. We need to continuously make new skin cells to replace the skin cells we lose. Did you know we lose 30,000 to 40,000 dead skin cells every minute? That means we lose around 50 million cells every day. This is a lot of skin cells to replace, making cell division in skin cells is so important. Other cells, like nerve and brain cells, divide much less often.

Part 1: Meiosis Bead Simulation



  1. Set up half of the beads exactly as follows, representing genes on the chromosome of a hypothetical critter. We will assume that the critter is diploid (2N) and has three different chromosomes. Because the critter has two copies of each of the three chromosomes, the diploid number is 6 (2 × 3 = 6).

This is what your critter’s chromosomes look like in the unreplicated form. Note that there are six chromosomes here consisting of three homologous pairs. Each chromosome pair consists of a maternal and paternal version of the chromosome. The maternal and paternal versions are represented by the respective bead color.

Do NOT proceed until you are comfortable with this! Don’t forget crossing over.


The process that produces haploid gametes is meiosis. Meiosis is a type of cell division in which the number of chromosomes is reduced by half. It occurs only in certain special cells of the organisms. During meiosis, homologous chromosomes separate, and haploid cells form that have only one chromosome from each pair. Two cell divisions occur during meiosis, and a total of four haploid cells are produced. The two cell divisions are called meiosis I and meiosis II. The overall process of meiosis is summarized in Figure below.

Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells. Notice the exchange of genetic material that occurs prior to the first cell division.

Phases of Meiosis

Meiosis I begins after DNA replicates during interphase of the cell cycle. In both meiosis I and meiosis II, cells go through the same four phases as mitosis - prophase, metaphase, anaphase and telophase. However, there are important differences between meiosis I and mitosis. The flowchart in Figure below shows what happens in both meiosis I and II.

Phases of Meiosis. This flowchart of meiosis shows meiosis I in greater detail than meiosis II. Meiosis I&mdashbut not meiosis II&mdashdiffers somewhat from mitosis. How does meiosis I differ from mitosis?

How does meiosis I differ from mitosis? Notice at the beginning of meiosis (prophase I), homologous chromosomes exchange segments of DNA. This is known as crossing-over, and is unique to this phase of meiosis.

Meiosis SE (2)

Vocabulary: ​ anaphase, chromosome, crossover, cytokinesis, diploid, DNA, dominant, gamete, genotype, germ cell, haploid, homologous chromosomes, interphase, meiosis, metaphase, mitosis, ovum, phenotype, prophase, recessive, sister chromatid, sperm cell, telophase, zygote

Prior Knowledge Questions ​(Do these BEFORE using the Gizmo.)

  1. During ​ mitosis ​, a single cell divides to produce two daughter cells. What must happen in the original cell so that each of the daughter cells has a complete set of ​ chromosomes ​?

It is important that the daughter cells have a copy of every chromosome, so the process involves copying the chromosomes first and then carefully separating the copies to give each new cell a full set.

  1. During sexual reproduction, two sex cells fuse to create a fertilized cell with a complete set of chromosomes. What must be true about the number of chromosomes in each sex cell?

The DNA must be copied so there is a full set of DNA to pass on to each daughter cell.

Gizmo Warm-up Meiosis ​ is a type of cell division that results in four daughter cells with half as many chromosomes as the parent cell. These daughter cells mature into ​ gametes ​, or sex cells. In the ​ Meiosis ​ Gizmo, you will learn the steps in meiosis and experiment to produce customized sex cells and offspring.

On the STEPS tab, click ​ Male ​.​ ​You are looking at a ​ germ

cell ​, or a cell that will undergo meiosis to become gametes.

  1. Read the description of ​ interphase ​ at the bottom of the Gizmo. What happens to the cell at

the beginning of interphase? cells grow synthesize mRNA and proteins required for

DNA synthesis

  1. Click on the ​ DNA ​ in the nucleus of the cell. Describe what happens. ​ DNA is copied and thedell grows some more
  1. Why is it necessary for the cell to grow and duplicate its DNA before the start of meiosis? ​ 2DNA sets

Introduction: ​ Unlike mitosis, which produces two identical daughter cells from one parent cell, meiosis creates four unique daughter cells with half the amount of DNA as the parent cell.

Question: How does meiosis create four daughter cells from one parent cell?

  1. Observe​: (​ Prophase ​ I) Click on the nucleus to break it down then click on the DNA to condense it into chromosomes. Drag the centrosomes to the top and bottom of the cell.

A. How many chromosomes does this cell have? 4 pairs

Each chromosome consists of a pair of ​ sister chromatids ​, two identical strands of DNA that formed when DNA replicated during interphase.

B. On the image to the right, draw two lines connecting the pairs of ​ homologous chromosomes (chromosomes of similar size with a matching set of genes).

In the Gizmo, drag the homologous chromosomes together. Click ​ Continue.

  1. Observe​: (​ Metaphase ​ I and ​ Anaphase ​ I) - Drag the groups of homologous chromosomes to the metaphase plate, then drag spindle fibers from each of the centrosomes to the chromosomes. Click the centrosome to pull the chromosomes apart.

How do the chromosomes separate in anaphase I? ​ sister chromatids het pulled to either end of the cell

A. How does anaphase I in meiosis differ from anaphase

in mitosis? mitosis breaks the chromatids

into 4. meiosis pulls 2 chromatids apart.

Steps in meiosis

Get the Gizmo ready​: ● Make sure the STEPS tab is selected. ● If necessary, choose the ​ Male ​ cell. Click on the DNA to copy it to proceed to prophase I.

Activity A (continued from previous page)

  1. Observe​: ​ Telophase ​ I and ​ cytokinesis ​ are the final steps of the first half of meiosis.

A. Describe what happens when you click on the chromosomes during telophase I.

chromosomes unravel and the nuclear envelope reforms around them

B. Click and drag on the contractile ring. Describe what happened during cytokinesis.

Structure made of actin and myosin filaments that forms a belt around a dividing cell, pinching it in two.

  1. Observe​: Go through the steps of the second half of meiosis until you reach the end of telophase II, following the instructions at the top right corner. As you proceed, answer the questions below. Use the ​ Back ​button if you need to see a step again.

A. Before prophase II begins, does the DNA in the cell duplicate itself? No

B. During metaphase II, do homologous chromosomes pair up as in metaphase I? No

C. How does anaphase II differ from anaphase I? anaphase I has chromosomes, anaphase II has sister chromatids

D. At the end of anaphase II, how many chromatids are on each side of the cell? ​ 2

E. After cytokinesis, how many cells have been formed from the parent cell? ​ 4

F. Are all of the cells the same size? yes

The original parent cell is called ​ diploid ​ because it contains a complete set of homologous chromosome pairs. Each of the four daughter cells is ​ haploid ​, meaning that each contains half of the original parent cell’s chromosomes. Each daughter cell contains one chromatid from each homologous pair.

