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11.1: What Are Genes? - Biology

11.1: What Are Genes? - Biology



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Learning Objectives

  • Explain the two functions of the genome
  • Explain the meaning of the central dogma of molecular biology
  • Differentiate between genotype and phenotype and explain how environmental factors influence phenotype

Clinical Focus: Part 1

Mark is 60-year-old software engineer who suffers from type II diabetes, which he monitors and keeps under control largely through diet and exercise. One spring morning, while doing some gardening, he scraped his lower leg while walking through blackberry brambles. He continued working all day in the yard and did not bother to clean the wound and treat it with antibiotic ointment until later that evening. For the next 2 days, his leg became increasingly red, swollen, and warm to the touch. It was sore not only on the surface, but deep in the muscle. After 24 hours, Mark developed a fever and stiffness in the affected leg. Feeling increasingly weak, he called a neighbor, who drove him to the emergency department.

Exercise (PageIndex{1})

  1. Did Mark wait too long to seek medical attention? At what point do his signs and symptoms warrant seeking medical attention?
  2. What types of infections or other conditions might be responsible for Mark’s symptoms?

DNA serves two essential functions that deal with cellular information. First, DNA is the genetic material responsible for inheritance and is passed from parent to offspring for all life on earth. To preserve the integrity of this genetic information, DNA must be replicated with great accuracy, with minimal errors that introduce changes to the DNA sequence. A genome contains the full complement of DNA within a cell and is organized into smaller, discrete units called genes that are arranged on chromosomes and plasmids. The second function of DNA is to direct and regulate the construction of the proteins necessary to a cell for growth and reproduction in a particular cellular environment.

A gene is composed of DNA that is “read” or transcribed to produce an RNA molecule during the process of transcription. One major type of RNA molecule, called messenger RNA (mRNA), provides the information for the ribosome to catalyze protein synthesis in a process called translation. The processes of transcription and translation are collectively referred to as gene expression. Gene expression is the synthesis of a specific protein with a sequence of amino acids that is encoded in the gene. The flow of genetic information from DNA to RNA to protein is described by the central dogma (Figure (PageIndex{1})). This central dogma of molecular biology further elucidates the mechanism behind Beadle and Tatum’s “one gene-one enzyme” hypothesis (see Using Microorganisms to Discover the Secrets of Life). Each of the processes of replication, transcription, and translation includes the stages of 1) initiation, 2) elongation (polymerization), and 3) termination. These stages will be described in more detail in this chapter.

A cell’s genotype is the full collection of genes it contains, whereas its phenotype is the set of observable characteristics that result from those genes. The phenotype is the product of the array of proteins being produced by the cell at a given time, which is influenced by the cell’s genotype as well as interactions with the cell’s environment. Genes code for proteins that have functions in the cell. Production of a specific protein encoded by an individual gene often results in a distinct phenotype for the cell compared with the phenotype without that protein. For this reason, it is also common to refer to the genotype of an individual gene and its phenotype. Although a cell’s genotype remains constant, not all genes are used to direct the production of their proteins simultaneously. Cells carefully regulate expression of their genes, only using genes to make specific proteins when those proteins are needed (Figure (PageIndex{2})).

Exercise (PageIndex{2})

  1. What are the two functions of DNA?
  2. Distinguish between the genotype and phenotype of a cell.
  3. How can cells have the same genotype but differ in their phenotype?

USE AND ABUSE OF GENOME DATA

Why can some humans harbor opportunistic pathogens like Haemophilus influenzae, Staphylococcus aureus, or Streptococcus pyogenes, in their upper respiratory tracts but remain asymptomatic carriers, while other individuals become seriously ill when infected? There is evidence suggesting that differences in susceptibility to infection between patients may be a result, at least in part, of genetic differences between human hosts. For example, genetic differences in human leukocyte antigens (HLAs) and red blood cell antigens among hosts have been implicated in different immune responses and resulting disease progression from infection with H. influenzae.

Because the genetic interplay between pathogen and host may contribute to disease outcomes, understanding differences in genetic makeup between individuals may be an important clinical tool. Ecological genomics is a relatively new field that seeks to understand how the genotypes of different organisms interact with each other in nature. The field answers questions about how gene expression of one organism affects gene expression of another. Medical applications of ecological genomics will focus on how pathogens interact with specific individuals, as opposed to humans in general. Such analyses would allow medical professionals to use knowledge of an individual’s genotype to apply more individualized plans for treatment and prevention of disease.

