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By what mechanism can two chromosomes fuse?

By what mechanism can two chromosomes fuse?


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What condition(s) would likely exist which could cause 2 chromosomes in the newly fertilized egg of a placental mammal to fuse? Assume this is more likely than having the same chromosome pair fuse in an individual egg, as well as an individual sperm.


Chromosome fusions (as well as fissions and other changes) usually take place during cell division or pairing, therefore, it's very unlikely that two chromosomes will fuse in an already fertilized egg. If they do such, this egg would probably be aborted.

One factor that may lead to chromosome fusion is loss of thelomeric sequences (telomere = chromosome extremity) (O'Sullivan and Karsleder, 2010) - but probably not in an fertilized egg.


Developmental Biology. 6th edition.

Recognition of sperm by the vitelline envelope or zona pellucida is followed by the lysis of that portion of the envelope or zona in the region of the sperm head by the acrosomal enzymes (Colwin and Colwin 1960 Epel 1980). This lysis is followed by the fusion of the sperm plasma membrane with the plasma membrane of the egg.

The entry of a sperm into a sea urchin egg is illustrated in Figure 7.19. Sperm-egg binding appears to cause the extension of several microvilli to form the fertilization cone (Summers et al. 1975 Schatten and Schatten 1980, 1983). Homology between the egg and the sperm is again demonstrated, because the transitory fertilization cone, like the acrosomal process, appears to be extended by the polymerization of actin. The sperm and egg plasma membranes then join together, and material from the sperm membrane can later be found on the egg membrane (Gundersen et al. 1986). The sperm nucleus and tail pass through the resulting cytoplasmic bridge, which is widened by the actin polymerization. A similar process occurs during the fusion of mammalian gametes (Yanagimachi and Noda 1970 Figure 7.20).

Figure 7.20

Entry of sperm into golden hamster egg. (A) Scanning electron micrograph of sperm fusing with egg. The �ld” spot (without microvilli) is where the polar body has budded off. (B) Close-up of sperm-zona binding. (C) Transmission electron (more. )

In the sea urchin, all regions of the egg plasma membrane are capable of fusing with sperm. In several other species, certain regions of the membrane are specialized for sperm recognition and fusion (Vacquier 1979). Fusion is an active process, often mediated by specific 𠇏usogenic” proteins. It seems that bindin plays a second role as a fusogenic protein. Glabe (1985) has shown that sea urchin bindin will cause phospholipid vesicles to fuse together and that, like viral fusogenic proteins, bindin contains a long stretch of hydrophobic amino acids near its amino terminus. This region is able to fuse phospholipid vesicles (Ulrich et al. 1998).

In mammals, the fertilin proteins in the sperm plasma membrane are essential for sperm membrane-egg membrane fusion (Primakoff et al. 1987 Blobel et al. 1992 Myles et al. 1994). Mouse fertilin is localized to the posterior plasma membrane of the sperm head (Hunnicut et al. 1997). It adheres the sperm to the egg by binding to the 㬖㬡 integrin protein on the egg plasma membrane (Evans et al. 1997 Chen and Sampson 1999). Moreover, like sea urchin bindin (to which it is not structurally related), fertilin has a hydrophobic region that could potentially mediate the union of the two membranes (Almeida et al. 1995). Thus, fertilin appears to bind the sperm plasma membrane to the egg plasma membrane and then to fuse the two of them together. Mice homozygous for mutant fertilin have sperm with several defects, one of them being the inability to fuse with the egg plasma membrane (Cho et al. 1998). When the membranes are fused, the sperm nucleus, mitochondria, centriole, and flagellum can enter the egg.


Implications for understanding our “becoming human”

The main implication from this work is that it places the fusion event well before the advent of our species. I’ve often chatted informally with Christians about evolution, and at times some have thought that this fusion event was what “started” our species, or made our species unable to interbreed with other groups. Some have even suggested that perhaps the fusion event was what produced the first human (i.e. Adam).