  1. Observe​: Click on the spermatids. Spermatids that formed from meiosis will develop into mature male gametes called ​ sperm cells ​. Sketch a mature sperm cell in the space to the right.

Mature sperm cells have only a small amount of cytoplasm and use their flagella, or “tails,” to propel themselves forward. Sperm are designed for one purpose, to deliver genetic material to the egg cell during fertilization.

Introduction: ​ ​Although both male and female gametes contain genetic material from the parent

organism, they perform different functions. A male gamete delivers genetic material to a female gamete. The fertilized female gamete, called a ​ zygote ​, then grows into the offspring.

Question: What are the differences in meiosis between male and female cells?Male meiosis takes place in the testicles, while female meiosis takes place in the ovaries.

  1. Compare​: Click on the ​ Female ​ button. For the female cell, proceed through meiosis until you reach the end of anaphase I.

Up to this point, did you notice any differences between the development of male and

female gametes? In species with two separate sexes, the sex that produces the

smaller and more motile sex cell or gamete is called the male. Explain: Male mammals

produce gametes called sperm while female mammals produce gametes called eggs.

A. What do you notice about the size of the two resulting cells? ​ 3 small, 1 large

B. How does this compare to the two cells at the end of telophase I and cytokinesis I in

male cells? Many cells that undergo rapid meiosis do not decondense the

chromosomes at the end of telophase I. Other cells do exhibit chromosome

decondensation at this time the chromosomes recondense in prophase II.

A. What do you notice about the four cells now? ​ All 3 of the cells are the same size an their is one larger one

B. What is the largest cell called? OVUM

The ovum is the largest cell in the human body. In contrast, the sperm cell is the smallest cell in the human body.

Comparing female and male gametes

Get the Gizmo ready​: ● Make sure the STEPS tab is selected.

● Click ​ Reset ​.

Introduction: ​ ​The activities above shows that organisms can produce at least four different

gametes. In reality, organisms can produce millions of genetically unique gametes.

Question: How can meiosis create an unlimited number of unique gametes?

  1. Experiment​: Use the following abbreviations for the chromosomes. Dark green – DG Light green – LG Dark purple – DP, Light purple – LP. Choose a ​ Male ​ or ​ Female ​ cell.

A. Proceed though meiosis to anaphase I. Which chromosomes went up and which

went down? Up: chromosomes Down: ​ anaphase

B. Click ​ Back ​ and run anaphase I again a few times. Did the results ever change?

Explain. Chromosomes are distributed randomly during anaphase I.

C. Chromosomes are distributed randomly during anaphase I. What are the possible chromosome combinations in the two daughter cells? (Use DG, LG, DP, and LP.)

There are (223) possible combinations of maternal and paternal chromosomes.

  1. Experiment​: Click ​ Reset ​. Choose a ​ Male ​ or ​ Female ​ cell. Proceed through meiosis until the chromosomes are condensed in Prophase I.

Drag the LG (light green) chromosome to the ​ Allele map ​ on the left. This shows the alleles (or variations of a gene) that are present on the chromosome. A ​ genotype ​ is a list of alleles. The genotype of the LG chromosome, for example, is EEFFGGHHJJ.

A. What are the genotypes of the remaining chromosomes? DG: ​ Light green

LP: Dark green DP: Light purple

B. After moving the centrosomes, drag the pairs of homologous chromosomes together.

Click on a chromosome. What happens? It creates a crossover

Genetic diversity

Get the Gizmo ready​: ● Make sure the STEPS tab is selected. ● Click ​ Reset ​.

When homologous chromosomes are paired up, they can exchange sections. This exchange of genes is called a ​ crossover ​.

C. Click on several segments to create crossovers, and then click ​ Continue ​. Proceed to anaphase I. Drag each chromosome to the Allele map and write its genotype.


(Activity C continued on next page)

Introduction: ​ Earlier, you learned how crossovers can result in genetically diverse gametes. In this activity, you will perform crossovers in parent cells undergoing meiosis and combine the resulting gametes to produce offspring with specific genotypes.

Question: How can offspring be created that have a specific phenotype and genotype?

  1. Explore​: The EXPERIMENTATION tab shows a simplified fruit fly genome, with a single pair of homologous chromosomes. Each chromosome has genes that control wing shape, body color, antenna type, and eye color. The uppercase alleles are ​ dominant ​ and the lower case alleles are ​ recessive ​. The allele key is given at lower left. (Note that real fruit flies have eight chromosomes and many more genes.)

A. Click ​ Reset ​. Without creating any crossovers, click ​ Divide into gametes ​. What are

the possible genotypes of the gametes? CBLR or cbrl

B. Drag a gamete from each parent into the box below to create a zygote. What are the

different combinations of possible offspring genotypes? Bb Aa Ab

C. Click ​ Show phenotype ​ for each combination. What are the resulting phenotypes?

  1. Experiment​: Click ​ Reset ​. You can create crossovers by clicking on the middle chromatids in each of the parent cells.

A. Create a gamete with the genotype C b l r. First, click on the c gene in one of the parent cells to create the crossover. Then, click ​ Divide into gametes ​.

Did you create a gamete with the genotype C b l r? ​ ​ yes

B. Click ​ Reset ​. Create a gamete with the genotype: c b L R. How many crossover were

needed to create this gamete? Just one

When a crossover occurs, the entire portion of genetic material is swapped between the two homologous chromosomes, so gene C is swapped along with gene B and gene R is swapped along with gene L.

C. Click ​ Reset ​. Create a c B L r gamete. How many crossovers were needed? ​ two

(Activity D continued on next page)

Activity D (continued from previous page)

  1. Challenge​: Select the ​ Challenge ​ radio button. Make sure that ​ Target offspring 1 ​ is selected in the dropdown menu.

Target offspring 1 is a fruit fly with normal wings (cc), a black body (bb), normal antenna (ll) and red eyes (Rr). Because the offspring receives one chromatid from each parent, each chromatid should come from a different parent.

A. Using the Gizmo, create a fruit fly with the correct genotype. Explain how you did it.

I crossed over with a losercase r and an uppercase.

B. Is there another way to get the correct phenotype, but not the correct genotype? ​ Yes

Explain. ​ Since some genes are recessive, dominant ones will show up on top.

  1. Challenge​: Use the dropdown menu to switch to the next target offspring. While creating target offspring 2-5, fill out the table below.

To produce target offspring 5, why were two crossovers needed on one chromatid arm?

Two crossover were needed because the chromosomes switched out the inside parts.