With the advent of next-generation sequencing, it is relatively easy to obtain the entire genomic sequences of pathogens; a bacterial genome can be sequenced in as little as a day.1 The speed and cost of sequencing the human genome has also been greatly reduced and, already, individuals can submit samples to receive extensive reports on their personal genetic traits, including ancestry and carrier status for various genetic diseases. As sequencing technologies progress further, such services will continue to become less expensive, more extensive, and quicker.

However, as this day quickly approaches, there are many ethical concerns with which society must grapple. For example, should genome sequencing be a standard practice for everybody? Should it be required by law or by employers if it will lower health-care costs? If one refuses genome sequencing, does he or she forfeit his or her right to health insurance coverage? For what purposes should the data be used? Who should oversee proper use of these data? If genome sequencing reveals predisposition to a particular disease, do insurance companies have the right to increase rates? Will employers treat an employee differently? Knowing that environmental influences also affect disease development, how should the data on the presence of a particular disease-causing allele in an individual be used ethically? The Genetic Information Nondiscrimination Act of 2008 (GINA) currently prohibits discriminatory practices based on genetic information by both health insurance companies and employers. However, GINA does not cover life, disability, or long-term care insurance policies. Clearly, all members of society must continue to engage in conversations about these issues so that such genomic data can be used to improve health care while simultaneously protecting an individual’s rights.

Key Concepts and Summary

  • DNA serves two important cellular functions: It is the genetic material passed from parent to offspring and it serves as the information to direct and regulate the construction of the proteins necessary for the cell to perform all of its functions.
  • The central dogma states that DNA organized into genes specifies the sequences of messenger RNA (mRNA), which, in turn, specifies the amino acid sequence of proteins.
  • The genotype of a cell is the full collection of genes a cell contains. Not all genes are used to make proteins simultaneously. The phenotype is a cell’s observable characteristics resulting from the proteins it is producing at a given time under specific environmental conditions.

Multiple Choice

DNA does all but which of the following?

A. serves as the genetic material passed from parent to offspring
B. remains constant despite changes in environmental conditions
C. provides the instructions for the synthesis of messenger RNA
D. is read by ribosomes during the process of translation

D

According to the central dogma, which of the following represents the flow of genetic information in cells?

A. protein to DNA to RNA
B. DNA to RNA to protein
C. RNA to DNA to protein
D. DNA to protein to RNA

B

True/False

Cells are always producing proteins from every gene they possess.

False

Fill in the Blank

The process of making an RNA copy of a gene is called ________.

transcription

A cell’s ________ remains constant whereas its phenotype changes in response to environmental influences.

genotype or genome

Short Answer

Can two observably different cells have the same genotype? Explain.

Critical Thinking

A pure culture of an unknown bacterium was streaked onto plates of a variety of media. You notice that the colony morphology is strikingly different on plates of minimal media with glucose compared to that seen on trypticase soy agar plates. How can you explain these differences in colony morphology?

Footnotes

  1. 1 D.J. Edwards, K.E. Holt. “Beginner’s Guide to Comparative Bacterial Genome Analysis Using Next-Generation Sequence Data.” Microbial Informatics and Experimentation 3 no. 1 (2013):2.

Chapter 11.1 - Gregor Mendel

TIP: In any cross that is dihybrid (AaBb x AaBb) you will always get a 9:3:3:1 ratio, if you memorize this, you can save the trouble of doing a giant square!

A Mathematical Alternative (LAWS OF PROBABILITY)

A punnet square is not needed to determine the ratios of genotypes and phenotypes. Simple statistics and math can save you the trouble of filling out a square.

In a monohybrid cross Pp x Pp, each parent produced P gametes and p gametes

If you wanted to determine how many of the offspring are pp: x =

H is dominate for long hair (h = short) and B is dominate for black eyes (b = red eyes). If the parents are.

HhBb x hhBb

How many off the offspring will be short haired and red eyed?

Task: Use mathematical analysis to determine the number of short haired, black eyed offspring from the cross above.

TWO-TRAIT TEST CROSS

Used to determine the genotype of an "unknown" by crossing it with an individual that is homozygous recessive for both traits.

In flies (Long wings is dominant to short wings, Gray body is dominant to black)

A L __ G ___ is test crossed.