Note that thinking this way suggests a misunderstanding of how chromosome fusions occur and what effect they have on their hosts. A fusion does not precipitate a speciation event, but rather the individual with the fusion remains a part of his or her population, and able to interbreed, even if with reduced fertility. Also, there is no necessary biological effect or change that the fusion produces on the appearance of the organism. These misunderstandings aside, however,what this new evidence shows is that this fusion event took place long before modern humans arose at around 200,000 years ago. Indeed, the 800,000 years ago date for the last human – Denisovan common ancestor means that this is the most recent date possible for the fusion. While it is an interesting piece of our evolutionary history, it doesn’t seem to have much to do with how we came to acquire the traits that set us apart from, and ultimately outcompete, other similar species.

Dennis Venema

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Observed patterns of chromosomal rearrangement

Dr. Miller's enthusiasm about this chromosomal rearrangement may be tied to the older notion that such mutations are the basis for speciation.8 This belief was shown to be overly simplistic decades ago when papers appeared describing chromosomal variations which were not eliminated by selection. One intriguing example is a single species of rodent (Holochilus brasiliensis) where 26 different karyotypes were identified in the 42 individuals tested.9 Chromosomal rearrangements have been identified within many ruminant species. There are examples in both goats and sheep where individuals with one or more centric fusions are phenotypically indistinguishable from other animals.10 One researcher who studied sheep carrying up to three different centric fusions concluded, “It is now considered that there is little or no evidence to suggest that centric fusions in a variety of combinations affect the total productive fitness of domestic sheep.”11 So, the bottom line is that centric fusions themselves do not inevitably result in a new species. It is conceivable that some apes exist with 46 chromosomes. Yet these animals will be distinctly apes they will not be “evolving” to become a human. If the observed evidence is really from a fusion, it is best explained by the fusion of two human chromosomes.


Sex Determination between Two Individuals of Same Species | Biology

The sex is hereditary difference between two individuals of same species. Sex is one of the most conspicuous and interesting kinds of hereditary differences observed among individuals of same species.

Determination of sex is determined at the time of fertilization, when the male and female gametes fuse together.

A. Male heterogamy:

Males form two types of gametes. One gamete possesses X chromosome and other lacks it. In some cases male may possess a Y chromosome. Such males are known as heterogametic. Females in such cases form only one type of gamete which contain X chromosome.

Male heterogamy is of two types:

By 1900, when microscope techniques had become quite well developed and chromosome behaviour was understood, it was noticed that there was one pair of chromosomes that differed from others. In females, the members of this pair were similar, but differ in appearance in other sex (males).

The two chromosomes, that were alike (in female) were the same as one of the members of the unlike pair in male. The chromosome which was present in pair in female and single in male was identified as X chromosome. In males, the other chromosome was called as Y. So, the two sexes can be characterized as under (Fig. 5.32).

X-chromosomes were first identified by Wilson and Stevens in 1905. The so called XY system occurs in wide variety of animals including Drosophila and mammals, as well as at least in some plants (e.g. Lychnis — an angiosperm).

X and Y chromosomes are called sex chromosomes (allosomes), the remaining ones of a given complement, which are same in both sexes are called autosomes. The type of system discussed above is called XX-XY system.

Another system reported is XX-XO. In 1902 an American McClung reported that the somatic cells of female grasshopper bears 24 chromosomes, whereas those of males had only 23. Thus in many insects there is a chromosomal difference between the sexes, female being referred to as XX having two X chromosomes and males as XO (“X – Oh” with one X chromosome).

As a result of meiosis all the eggs of such species carry on X chromosome, whereas only half of the sperms have one, the other half having none. Since the males produce two types of gametes X or O in XO type and X and Y in XY type, they are called heterogametic. Females are homogametic producing only one type of gamete with X chromosome.

B. Female heterogamy:

In such cases females produce two types of gametes. One egg contains X and other lacking it (X) or containing Y chromosome. Thus the male is AAXX and female is AAXO or AAXY. For the sake of convenience and to avoid confusion the standard practice is that in these organisms X is denoted to as Z and Y as W.

Female heterogamy is also of two types.