5. Think and discuss​: ​ ​ Suppose there are two homologous chromosomes. Each chromosome

contains a single mutant allele in different parts of the chromosome. How can crossovers be beneficial in this situation? (Hint: How can you create a single, mutation-free chromosome?)

If the crossover results in the pair of the chromosome, in which one contains no mutant allele while others contain two mutant alleles.

Biology 171

The ability to reproduce is a basic characteristic of all organisms: Hippopotamuses give birth to hippopotamus calves Joshua trees produce seeds from which Joshua tree seedlings emerge and adult flamingos lay eggs that hatch into flamingo chicks. However, unlike the organisms shown above, offspring may or may not resemble their parents. For example, in the case of most insects such as butterflies (with a complete metamorphosis), the larval forms rarely resemble the adult forms.

Although many unicellular organisms and a few multicellular organisms can produce genetically identical clones of themselves through asexual reproduction, many single-celled organisms and most multicellular organisms reproduce regularly using another method—sexual reproduction. This highly evolved method involves the production by parents of two haploid cells and the fusion of two haploid cells to form a single, genetically recombined diploid cell—a genetically unique organism. Haploid cells that are part of the sexual reproductive cycle are produced by a type of cell division called meiosis . Sexual reproduction, involving both meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms, both multicellular and unicellular, can or must employ some form of meiosis and fertilization to reproduce.

In most plants and animals, through thousands of rounds of mitotic cell division, diploid cells (whether produced by asexual or sexual reproduction) will develop into an adult organism.

Learning Objectives

By the end of this section, you will be able to do the following:

  • Describe the behavior of chromosomes during meiosis, and the differences between the first and second meiotic divisions
  • Describe the cellular events that take place during meiosis
  • Explain the differences between meiosis and mitosis
  • Explain the mechanisms within the meiotic process that produce genetic variation among the haploid gametes

Sexual reproduction requires the union of two specialized cells, called gametes , each of which contains one set of chromosomes. When gametes unite, they form a zygote, or fertilized egg that contains two sets of chromosomes. (Note: Cells that contain one set of chromosomes are called haploid cells containing two sets of chromosomes are called diploid .) If the reproductive cycle is to continue for any sexually reproducing species, then the diploid cell must somehow reduce its number of chromosome sets to produce haploid gametes otherwise, the number of chromosome sets will double with every future round of fertilization. Therefore, sexual reproduction requires a nuclear division that reduces the number of chromosome sets by half.

Most animals and plants and many unicellular organisms are diploid and therefore have two sets of chromosomes. In each somatic cell of the organism (all cells of a multicellular organism except the gametes or reproductive cells), the nucleus contains two copies of each chromosome, called homologous chromosomes. Homologous chromosomes are matched pairs containing the same genes in identical locations along their lengths. Diploid organisms inherit one copy of each homologous chromosome from each parent.

Meiosis is the nuclear division that forms haploid cells from diploid cells, and it employs many of the same cellular mechanisms as mitosis. However, as you have learned, mitosis produces daughter cells whose nuclei are genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are at the same “ploidy level”—diploid in the case of most multicellular most animals. Plants use mitosis to grow as sporophytes, and to grow and produce eggs and sperm as gametophytes so they use mitosis for both haploid and diploid cells (as well as for all other ploidies). In meiosis, the starting nucleus is always diploid and the daughter nuclei that result are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome replication followed by two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Likewise, Meiosis II (during which the second round of meiotic division takes place) includes prophase II, prometaphase II, and so on.

Meiosis I

Meiosis is preceded by an interphase consisting of G1, S, and G2 phases, which are nearly identical to the phases preceding mitosis. The G1 phase (the “first gap phase”) is focused on cell growth. During the S phase—the second phase of interphase—the cell copies or replicates the DNA of the chromosomes. Finally, in the G2 phase (the “second gap phase”) the cell undergoes the final preparations for meiosis.

During DNA duplication in the S phase, each chromosome is replicated to produce two identical copies—sister chromatids that are held together at the centromere by cohesin proteins, which hold the chromatids together until anaphase II.

Prophase I

Early in prophase I, before the chromosomes can be seen clearly with a microscope, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair closer together. Recall that in mitosis, homologous chromosomes do not pair together. The synaptonemal complex , a lattice of proteins between the homologous chromosomes, first forms at specific locations and then spreads outward to cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis. In synapsis , the genes on the chromatids of the homologous chromosomes are aligned precisely with each other. The synaptonemal complex supports the exchange of chromosomal segments between homologous nonsister chromatids—a process called crossing over . Crossing over can be observed visually after the exchange as chiasmata (singular = chiasma) ((Figure)).

In humans, even though the X and Y sex chromosomes are not completely homologous (that is, most of their genes differ), there is a small region of homology that allows the X and Y chromosomes to pair up during prophase I. A partial synaptonemal complex develops only between the regions of homology.

Located at intervals along the synaptonemal complex are large protein assemblies called recombination nodules . These assemblies mark the points of later chiasmata and mediate the multistep process of crossover —or genetic recombination—between the nonsister chromatids. Near the recombination nodule, the double-stranded DNA of each chromatid is cleaved, the cut ends are modified, and a new connection is made between the nonsister chromatids. As prophase I progresses, the synaptonemal complex begins to break down and the chromosomes begin to condense. When the synaptonemal complex is gone, the homologous chromosomes remain attached to each other at the centromere and at chiasmata. The chiasmata remain until anaphase I. The number of chiasmata varies according to the species and the length of the chromosome. There must be at least one chiasma per chromosome for proper separation of homologous chromosomes during meiosis I, but there may be as many as 25. Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is removed. At the end of prophase I, the pairs are held together only at the chiasmata ((Figure)). These pairs are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.

The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A single crossover event between homologous nonsister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. When a recombinant sister chromatid is moved into a gamete cell it will carry some DNA from one parent and some DNA from the other parent. The recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover. Crossover events can occur almost anywhere along the length of the synapsed chromosomes. Different cells undergoing meiosis will therefore produce different recombinant chromatids, with varying combinations of maternal and parental genes. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of DNA to produce genetically recombined chromosomes.

Prometaphase I

The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. Kinetochore proteins are multiprotein complexes that bind the centromeres of a chromosome to the microtubules of the mitotic spindle. Microtubules grow from microtubule-organizing centers (MTOCs). In animal cells, MTOCs are centrosomes located at opposite poles of the cell. The microtubules from each pole move toward the middle of the cell and attach to one of the kinetochores of the two fused homologous chromosomes. Each member of the homologous pair attaches to a microtubule extending from opposite poles of the cell so that in the next phase, the microtubules can pull the homologous pair apart. A spindle fiber that has attached to a kinetochore is called a kinetochore microtubule. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. The homologous chromosomes are still held together at the chiasmata. In addition, the nuclear membrane has broken down entirely.