The offspring are 1:1:1:1 --> What is the genotype of the unknown parent?
If the offspring are half long winged & gray, and half long winged and black --> What is the genotype of the unknown parent?


11.1 The Process of Meiosis

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 11.2).

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 11.3). 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 11.4).

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 11.5).

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.

Link to Learning

Review the process of meiosis, observing how chromosomes align and migrate, at Meiosis: An Interactive Animation.

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 11.6.

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 11.7). 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.

Evolution Connection

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.

Link to Learning

Click through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis at How Cells Divide.


Mean genes and the biology of aggression: a critical review of recent animal and human research

Recent genetic work has suggested that abnormalities in serotonin biochemistry are directly causally linked to aggressive behavior, and there appears to be a consensus in the psychiatric literature that low levels of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) in cerebrospinal fluid are specifically associated with impulsive violent behavior. We review the limitations of the genetic studies and conduct a meta-analysis of 39 studies linking 5-HIAA to aggression in humans. No differences in mean 5-HIAA levels were found between groups of violent impulsive psychiatric patients and groups of subjects diagnosed with other psychiatric or medical conditions not considered to involve violence once these levels had been corrected for three nonpsychiatric sources of variation (age, sex and height). However, mean 5-HIAA levels in both of these groups were lower than the mean corrected level in groups of normal healthy volunteers. The results confirm an association between low 5-HIAA levels and psychiatric disorders, but fail to support any specific relationship between low 5-HIAA levels and impulsive aggression or criminality. It is premature and misleading to speak of "mean genes" (Hen 1996) or a specific neurochemistry of aggressive behavior.


Carbohydrate Chains: Enzymatic and Chemical Synthesis☆

Thomas J. Tolbert , Chi-Huey Wong , in Reference Module in Life Sciences , 2020

Recombinant Expression of Glycosyltransferases

Genomic sequencing efforts have made the DNA sequences of many mammalian and bacterial glycosyltransferases freely available. This has enabled efforts to recombinantly overexpress glycosyltransferases for use in the synthesis of carbohydrate chains. The use of bacterial and yeast expression systems has allowed several mammalian glycosyltransferases to be produced on relatively large scale, and it has also been recognized that many bacterial glycosyltransferases, which are often easier to express, can be utilized to produce mammalian-type carbohydrate structures. These efforts are ongoing and are continually increasing the range of carbohydrate structures that can be formed using glycosyltransferases.


Testing

A few ways exist to test people as mutants:

  • Looking for readings with Cerebro, ⎶]
  • Looking for the X-Gene (and possibly for altered DNA helix or hereditary markers, ⎶] although they are presumably not specific to mutants).
    • There more extensive tests that could be done, involving cellular search in bone marrow. ⎶]

    In Earth-11326, X-Gene was proceeded in order to detect and arrest mutants. ⎷]

    False negatives

    On some cases, X-Gene testing at birth have revealed negative results for people actually proved to be mutant afterwards, such as Molly Hayes. ⎸]

    In Earth-4935, similarly, Blaquesmith's genescan stated him to be X-Factor Negative and his appearance to be the result of a genetic defect and not an X-Factor mutation, but that test was seemingly wrong, as Blaquesmith exhibited a telepathic gift ⎹] (he was later confirmed to be a mutant). ⎺]


    Chapter 11 - Cell Communication

    • What messages are passed from cell to cell? How do cells respond to these messages?
    • We will first consider communication in microbes, to gain insight into the evolution of cell signaling.

    Cell signaling evolved early in the history of life.

    • One topic of cell “conversation” is sex.
    • Saccharomyces cerevisiae, the yeast of bread, wine, and beer, identifies potential mates by chemical signaling.
      • There are two sexes, a and ?, each of which secretes a specific signaling molecule, a factor and ? factor, respectively.
      • These factors each bind to receptor proteins on the other mating type.
      • The molecular details of these pathways are strikingly similar in yeast and animal cells, even though their last common ancestor lived more than a billion years ago.
      • Signaling systems of bacteria and plants also share similarities.

      Communicating cells may be close together or far apart.