Another interesting ZZ-ZW system has been found in some birds including domestic .fowl, butterflies and some fishes. In this case females are heterogametic and males are homogametic. Sex chromosomes here have been designated as Z and W to avoid confusion with instances where female is homogametic. Females here are ZW and males ZZ (Fig. 5.34).

In ZO-ZZ type of sex determination which occurs in some butterflies and moths. It is opposite to that found in Cockroaches and Grasshoppers. Here females have odd sex chromosome (AA+Z) while the males have two homomorphic sex chromosomes (AA+ZZ).

The females are heterogametic. They produce two types of eggs, male forming with one sex chromosome (A + Z) and female forming without the sex chromosome (A + O). The males are homographic, forming similar types of sperms (A + Z). The two sexes are obtained in the progeny in equal ratio (Fig. 5.35) as both the types of eggs are produced in equal ratio.

Sex determination in man:

A human male has an X chromosome and a Y chromosome and 22 pairs of autosomes, making a total of 46. The females have a pair of X chromosomes and 22 pairs of autosomes, again making a total of 46. The sex chromosomes segregate at meiosis just as the other chromosomes do, s6 this means that each sperm cell will receive only one sex chromosome.

Hence at the time of spermatogenesis, there will be two types of sperm cells produced in equal numbers, those containing an X chromosome and those containing a Y chromosome. Each of the eggs produced by female will contain one X chromosome. Therefore, the sex of progeny is determined at the time of fertilization of egg.

If the egg is fertilized by a sperm bearing a Y chromosome (along with 22 ordinary chromosomes in man), the zygote will have an X and Y and will develop male. If egg is fertilized by an X sperm, the zygote will have two X chromosomes and will develop into female (Fig. 5.36).

Gynandromorphs:

Few Drosophila individuals were found to be having half of the body as male and other half as female. They are called as gynandromorphs.

Three types of gynanders or gynandromorphs can be differentiated:

1. Bilateral gynanders:

Here the half lateral side is of male and other half is of female.

2. Antero-posterior gynanders:

Here the anterior end of the animal is of one sex and posterior of other.

In this case, female fly bears irregularly scattered spots of male tissue. Morgan and Bridges (1919) explained that in Drosophila, zygote developing into female has two X-chromosomes. Due to loss or disappearance of one X-chromosome during cleavage of fertilized egg, a gynandromorph is formed.

Sex Determination in Plants:

Allen (1940) gave a list of plant species where sex chromosomes had been reported. Wastergard (1950) prepared a list of plant species where the presence of a pair of heteromorphic sex chromosomes was well established and also of those where it was not established.

One of the methods for determining heterogametic sex in plants has been studied in plants like Cannabis and Melandrium. If sex ratios in the progenies from sparse vs. excess pollen differ, it suggests that male sex is heterogametic.

For example, in hemp (Cannabis), sparse pollination gave excess males while in Melandrium, sparse pollination gave excess females, suggesting that the male sex is heterogametic in both the cases. If female is heterogametic sparse pollination should give male and females in equal proportion.

In Melandrium album, diploids, triploids and tetraploids having different doses of X and Y chromosomes were noticed by Warmeke (1946). It was found that plant is male when one or more Y chromosomes are present and in females Y chromosome is absent.

The number of autosome did not visibly affect sex expression. In Melandrium, Y chromosome is longer than X chromosome and they form a heteromorphic bivalve at meiosis.

Genetic balance theory of sex-determination:

Genie balance theory of sex determination was proposed by Bridges (1923) who believed that “interaction of genes present in Sex chromosome and autosomes, which governs female and male potency respectively, determines the sex of an offspring. In Drosophila Genie Balance Theory Operates (Y-has no role)

X/A = between 0.5 to 1 = Inter sex

More than 1 = Super female

The mechanisms of sex determination in plants are similar to those found in animals. Mostly plants are hermaphrodite and it is only in Dioecious plants separate male and female plants are found in papaya, spinach, Vitis, Asparagus etc. It is governed by single gene.