Metaphase I

During metaphase I, the homologous chromosomes are arranged at the metaphase plate—roughly in the midline of the cell, with the kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. For example, if the two homologous members of chromosome 1 are labeled a and b, then the chromosomes could line up a-b or b-a. This is important in determining the genes carried by a gamete, as each will only receive one of the two homologous chromosomes. (Recall that after crossing over takes place, homologous chromosomes are not identical. They contain slight differences in their genetic information, causing each gamete to have a unique genetic makeup.)

The randomness in the alignment of recombined chromosomes at the metaphase plate, coupled with the crossing over events between nonsister chromatids, are responsible for much of the genetic variation in the offspring. To clarify this further, remember that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent. Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis, the homologous chromosomes form the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate. Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Thus, any maternally inherited chromosome may face either pole. Likewise, any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.

This event—the random (or independent) assortment of homologous chromosomes at the metaphase plate—is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate the possible number of alignments therefore equals 2 n in a diploid cell, where n is the number of chromosomes per haploid set. Humans have 23 chromosome pairs, which results in over eight million (2 23 ) possible genetically-distinct gametes just from the random alignment of chromosomes at the metaphase plate. This number does not include the variability that was previously produced by crossing over between the nonsister chromatids. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition ((Figure)).

To summarize, meiosis I creates genetically diverse gametes in two ways. First, during prophase I, crossover events between the nonsister chromatids of each homologous pair of chromosomes generate recombinant chromatids with new combinations of maternal and paternal genes. Second, the random assortment of tetrads on the metaphase plate produces unique combinations of maternal and paternal chromosomes that will make their way into the gametes.

Anaphase I

In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart ((Figure)).

Telophase I and Cytokinesis

In telophase, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur, depending on the species. In some organisms, the chromosomes “decondense” and nuclear envelopes form around the separated sets of chromatids produced during telophase I. In other organisms, cytokinesis—the physical separation of the cytoplasmic components into two daughter cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic division). In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells.

Two haploid cells are the result of the first meiotic division of a diploid cell. The cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though each chromosome still consists of two sister chromatids. Recall that sister chromatids are merely duplicates of one of the two homologous chromosomes (except for changes that occurred during crossing over). In meiosis II, these two sister chromatids will separate, creating four haploid daughter cells.

Review the process of meiosis, observing how chromosomes align and migrate, with Animal Cell Meiosis (Javascript interactive).

Meiosis II

In some species, cells enter a brief interphase, or interkinesis , before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II in synchrony. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II are similar to mitosis, except that each dividing cell has only one set of homologous chromosomes, each with two chromatids. Therefore, each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis. In terms of chromosomal content, cells at the start of meiosis II are similar to haploid cells in G2, preparing to undergo mitosis.

Prophase II

If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The MTOCs that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed.

Prometaphase II

The nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.

Metaphase II

The sister chromatids are maximally condensed and aligned at the equator of the cell.

Anaphase II

The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Nonkinetochore microtubules elongate the cell.

Telophase II and Cytokinesis

The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. If the parent cell was diploid, as is most commonly the case, then cytokinesis now separates the two cells into four unique haploid cells. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombination of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover. The entire process of meiosis is outlined in (Figure).

Comparing Meiosis and Mitosis

Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but also exhibit a number of important and distinct differences that lead to very different outcomes ((Figure)). Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the same number of sets of chromosomes: one set in the case of haploid cells and two sets in the case of diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new, genetically distinct cells. The four nuclei produced during meiosis are not genetically identical, and they contain one chromosome set only. This is half the number of chromosome sets in the original cell, which is diploid.

The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs physically meet and are bound together with the synaptonemal complex. Following this, the chromosomes develop chiasmata and undergo crossover between nonsister chromatids. In the end, the chromosomes line up along the metaphase plate as tetrads—with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog to form a tetrad. All of these events occur only in meiosis I.

When the chiasmata resolve and the tetrad is broken up with the homologs moving to one pole or another, the ploidy level—the number of sets of chromosomes in each future nucleus—has been reduced from two to one. For this reason, meiosis I is referred to as a reductional division . There is no such reduction in ploidy level during mitosis.

Meiosis II is analogous to a mitotic division. In this case, the duplicated chromosomes (only one set of them) line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles. During anaphase II, as in mitotic anaphase, the kinetochores divide and one sister chromatid—now referred to as a chromosome—is pulled to one pole while the other sister chromatid is pulled to the other pole. If it were not for the fact that there had been crossover, the two products of each individual meiosis II division would be identical (as in mitosis). Instead, they are different because there has always been at least one crossover per chromosome. Meiosis II is not a reduction division because although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I.

The Mystery of the Evolution of Meiosis Some characteristics of organisms are so widespread and fundamental that it is sometimes difficult to remember that they evolved like other simple traits. Meiosis is such an extraordinarily complex series of cellular events that biologists have had trouble testing hypotheses concerning how it may have evolved. Although meiosis is inextricably entwined with sexual reproduction and its advantages and disadvantages, it is important to separate the questions of the evolution of meiosis and the evolution of sex, because early meiosis may have been advantageous for different reasons than it is now. Thinking outside the box and imagining what the early benefits from meiosis might have been is one approach to uncovering how it may have evolved.

Meiosis and mitosis share obvious cellular processes, and it makes sense that meiosis evolved from mitosis. The difficulty lies in the clear differences between meiosis I and mitosis. Adam Wilkins and Robin Holliday 1 summarized the unique events that needed to occur for the evolution of meiosis from mitosis. These steps are homologous chromosome pairing and synapsis, crossover exchanges, sister chromatids remaining attached during anaphase, and suppression of DNA replication in interphase. They argue that the first step is the hardest and most important and that understanding how it evolved would make the evolutionary process clearer. They suggest genetic experiments that might shed light on the evolution of synapsis.

There are other approaches to understanding the evolution of meiosis in progress. Different forms of meiosis exist in single-celled protists. Some appear to be simpler or more “primitive” forms of meiosis. Comparing the meiotic divisions of different protists may shed light on the evolution of meiosis. Marilee Ramesh and colleagues 2 compared the genes involved in meiosis in protists to understand when and where meiosis might have evolved. Although research is still ongoing, recent scholarship into meiosis in protists suggests that some aspects of meiosis may have evolved later than others. This kind of genetic comparison can tell us what aspects of meiosis are the oldest and what cellular processes they may have borrowed from in earlier cells.

Click through How Cells Divide (website, interactive) to compare the meiotic process of cell division to that of mitosis.