      • Multicellular organisms release signaling molecules that target other cells.
      • Cells may communicate by direct contact.
        • Both animals and plants have cell junctions that connect to the cytoplasm of adjacent cells.
        • Signaling substances dissolved in the cytosol can pass freely between adjacent cells.
        • Animal cells can communicate by direct contact between membrane-bound cell surface molecules.
        • Such cell-cell recognition is important to such processes as embryonic development and the immune response.
        • Some transmitting cells release local regulators that influence cells in the local vicinity.
        • One class of local regulators in animals, growth factors, includes compounds that stimulate nearby target cells to grow and multiply.
        • This is an example of paracrine signaling, which occurs when numerous cells simultaneously receive and respond to growth factors produced by a single cell in their vicinity.
        • The neurotransmitter stimulates the target cell.
        • The transmission of a signal through the nervous system can also be considered an example of long-distance signaling.
        • In animals, specialized endocrine cells release hormones into the circulatory system, by which they travel to target cells in other parts of the body.
        • Plant hormones, called growth regulators, may travel in vessels but more often travel from cell to cell or move through air by diffusion.
        • The plant hormone ethylene (C2H4), which promotes fruit ripening and regulates growth, is a hydrocarbon of only six atoms, capable of passing through cell walls.
        • Insulin, which regulates blood sugar levels in mammals, is a protein with thousands of atoms.
        • The signal must be recognized by a specific receptor molecule, and the information it carries must be changed into another form, or transduced, inside the cell before the cell can respond.

        The three stages of cell signaling are reception, transduction, and response.

        • E. W. Sutherland and his colleagues pioneered our understanding of cell signaling.
          • Their work investigated how the animal hormone epinephrine stimulates breakdown of the storage polysaccharide glycogen in liver and skeletal muscle.
          • Breakdown of glycogen releases glucose derivatives that can be used for fuel in glycolysis or released as glucose in the blood for fuel elsewhere.
          • Thus one effect of epinephrine, which is released from the adrenal gland during times of physical or mental stress, is mobilization of fuel reserves.
          • However, epinephrine did not activate the phosphorylase directly in vitro but could only act via intact cells.
          • Therefore, there must be an intermediate step or steps occurring inside the cell.
          • The plasma membrane must be involved in transmitting the epinephrine signal.
          • In reception, a chemical signal binds to a cellular protein, typically at the cell’s surface or inside the cell.
          • In transduction, binding leads to a change in the receptor that triggers a series of changes in a series of different molecules along a signal-transduction pathway. The molecules in the pathway are called relay molecules.
          • In response, the transduced signal triggers a specific cellular activity.

          Concept 11.2 Reception: A signal molecule binds to a receptor protein, causing it to change shape

          • The cell targeted by a particular chemical signal has a receptor protein on or in the target cell that recognizes the signal molecule.
            • Recognition occurs when the signal binds to a specific site on the receptor that is complementary in shape to the signal.
            • For other receptors, this causes aggregation of receptor molecules, leading to further molecular events inside the cell.

            Some receptor proteins are intracellular.

            • Some signal receptors are dissolved in the cytosol or nucleus of target cells.
              • To reach these receptors, the signals pass through the target cell’s plasma membrane.
              • Such chemical messengers are either hydrophobic enough or small enough to cross the phospholipid interior of the plasma membrane.
              • The cytosol of target cells contains receptor molecules that bind testosterone, activating the receptor.
              • These activated proteins enter the nucleus and turn on specific genes that control male sex characteristics.

              Most signal receptors are plasma membrane proteins.

              • Most signal molecules are water-soluble and too large to pass through the plasma membrane.
              • They influence cell activities by binding to receptor proteins on the plasma membrane.
                • Binding leads to changes in the shape of the receptor or to the aggregation of receptors.
                • These cause changes in the intracellular environment.
                • Seven alpha helices span the membrane.
                • G-protein-linked receptors bind many different signal molecules, including yeast mating factors, epinephrine and many other hormones, and neurotransmitters.
                • If GDP is bound to the G protein, the G protein is inactive.
                • When the appropriate signal molecule binds to the extracellular side of the receptor, the G protein binds GTP (instead of GDP) and becomes active.
                • The activated G protein dissociates from the receptor and diffuses along the membrane, where it binds to an enzyme, altering its activity.
                • The activated enzyme triggers the next step in a pathway leading to a cellular response.
                • This change turns the G protein off.
                • They play important roles during embryonic development.
                • Vision and smell in humans depend on these proteins.
                • Bacterial infections causing cholera and botulism interfere with G-protein function.
                • This system helps the cell regulate and coordinate many aspects of cell growth and reproduction.
                • A kinase is an enzyme that catalyzes the transfer of phosphate groups.
                • The cytoplasmic side of these receptors functions as a tyrosine kinase, transferring a phosphate group from ATP to tyrosine on a substrate protein.
                • An extracellular signal-binding site.
                • A single alpha helix spanning the membrane.
                • An intracellular tail with several tyrosines.
                • Ligands bind to two receptors, causing the two receptors to aggregate and form a dimer.
                • One tyrosine-kinase receptor dimer may activate ten or more different intracellular proteins simultaneously.
                • These activated relay proteins trigger many different transduction pathways and responses.
                • Binding by a ligand to the extracellular side changes the protein’s shape and opens the channel.
                • When the ligand dissociates from the receptor protein, the channel closes.
                • For example, neurotransmitter molecules released at a synapse between two neurons bind as ligands to ion channels on the receiving cell, causing the channels to open.
                • Ions flow in and trigger an electrical signal that propagates down the length of the receiving cell.