In papaya, single, gene with Bires alleles (m, M1, and M2) is suggested to control sex differentiation. Female plants are homozygous (mm.) The males are heterozygous (M1m) and heterozygous (M2m) produces hermaphrodite. In plants, sex is determined by Y chromosome. If Y chromosome is present, the plant is male, otherwise female.

Morgan and Drosophila:

Drosophila melanogaster (fruit fly) meaning “black- bellied dew lover” was first investigated intensively in the laboratories of Columbia University in New York City, where Walter Sutton had earlier been a graduate student. Here Thomas Hunt Morgan in 1910 discovered a fruit fly with white eyes in a vial of flies with normal red eyes.

Thomas Hunt Morgan (1866-1945). Morgan’s discovery of sex-linked traits in Drosophila led to experiments that collectively yielded chromosome “maps”—identification of the genes carried by each chromosome, and the approximate location of each gene on a chromosome.

He was raising thousands of red eyed flies in bottles, supplying mashed bananas as food. What was the basis for this variation? The gene for white eyes arose as a mutation of a gene which is on the X-chromosome and which is involved in the production of eye pigment.

Mounting evidence for the chromosome theory of heredity, came mostly from the study of Drosophila. Mutations ought to involve changes in chromosome structure because genes were present on chromosome as discussed in chromosomal theory of heredity.

T. H. Morgan mainly studied the inheritance of mutant traits in Drosophila because for him, they were less expensive to rear than other animals like mice and rabbit. However, his study to work on Drosophila proved most rewarding for the investigations in genetics.

Drosophila is a suitable material for the genetical experiments due to the following reasons:

(a) Its generation time is 12-14 days, which is helpful in rapid study and analysis of results in laboratory.

(b) It can be multiplied in large number under laboratory conditions.

(c) A large number of flies are produced in each progeny. A pair of flies in a small milk is able to produce hundreds of progeny in a single mating.

(d) Fly breeding could be done throughout the year in a laboratory with inexpensive material.

(e) Each cell of Drosophila melanogaster has four pairs of chromosomes. Out of which three pairs of chromosomes are similar in male and female and are called autosomes. The males possess one X chromosome and one Y chromosome producing two kinds of sperms half with X chromosome and half with Y chromosome.

The Y chromosome is typically J shaped. Female possesses two homomorphic X chromosomes in their body cells, hence referred as XX. Being homogametic, females produce only one kind of eggs, each with one X chromosome.


Cells simply avoid chromosome confusion

This natural safeguard prevents incorrect chromosome counts and misalignments that lead to infertility, miscarriage, or congenital conditions.

"Mistakes during reproductive cell division cause these problems, but what exactly goes wrong is often not understood," said Adele Marston of the Wellcome Trust Center for Cell Biology at the University of Edinburgh in Scotland and lead author of the study. Understanding normal protective mechanisms like the one newly discovered might suggest where things can go awry.

Marston is part of an international team studying meiosis -- the type of cell division that splits an organism's original number of chromosomes in half for sexual reproduction. Meiosis occurs, for example, to create sperm or egg cells. The reduction allows offspring to inherit half their chromosomes from their father, and half from their mother.

"During cell division," she said, "chromosomes must be precisely sorted in an elaborate choreography where chromosomes pair up and then part in a sequence."

However, the arrangement gets complicated during the early stages of reproductive cell division. Instead of just pairs of chromosomes, the spindle-like apparatus in cells that pulls chromosomes apart has to deal with quartets. Each contains two 'sister chromatids' coming from the mother linked to two coming from the father A chromatid is either of the two strands formed when a chromosome is duplicated sister chromatids are identical copies.

"The correct outcome for the first stage of meiosis," explained Dr. Charles L. Asbury, professor of physiology and biophysics at the University of Washington, "is for the sister chromatids to migrate together rather than to separate." Asbury is the senior author of the study.

In all types of cell division, he noted, sister chromatids are held together at first by cohesion. But in the earlier stages of reproductive cell division, the research team discovered that a strong, extra-tight linkage joins the sister chromatids.

When cells prepare to divide, molecular machines, called kinetochores, show up and assume several roles. They both control and drive chromosome movement. They set the timing for other cell division events, including the actual splitting of the chromosomes.