Section Summary

Sexual reproduction requires that organisms produce cells that can fuse during fertilization to produce offspring. In most animals, meiosis is used to produce haploid eggs and sperm from diploid parent cells so that the fusion of an egg and sperm produces a diploid zygote. As with mitosis, DNA replication occurs prior to meiosis during the S-phase of the cell cycle so that each chromosome becomes a pair of sister chromatids. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell. The first division separates homologs, and the second—like mitosis—separates chromatids into individual chromosomes. Meiosis generates variation in the daughter nuclei during crossover in prophase I as well as during the random alignment of tetrads at metaphase I. The cells that are produced by meiosis are genetically unique.

Meiosis and mitosis share similar processes, but have distinct outcomes. Mitotic divisions are single nuclear divisions that produce genetically identical daughter nuclei (i.e., each daughter nucleus has the same number of chromosome sets as the original cell). In contrast, meiotic divisions include two nuclear divisions that ultimately produce four genetically different daughter nuclei that have only one chromosome set (instead of the two sets of chromosomes in the parent cell). The main differences between the two nuclear division processes take place during the first division of meiosis: homologous chromosomes pair, crossover, and exchange homologous nonsister chromatid segments. The homologous chromosomes separate into different nuclei during meiosis I, causing a reduction of ploidy level in the first division. The second division of meiosis is similar to a mitotic division, except that the daughter cells do not contain identical genomes because of crossover and chromosome recombination in prophase I.

Free Response

Describe the process that results in the formation of a tetrad.

During the meiotic interphase, each chromosome is duplicated. The sister chromatids that are formed during synthesis are held together at the centromere region by cohesin proteins. All chromosomes are attached to the nuclear envelope by their tips. As the cell enters prophase I, the nuclear envelope begins to fragment and the proteins holding homologous chromosomes locate each other. The four sister chromatids align lengthwise, and a protein lattice called the synaptonemal complex is formed between them to bind them together. The synaptonemal complex facilitates crossover between nonsister chromatids, which is observed as chiasmata along the length of the chromosome. As prophase I progresses, the synaptonemal complex breaks down and the sister chromatids become free, except where they are attached by chiasmata. At this stage, the four chromatids are visible in each homologous pairing and are called a tetrad.

Explain how the random alignment of homologous chromosomes during metaphase I contributes to the variation in gametes produced by meiosis.

Random alignment leads to new combinations of traits. The chromosomes that were originally inherited by the gamete-producing individual came equally from the egg and the sperm. In metaphase I, the duplicated copies of these maternal and paternal homologous chromosomes line up across the center of the cell. The orientation of each tetrad is random. There is an equal chance that the maternally derived chromosomes will be facing either pole. The same is true of the paternally derived chromosomes. The alignment should occur differently in almost every meiosis. As the homologous chromosomes are pulled apart in anaphase I, any combination of maternal and paternal chromosomes will move toward each pole. The gametes formed from these two groups of chromosomes will have a mixture of traits from the individual’s parents. Each gamete is unique.

What is the function of the fused kinetochore found on sister chromatids in prometaphase I?

In metaphase I, the homologous chromosomes line up at the metaphase plate. In anaphase I, the homologous chromosomes are pulled apart and move to opposite poles. Sister chromatids are not separated until meiosis II. The fused kinetochore formed during meiosis I ensures that each spindle microtubule that binds to the tetrad will attach to both sister chromatids.

In a comparison of the stages of meiosis to the stages of mitosis, which stages are unique to meiosis and which stages have the same events in both meiosis and mitosis?

All of the stages of meiosis I, except possibly telophase I, are unique because homologous chromosomes are separated, not sister chromatids. In some species, the chromosomes do not decondense and the nuclear envelopes do not form in telophase I. All of the stages of meiosis II have the same events as the stages of mitosis, with the possible exception of prophase II. In some species, the chromosomes are still condensed and there is no nuclear envelope. Other than this, all processes are the same.

Why would an individual with a mutation that prevented the formation of recombination nodules be considered less fit than other members of its species?

The chromosomes of the individual cannot cross over during meiosis if the individual cannot make recombination nodules. This limits the genetic diversity of the individual’s gametes to what occurs during independent assortment, with all daughter cells receiving complete maternal or paternal chromatids. An individual who cannot produce diverse offspring is considered less fit than individuals who do produce diverse offspring.

Does crossing over occur during prophase II? From an evolutionary perspective, why is this advantageous?

Crossing over does not occur during prophase II it only occurs during prophase I. In prophase II, there are still two copies of each gene, but they are on sister chromatids within a single chromosome (rather than homologous chromosomes as in prophase I). Therefore, any crossover event would still produce two identical chromatids. Because it is advantageous to avoid wasting energy on events that will not increase genetic diversity, crossing over does not occur.


    Adam S. Wilkins and Robin Holliday, “The Evolution of Meiosis from Mitosis,” Genetics 181 (2009): 3–12. Marilee A. Ramesh, Shehre-Banoo Malik and John M. Logsdon, Jr, “A Phylogenetic Inventory of Meiotic Genes: Evidence for Sex in Giardia and an Early Eukaryotic Origin of Meiosis,” Current Biology 15 (2005):185–91.



Although the process of meiosis is related to the more general cell division process of mitosis, it differs in two important respects:

usually occurs between identical sister chromatids and does not result in genetic changes

Meiosis begins with a diploid cell, which contains two copies of each chromosome, termed homologs. First, the cell undergoes DNA replication, so each homolog now consists of two identical sister chromatids. Then each set of homologs pair with each other and exchange genetic information by homologous recombination often leading to physical connections (crossovers) between the homologs. In the first meiotic division, the homologs are segregated to separate daughter cells by the spindle apparatus. The cells then proceed to a second division without an intervening round of DNA replication. The sister chromatids are segregated to separate daughter cells to produce a total of four haploid cells. Female animals employ a slight variation on this pattern and produce one large ovum and two small polar bodies. Because of recombination, an individual chromatid can consist of a new combination of maternal and paternal genetic information, resulting in offspring that are genetically distinct from either parent. Furthermore, an individual gamete can include an assortment of maternal, paternal, and recombinant chromatids. This genetic diversity resulting from sexual reproduction contributes to the variation in traits upon which natural selection can act.

Meiosis uses many of the same mechanisms as mitosis, the type of cell division used by eukaryotes to divide one cell into two identical daughter cells. In some plants, fungi, and protists meiosis results in the formation of spores: haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, like bdelloid rotifers, do not have the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis.

Meiosis does not occur in archaea or bacteria, which generally reproduce asexually via binary fission. However, a "sexual" process known as horizontal gene transfer involves the transfer of DNA from one bacterium or archaeon to another and recombination of these DNA molecules of different parental origin.