                Concept 11.3 Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell

                • The transduction stage of signaling is usually a multistep pathway.
                • These pathways often greatly amplify the signal.
                  • If some molecules in a pathway transmit a signal to multiple molecules of the next component in the series, the result can be large numbers of activated molecules at the end of the pathway.

                  Pathways relay signals from receptors to cellular responses.

                  • Signal-transduction pathways act like falling dominoes.
                    • The signal-activated receptor activates another protein, which activates another, and so on, until the protein that produces the final cellular response is activated.
                    • The interaction of proteins is a major theme of cell signaling.
                    • Protein interaction is a unifying theme of all cellular regulation.
                    • It passes on information.
                    • At each step, the signal is transduced into a different form, often by a conformational change in a protein.
                    • The conformational change is often brought about by phosphorylation.

                    Protein phosphorylation, a common mode of regulation in cells, is a major mechanism of signal transduction.

                    • The phosphorylation of proteins by a specific enzyme (a protein kinase) is a widespread cellular mechanism for regulating protein activity.
                      • Most protein kinases act on other substrate proteins, unlike tyrosine kinases that act on themselves.
                      • Rarely, phosphorylation inactivates protein activity.
                      • Fully 2% of our genes are thought to code for protein kinases.
                      • Together, they regulate a large proportion of the thousands of cell proteins.
                      • These enzymes rapidly remove phosphate groups from proteins, a process called dephosphorylation.
                      • Phosphatases also make the protein kinases available for reuse, enabling the cell to respond again to a signal.

                      Certain signal molecules and ions are key components of signaling pathways (second messengers).

                      • Many signaling pathways involve small, water-soluble, nonprotein molecules or ions called second messengers.
                        • These molecules rapidly diffuse throughout the cell.
                        • Two of the most widely used second messengers are cyclic AMP and Ca2+.
                        • This occurs because the activated receptor activates adenylyl cyclase, which converts ATP to cAMP.
                        • The normal cellular concentration of cAMP can be boosted twentyfold within seconds.
                        • cAMP is short-lived, as phosphodiesterase converts it to AMP.
                        • Another surge of epinephrine is needed to reboost the cytosolic concentration of cAMP.
                        • Caffeine blocks the conversion of cAMP to AMP, maintaining the system in a state of activation in the absence of epinephrine.
                        • G-protein-linked receptors, G proteins, and protein kinases are other components of cAMP pathways.
                        • cAMP diffuses through the cell and activates a serine/threonine kinase called protein kinase A.
                        • The activated kinase phosphorylates various other proteins.
                        • These use a different signal molecule to activate a different receptor that activates an inhibitory G protein.
                        • The cholera bacterium, Vibrio cholerae, may be present in water contaminated with human feces.
                        • This bacterium colonizes the small intestine and produces a toxin that modifies a G protein that regulates salt and water secretion.
                        • The modified G protein is unable to hydrolyze GTP to GDP and remains stuck in its active form, continuously stimulating adenylyl cyclase to make cAMP.
                        • The resulting high concentration of cAMP causes the intestinal cells to secrete large amounts of water and salts into the intestines, leading to profuse diarrhea and death from loss of water and salts.
                        • One pathway uses cyclic GMP, or cGMP, as a signaling molecule. Its effects include the relaxation of smooth muscle cells in artery walls.
                        • A compound was developed to treat chest pains. This compound inhibits the hydrolysis of cGMP to GMP, prolonging the signal and increasing blood flow to the heart muscle.
                        • Under the trade name Viagra, this compound is now widely used as a treatment for erectile dysfunction. Viagra causes dilation of blood vessels, allowing increased blood flow to the penis.
                        • In animal cells, increases in Ca2+ may cause contraction of muscle cells, secretion of certain substances, and cell division.
                        • In plant cells, increases in Ca2+ trigger responses such as the pathway for greening in response to light.
                        • Various protein pumps transport Ca2+ outside the cell or into the endoplasmic reticulum or other organelles.
                        • As a result, the concentration of Ca2+ in the ER is usually much higher than the concentration in the cytosol.
                        • DAG and IP3 are created when a phospholipase cleaves membrane phospholipid PIP2.
                        • The phospholipase may be activated by a G protein or by a tyrosine-kinase receptor.
                        • IP3 activates a gated-calcium channel, releasing Ca2+ from the ER.