Meiosis was discovered and described for the first time in sea urchin eggs in 1876 by the German biologist Oscar Hertwig. It was described again in 1883, at the level of chromosomes, by the Belgian zoologist Edouard Van Beneden, in Ascaris roundworm eggs. The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann, who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. In 1911, the American geneticist Thomas Hunt Morgan detected crossovers in meiosis in the fruit fly Drosophila melanogaster, which helped to establish that genetic traits are transmitted on chromosomes.

The term "meiosis" is derived from the Greek word μείωσις , meaning 'lessening'. It was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905, using the idiosyncratic rendering "maiosis":

We propose to apply the terms Maiosis or Maiotic phase to cover the whole series of nuclear changes included in the two divisions that were designated as Heterotype and Homotype by Flemming. [8]

The spelling was changed to "meiosis" by Koernicke (1905) and by Pantel and De Sinety (1906) to follow the usual conventions for transliterating Greek. [9]

Meiosis is divided into meiosis I and meiosis II which are further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively. The preparatory steps that lead up to meiosis are identical in pattern and name to interphase of the mitotic cell cycle. [10] Interphase is divided into three phases:

    : In this very active phase, the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In G1, each of the chromosomes consists of a single linear molecule of DNA. : The genetic material is replicated each of the cell's chromosomes duplicates to become two identical sister chromatids attached at a centromere. This replication does not change the ploidy of the cell since the centromere number remains the same. The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the light microscope. This will take place during prophase I in meiosis. : G2 phase as seen before mitosis is not present in meiosis. Meiotic prophase corresponds most closely to the G2 phase of the mitotic cell cycle.

Interphase is followed by meiosis I and then meiosis II. Meiosis I separates replicated homologous chromosomes, each still made up of two sister chromatids, into two daughter cells, thus reducing the chromosome number by half. During meiosis II, sister chromatids decouple and the resultant daughter chromosomes are segregated into four daughter cells. For diploid organisms, the daughter cells resulting from meiosis are haploid and contain only one copy of each chromosome. In some species, cells enter a resting phase known as interkinesis between meiosis I and meiosis II.

Meiosis I and II are each divided into prophase, metaphase, anaphase, and telophase stages, similar in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis includes the stages of meiosis I (prophase I, metaphase I, anaphase I, telophase I) and meiosis II (prophase II, metaphase II, anaphase II, telophase II).

During meiosis, specific genes are more highly transcribed. [11] [12] In addition to strong meiotic stage-specific expression of mRNA, there are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating the ultimate meiotic stage-specific protein expression of genes during meiosis. [13] Thus, both transcriptional and translational controls determine the broad restructuring of meiotic cells needed to carry out meiosis.

Meiosis I Edit

Meiosis I segregates homologous chromosomes, which are joined as tetrads (2n, 4c), producing two haploid cells (n chromosomes, 23 in humans) which each contain chromatid pairs (1n, 2c). Because the ploidy is reduced from diploid to haploid, meiosis I is referred to as a reductional division. Meiosis II is an equational division analogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c). [14]

Prophase I Edit

Prophase I is by far the longest phase of meiosis (lasting 13 out of 14 days in mice [15] ). During prophase I, homologous maternal and paternal chromosomes pair, synapse, and exchange genetic information (by homologous recombination), forming at least one crossover per chromosome. [16] These crossovers become visible as chiasmata (plural singular chiasma). [17] This process facilitates stable pairing between homologous chromosomes and hence enables accurate segregation of the chromosomes at the first meiotic division. The paired and replicated chromosomes are called bivalents (two chromosomes) or tetrads (four chromatids), with one chromosome coming from each parent. Prophase I is divided into a series of substages which are named according to the appearance of chromosomes.

Leptotene Edit

The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads". [18] : 27 In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—become "individualized" to form visible strands within the nucleus. [18] : 27 [19] : 353 The chromosomes each form a linear array of loops mediated by cohesin, and the lateral elements of the synaptonemal complex assemble forming an "axial element" from which the loops emanate. [20] Recombination is initiated in this stage by the enzyme SPO11 which creates programmed double strand breaks (around 300 per meiosis in mice). [21] This process generates single stranded DNA filaments coated by RAD51 and DMC1 which invade the homologous chromosomes, forming inter-axis bridges, and resulting in the pairing/co-alignment of homologues (to a distance of

Zygotene Edit

Leptotene is followed by the zygotene stage, also known as zygonema, from Greek words meaning "paired threads", [18] : 27 which in some organisms is also called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. [23] In this stage the homologous chromosomes become much more closely (

100 nm) and stably paired (a process called synapsis) mediated by the installation of the transverse and central elements of the synaptonemal complex. [20] Synapsis is thought to occur in a zipper-like fashion starting from a recombination nodule. The paired chromosomes are called bivalent or tetrad chromosomes.

Pachytene Edit

The pachytene stage ( / ˈ p æ k ɪ t iː n / PAK -i-teen), also known as pachynema, from Greek words meaning "thick threads". [18] : 27 is the stage at which all autosomal chromosomes have synapsed. In this stage homologous recombination, including chromosomal crossover (crossing over), is completed through the repair of the double strand breaks formed in leptotene. [20] Most breaks are repaired without forming crossovers resulting in gene conversion. [24] However, a subset of breaks (at least one per chromosome) form crossovers between non-sister (homologous) chromosomes resulting in the exchange of genetic information. [25] Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology called the pseudoautosomal region. [26] The exchange of information between the homologous chromatids results in a recombination of information each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through an ordinary light microscope, and chiasmata are not visible until the next stage.

Diplotene Edit

During the diplotene stage, also known as diplonema, from Greek words meaning "two threads", [18] : 30 the synaptonemal complex disassembles and homologous chromosomes separate from one another a little. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed at the transition to anaphase I to allow homologous chromosomes to move to opposite poles of the cell.

In human fetal oogenesis, all developing oocytes develop to this stage and are arrested in prophase I before birth. [27] This suspended state is referred to as the dictyotene stage or dictyate. It lasts until meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later.

Diakinesis Edit

Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through". [18] : 30 This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.

Meiotic spindle formation Edit

Unlike mitotic cells, human and mouse oocytes do not have centrosomes to produce the meiotic spindle. In mice, approximately 80 MicroTubule Organizing Centers (MTOCs) form a sphere in the ooplasm and begin to nucleate microtubules that reach out towards chromosomes, attaching to the chromosomes at the kinetochore. Over time the MTOCs merge until two poles have formed, generating a barrel shaped spindle. [28] In human oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that eventually expands to surround the chromosomes. [29] Chromosomes then slide along the microtubules towards the equator of the spindle, at which point the chromosome kinetochores form end-on attachments to microtubules. [30]

Metaphase I Edit

Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both spindle poles attach to their respective kinetochores, the paired homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. This attachment is referred to as a bipolar attachment. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line. [17] The protein complex cohesin holds sister chromatids together from the time of their replication until anaphase. In mitosis, the force of kinetochore microtubules pulling in opposite directions creates tension. The cell senses this tension and does not progress with anaphase until all the chromosomes are properly bi-oriented. In meiosis, establishing tension ordinarily requires at least one crossover per chromosome pair in addition to cohesin between sister chromatids (see Chromosome segregation).