                        Concept 11.4 Response: Cell signaling leads to regulation of cytoplasmic activities or transcription

                        • Ultimately, a signal-transduction pathway leads to the regulation of one or more cellular activities.
                          • This may be the opening or closing of an ion channel or a change in cell metabolism.
                          • For example, epinephrine helps regulate cellular energy metabolism by activating enzymes that catalyze the breakdown of glycogen.

                          Elaborate pathways amplify and specify the cell’s response to signals.

                          • Signaling pathways with multiple steps have two benefits.
                            1. They amplify the response to a signal.
                            2. They contribute to the specificity of the response.
                          • At each catalytic step in a cascade, the number of activated products is much greater than in the preceding step.
                            • In the epinephrine-triggered pathway, binding by a small number of epinephrine molecules can lead to the release of hundreds of millions of glucose molecules.
                            • For example, epinephrine triggers liver or striated muscle cells to break down glycogen, but stimulates cardiac muscle cells to contract, leading to a rapid heartbeat.
                            • The response of a particular cell to a signal depends on its particular collection of receptor proteins, relay proteins, and proteins needed to carry out the response.
                            • Two cells that respond differently to the same signal differ in one or more of the proteins that handle and respond to the signal.
                            • Scaffolding proteins may themselves be relay proteins to which several other relay proteins attach.
                            • This hardwiring enhances the speed, accuracy, and efficiency of signal transfer between cells.
                            • The inherited disorder Wiskott-Aldrich syndrome (WAS) is caused by the absence of a single relay protein.
                            • Symptoms include abnormal bleeding, eczema, and a predisposition to infections and leukemia, due largely to the absence of the protein in the cells of the immune system.
                            • The WAS protein is located just beneath the cell surface, where it interacts with the microfilaments of the cytoskeleton and with several signaling pathways, including those that regulate immune cell proliferation.
                            • When the WAS protein is absent, the cytoskeleton is not properly organized and signaling pathways are disrupted.
                            • For a cell to remain alert and capable of responding to incoming signals, each molecular change in its signaling pathways must last only a short time.
                            • If signaling pathway components become locked into one state, whether active or inactive, the proper function of the cell can be disrupted.
                            • Binding of signal molecules to receptors must be reversible, allowing the receptors to return to their inactive state when the signal is released.
                            • Similarly, activated signals (cAMP and phosphorylated proteins) must be inactivated by appropriate enzymes to prepare the cell for a fresh signal.

                            Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 11-1


                            Alternative RNA Splicing

                            In the 1970s, genes were first observed that exhibited alternative RNA splicing. Alternative RNA splicing is a mechanism that allows different protein products to be produced from one gene when different combinations of introns (and sometimes exons) are removed from the transcript (Figure 9.23). This alternative splicing can be haphazard, but more often it is controlled and acts as a mechanism of gene regulation, with the frequency of different splicing alternatives controlled by the cell as a way to control the production of different protein products in different cells, or at different stages of development. Alternative splicing is now understood to be a common mechanism of gene regulation in eukaryotes according to one estimate, 70% of genes in humans are expressed as multiple proteins through alternative splicing.

                            Figure 9.23 There are five basic modes of alternative splicing. Segments of pre-mRNA with exons shown in blue, red, orange, and pink can be spliced to produce a variety of new mature mRNA segments.