Anaphase I Edit

Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of a pair of sister chromatids) to opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes farther apart. The cell elongates in preparation for division down the center. [17] Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected by a protein named Shugoshin (Japanese for "guardian spirit"), what prevents the sister chromatids from separating. [31] This allows the sister chromatids to remain together while homologs are segregated.

Telophase I Edit

The first meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. However, cytokinesis does not fully complete resulting in "cytoplasmic bridges" which enable the cytoplasm to be shared between daughter cells until the end of meiosis II. [32] Sister chromatids remain attached during telophase I.

Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.

Meiosis II Edit

Meiosis II is the second meiotic division, and usually involves equational segregation, or separation of sister chromatids. Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different. The end result is production of four haploid cells (n chromosomes, 23 in humans) from the two haploid cells (with n chromosomes, each consisting of two sister chromatids) produced in meiosis I. The four main steps of meiosis II are: prophase II, metaphase II, anaphase II, and telophase II.

In prophase II, we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrosomes move to the polar regions and arrange spindle fibers for the second meiotic division.

In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes at opposite poles. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate. [33]

This is followed by anaphase II, in which the remaining centromeric cohesin, not protected by Shugoshin anymore, is cleaved, allowing the sister chromatids to segregate. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles. [31]

The process ends with telophase II, which is similar to telophase I, and is marked by decondensation and lengthening of the chromosomes and the disassembly of the spindle. Nuclear envelopes re-form and cleavage or cell plate formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.

Meiosis is now complete and ends up with four new daughter cells.

The origin and function of meiosis are currently not well understood scientifically, and would provide fundamental insight into the evolution of sexual reproduction in eukaryotes. There is no current consensus among biologists on the questions of how sex in eukaryotes arose in evolution, what basic function sexual reproduction serves, and why it is maintained, given the basic two-fold cost of sex. It is clear that it evolved over 1.2 billion years ago, and that almost all species which are descendants of the original sexually reproducing species are still sexual reproducers, including plants, fungi, and animals.

Meiosis is a key event of the sexual cycle in eukaryotes. It is the stage of the life cycle when a cell gives rise to haploid cells (gametes) each having half as many chromosomes as the parental cell. Two such haploid gametes, ordinarily arising from different individual organisms, fuse by the process of fertilization, thus completing the sexual cycle.

Meiosis is ubiquitous among eukaryotes. It occurs in single-celled organisms such as yeast, as well as in multicellular organisms, such as humans. Eukaryotes arose from prokaryotes more than 2.2 billion years ago [34] and the earliest eukaryotes were likely single-celled organisms. To understand sex in eukaryotes, it is necessary to understand (1) how meiosis arose in single celled eukaryotes, and (2) the function of meiosis.

The new combinations of DNA created during meiosis are a significant source of genetic variation alongside mutation, resulting in new combinations of alleles, which may be beneficial. Meiosis generates gamete genetic diversity in two ways: (1) Law of Independent Assortment. The independent orientation of homologous chromosome pairs along the metaphase plate during metaphase I and orientation of sister chromatids in metaphase II, this is the subsequent separation of homologs and sister chromatids during anaphase I and II, it allows a random and independent distribution of chromosomes to each daughter cell (and ultimately to gametes) [35] and (2) Crossing Over. The physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of genetic information within chromosomes. [36]

Prophase I arrest Edit

Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of meiosis. [37] In humans, as an example, oocytes are formed between three and four months of gestation within the fetus and are therefore present at birth. During this prophase I arrested stage (dictyate), which may last for decades, four copies of the genome are present in the oocytes. The arrest of ooctyes at the four genome copy stage was proposed to provide the informational redundancy needed to repair damage in the DNA of the germline. [37] The repair process used appears to involve homologous recombinational repair [37] [38] Prophase I arrested oocytes have a high capability for efficient repair of DNA damages, particularly exogenously induced double-strand breaks. [38] DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility. [38]

In life cycles Edit

Meiosis occurs in eukaryotic life cycles involving sexual reproduction, consisting of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. In multicellular organisms, there is an intermediary step between the diploid and haploid transition where the organism grows. At certain stages of the life cycle, germ cells produce gametes. Somatic cells make up the body of the organism and are not involved in gamete production.

Cycling meiosis and fertilization events produces a series of transitions back and forth between alternating haploid and diploid states. The organism phase of the life cycle can occur either during the diploid state (diplontic life cycle), during the haploid state (haplontic life cycle), or both (haplodiplontic life cycle, in which there are two distinct organism phases, one during the haploid state and the other during the diploid state). In this sense there are three types of life cycles that utilize sexual reproduction, differentiated by the location of the organism phase(s). [ citation needed ]

In the diplontic life cycle (with pre-gametic meiosis), of which humans are a part, the organism is diploid, grown from a diploid cell called the zygote. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes (the spermatozoa for males and ova for females), which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism.

In the haplontic life cycle (with post-zygotic meiosis), the organism is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. Two organisms of opposing sex contribute their haploid gametes to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism. Many fungi and many protozoa utilize the haplontic life cycle. [ citation needed ]

Finally, in the haplodiplontic life cycle (with sporic or intermediate meiosis), the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the alternation of generations. The diploid organism's germ-line cells undergo meiosis to produce spores. The spores proliferate by mitosis, growing into a haploid organism. The haploid organism's gamete then combines with another haploid organism's gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become a diploid organism again. The haplodiplontic life cycle can be considered a fusion of the diplontic and haplontic life cycles. [39] [ citation needed ]

In plants and animals Edit

Meiosis occurs in all animals and plants. The end result, the production of gametes with half the number of chromosomes as the parent cell, is the same, but the detailed process is different. In animals, meiosis produces gametes directly. In land plants and some algae, there is an alternation of generations such that meiosis in the diploid sporophyte generation produces haploid spores. These spores multiply by mitosis, developing into the haploid gametophyte generation, which then gives rise to gametes directly (i.e. without further meiosis). In both animals and plants, the final stage is for the gametes to fuse, restoring the original number of chromosomes. [40]

In mammals Edit

In females, meiosis occurs in cells known as oocytes (singular: oocyte). Each primary oocyte divides twice in meiosis, unequally in each case. The first division produces a daughter cell, and a much smaller polar body which may or may not undergo a second division. In meiosis II, division of the daughter cell produces a second polar body, and a single haploid cell, which enlarges to become an ovum. Therefore, in females each primary oocyte that undergoes meiosis results in one mature ovum and one or two polar bodies.

Note that there are pauses during meiosis in females. Maturing oocytes are arrested in prophase I of meiosis I and lie dormant within a protective shell of somatic cells called the follicle. At the beginning of each menstrual cycle, FSH secretion from the anterior pituitary stimulates a few follicles to mature in a process known as folliculogenesis. During this process, the maturing oocytes resume meiosis and continue until metaphase II of meiosis II, where they are again arrested just before ovulation. If these oocytes are fertilized by sperm, they will resume and complete meiosis. During folliculogenesis in humans, usually one follicle becomes dominant while the others undergo atresia. The process of meiosis in females occurs during oogenesis, and differs from the typical meiosis in that it features a long period of meiotic arrest known as the dictyate stage and lacks the assistance of centrosomes. [41] [42]

In males, meiosis occurs during spermatogenesis in the seminiferous tubules of the testicles. Meiosis during spermatogenesis is specific to a type of cell called spermatocytes, which will later mature to become spermatozoa. Meiosis of primordial germ cells happens at the time of puberty, much later than in females. Tissues of the male testis suppress meiosis by degrading retinoic acid, proposed to be a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their own retinoic acid. Sensitivity to retinoic acid is also adjusted by proteins called nanos and DAZL. [43] [44] Genetic loss-of-function studies on retinoic acid-generating enzymes have shown that retinoic acid is required postnatally to stimulate spermatogonia differentiation which results several days later in spermatocytes undergoing meiosis, however retinoic acid is not required during the time when meiosis initiates. [45]

In female mammals, meiosis begins immediately after primordial germ cells migrate to the ovary in the embryo. Some studies suggest that retinoic acid derived from the primitive kidney (mesonephros) stimulates meiosis in embryonic ovarian oogonia and that tissues of the embryonic male testis suppress meiosis by degrading retinoic acid. [46] However, genetic loss-of-function studies on retinoic acid-generating enzymes have shown that retinoic acid is not required for initiation of either female meiosis which occurs during embryogenesis [47] or male meiosis which initiates postnatally. [45]

Flagellates Edit

While the majority of eukaryotes have a two-divisional meiosis (though sometimes achiasmatic), a very rare form, one-divisional meiosis, occurs in some flagellates (parabasalids and oxymonads) from the gut of the wood-feeding cockroach Cryptocercus. [48]

Recombination among the 23 pairs of human chromosomes is responsible for redistributing not just the actual chromosomes, but also pieces of each of them. There is also an estimated 1.6-fold more recombination in females relative to males. In addition, average, female recombination is higher at the centromeres and male recombination is higher at the telomeres. On average, 1 million bp (1 Mb) correspond to 1 cMorgan (cm = 1% recombination frequency). [49] The frequency of cross-overs remain uncertain. In yeast, mouse and human, it has been estimated that ≥200 double-strand breaks (DSBs) are formed per meiotic cell. However, only a subset of DSBs (

5–30% depending on the organism), go on to produce crossovers, [50] which would result in only 1-2 cross-overs per human chromosome.

Nondisjunction Edit

The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction. When the segregation is not normal, it is called nondisjunction. This results in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.

Most monosomic and trisomic human embryos are not viable, but some aneuploidies can be tolerated, such as trisomy for the smallest chromosome, chromosome 21. Phenotypes of these aneuploidies range from severe developmental disorders to asymptomatic. Medical conditions include but are not limited to:

    – trisomy of chromosome 21 – trisomy of chromosome 13 – trisomy of chromosome 18 – extra X chromosomes in males – i.e. XXY, XXXY, XXXXY, etc. – lacking of one X chromosome in females – i.e. X0 – an extra X chromosome in females – an extra Y chromosome in males.

The probability of nondisjunction in human oocytes increases with increasing maternal age, [51] presumably due to loss of cohesin over time. [52]

In order to understand meiosis, a comparison to mitosis is helpful. The table below shows the differences between meiosis and mitosis. [53]

Meiosis Mitosis
End result Normally four cells, each with half the number of chromosomes as the parent Two cells, having the same number of chromosomes as the parent
Function Production of gametes (sex cells) in sexually reproducing eukaryotes with diplont life cycle Cellular reproduction, growth, repair, asexual reproduction
Where does it happen? Almost all eukaryotes (animals, plants, fungi, and protists) [54] [48]
In gonads, before gametes (in diplontic life cycles)
After zygotes (in haplontic)
Before spores (in haplodiplontic)
All proliferating cells in all eukaryotes
Steps Prophase I, Metaphase I, Anaphase I, Telophase I,
Prophase II, Metaphase II, Anaphase II, Telophase II
Prophase, Prometaphase, Metaphase, Anaphase, Telophase
Genetically same as parent? No Yes
Crossing over happens? Yes, normally occurs between each pair of homologous chromosomes Very rarely
Pairing of homologous chromosomes? Yes No
Cytokinesis Occurs in Telophase I and Telophase II Occurs in Telophase
Centromeres split Does not occur in Anaphase I, but occurs in Anaphase II Occurs in Anaphase

How a cell proceeds to meiotic division in meiotic cell division is not well known. Maturation promoting factor (MPF) seemingly have role in frog Oocyte meiosis. In the fungus S. pombe. there is a role of MeiRNA binding protein for entry to meiotic cell division. [55]

It has been suggested that Yeast CEP1 gene product, that binds centromeric region CDE1, may play a role in chromosome pairing during meiosis-I. [56]

Meiotic recombination is mediated through double stranded break, which is catalyzed by Spo11 protein. Also Mre11, Sae2 and Exo1 play role in breakage and recombination. After the breakage happen, recombination take place which is typically homologous. The recombination may go through either a double Holliday junction (dHJ) pathway or synthesis-dependent strand annealing (SDSA). (The second one gives to noncrossover product). [57]

Seemingly there are checkpoints for meiotic cell division too. In S. pombe, Rad proteins, S. pombe Mek1 (with FHA kinase domain), Cdc25, Cdc2 and unknown factor is thought to form a checkpoint. [58]

In vertebrate oogenesis, maintained by cytostatic factor (CSF) has role in switching into meiosis-II. [56]

Watch the video: Meiosis Updated (July 2022).


  1. Grindan

    the remarkable phrase and is timely

  2. Zechariah

    If I were you, I would not do that.

  3. Mikakree

    How absurd

  4. Gukree

    I am sure this is the wrong path.

  5. Goltirn

    Just think about it!

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