                            How could alternative splicing evolve? Introns have a beginning and ending recognition sequence, and it is easy to imagine the failure of the splicing mechanism to identify the end of an intron and find the end of the next intron, thus removing two introns and the intervening exon. In fact, there are mechanisms in place to prevent such exon skipping, but mutations are likely to lead to their failure. Such “mistakes” would more than likely produce a nonfunctional protein. Indeed, the cause of many genetic diseases is alternative splicing rather than mutations in a sequence. However, alternative splicing would create a protein variant without the loss of the original protein, opening up possibilities for adaptation of the new variant to new functions. Gene duplication has played an important role in the evolution of new functions in a similar way—by providing genes that may evolve without eliminating the original functional protein.


                            Blood iron levels could be key to slowing aging, gene study shows

                            Genes linked to ageing that could help explain why some people age at different rates to others have been identified by scientists.

                            The international study using genetic data from more than a million people suggests that maintaining healthy levels of iron in the blood could be a key to ageing better and living longer.

                            The findings could accelerate the development of drugs to reduce age-related diseases, extend healthy years of life and increase the chances of living to old age free of disease, the researchers say.

                            Scientists from the University of Edinburgh and the Max Planck Institute for Biology of Ageing in Germany focused on three measures linked to biological ageing -- lifespan, years of life lived free of disease (healthspan), and being extremely long-lived (longevity).

                            Biological ageing -- the rate at which our bodies decline over time -- varies between people and drives the world's most fatal diseases, including heart disease, dementia and cancers.

                            The researchers pooled information from three public datasets to enable an analysis in unprecedented detail. The combined dataset was equivalent to studying 1.75 million lifespans or more than 60,000 extremely long-lived people.

                            The team pinpointed ten regions of the genome linked to long lifespan, healthspan and longevity. They also found that gene sets linked to iron were overrepresented in their analysis of all three measures of ageing.

                            The researchers confirmed this using a statistical method -- known as Mendelian randomisation -- that suggested that genes involved in metabolising iron in the blood are partly responsible for a healthy long life.

                            Blood iron is affected by diet and abnormally high or low levels are linked to age-related conditions such as Parkinson's disease, liver disease and a decline in the body's ability to fight infection in older age.

                            The researchers say that designing a drug that could mimic the influence of genetic variation on iron metabolism could be a future step to overcome some of the effects of ageing, but caution that more work is required.

                            The study was funded by the Medical Research Council and is published in the journal Nature Communications.

                            Anonymised datasets linking genetic variation to healthspan, lifespan, and longevity were downloaded from the publically available Zenodo, Edinburgh DataShare and Longevity Genomics servers.

                            Dr Paul Timmers from the Usher Institute at the University of Edinburgh, said: "We are very excited by these findings as they strongly suggest that high levels of iron in the blood reduces our healthy years of life, and keeping these levels in check could prevent age-related damage. We speculate that our findings on iron metabolism might also start to explain why very high levels of iron-rich red meat in the diet has been linked to age-related conditions such as heart disease."

                            Dr Joris Deelen from the Max Planck Institute for Biology of Ageing in Germany, said: "Our ultimate aim is to discover how ageing is regulated and find ways to increase health during ageing. The ten regions of the genome we have discovered that are linked to lifespan, healthspan and longevity are all exciting candidates for further studies."


                            Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product.

                            These products are often proteins, but in non-protein coding genes such as transfer RNA (tRNA) or small nuclear RNA (snRNA) genes, the product is a functional RNA.

                            As shown by this picture, gene expression consist of genetic transcription, that results in a mRNA, maturation of the mRNA (splicing, incorporation of a poly(A) tail, capping, . ) and finally protein synthesis by means of translation of the mature mRNA.

                            Again, the homonymous Wikipedia article tells us what is protein synthesis:

                            Protein synthesis is the process whereby biological cells generate new proteins [. ]. Translation, the assembly of amino acids by ribosomes, is an essential part of the biosynthetic pathway, along with generation of messenger RNA (mRNA), aminoacylation of transfer RNA (tRNA), co-translational transport, and post-translational modification.

                            The following picture describes the process of mRNA translation by ribosomes that results in a polypeptide.

                            Once folded in its proper 3D structure, the polypeptide becomes a functional protein.

                            In order to work properly, some proteins need post-translational modifications, which is (citing Wikipedia again) the covalent modification of proteins following protein biosynthesis and goes beyond the scope of this answer.


                            Watch the video: 5 5 Κληρονομικότητα (August 2022).