The kinetochores consist of an array of proteins that bind to the tips of miniscule, fiber-like structures called microtubules. The tips act as motors. The kinetochore converts the lengthening and shortening of the microtubules tips into useful force to move chromosomes.

The researchers determined that, during the early stages of meiosis, kinetochores between sister chromosomes mechanically fuse. The tethering keeps chromosomes from separating prematurely and ending up misplaced.

The fused kinetochores contain more binding elements than do single kinetochores, and form sturdy, hard-to-rupture attachments. A protein complex called monopolin is found inside cells during the early stages of reproductive cell division. It appears to be behind this modification. Monopolin alone was able to fuse kinetochore particles in a lab dish in the absence of other factors.

The researchers believe that the kinetochore fusion is a basic mechanism for the proper distribution of chromosomes in healthy cells. This feature of reproductive cell division is conserved across species and fundamental to carrying out expected patterns of genetic inheritance.

In this study, the researches worked with a simple life form, baker's yeast, and used advanced, highly sophisticated techniques. These included genetic manipulation, laser trapping and fluorescence microscopy.

"We combined genetic control of the cell cycle with biophysical manipulation of a complex protein machine -- the kinetochore -- at a single particle level," Asbury said. "I think our work will guide others who are studying molecular machineries that are regulated according to the cell cycle."


The SAC and cell cycle control at the oocyte to embryo transition

Not only spindle assembly and function, but also the control of faithful chromosome segregation and duration of the cell division phase (M-phase), are different in the oocyte and early embryo compared to somatic cells (Fig 1). In all cells, M-phase entry is governed by an increase in cyclin-dependent kinase 1 (CDK1) activity, which is mediated by synthesis and binding of cyclin B and activating the kinase through a change in its phosphorylation state [43]. Conversely, exiting M-phase requires that CDK1 activity decreases. This is mediated by degradation of cyclin B and reverse phosphoregulation of the kinase. For cyclin B to be degraded, the E3 ligase anaphase promoting complex or cyclosome (APC/C) polyubiquitinylates the protein and thereby targets it for proteasomal degradation. Upon activation of the APC/C through binding of cell division cycle protein 20 (Cdc20), the APC/C modifies key substrates, such as cyclin B and securin with ubiquitin chains, and thereby drives progression into anaphase.

In somatic cells, mitotic duration and cyclin B–dependent CDK1 activity is kept short, typically lasting only approximately 40 minutes [44,45]. Only under certain conditions can CDK1 activity be stabilized. This is primarily mediated through the SAC, which delays anaphase onset until all kinetochore pairs from sister chromatids are stably attached to spindle microtubules from opposite spindle poles [46,47]. The SAC thereby ensures that chromosomes segregate faithfully. Molecularly, the SAC achieves this by blocking APC/C activation when kinetochores are unattached, they recruit SAC components—such as mitotic arrest deficient 2 (Mad2), budding uninhibited by benzimidazoles 3 (Bub3), and budding uninhibited by benzimidazoles-related 1 (BubR1)—that, together with Cdc20, form the mitotic checkpoint complex, which even binds a second Cdc20 molecule. Sequestering this activator of the APC/C, the SAC delays cyclin B degradation and CDK1 inactivation and thereby delays cell cycle progression into anaphase. Once all sister chromatids are correctly bioriented to microtubules from opposite spindle poles, the kinetochores stop recruiting SAC components, switching the checkpoint off [48,49].

Compared to somatic cells, M-phase in mouse oocyte meiosis I is much longer, lasting up to 10 hours. Starting with oocyte maturation, cyclin B levels increase only gradually when the protein becomes translated from previously stored and silenced mRNA [50–52]. Consequently, CDK1 activity also increases gradually. This gradual increase as well as the lengthy M-phase are thought to be essential to generate stable microtubule–kinetochore attachments in the oocyte [53]. Also, the control of faithful chromosome segregation in meiosis I differs from the control in somatic mitosis: the SAC components are present, and the checkpoint is generally active [54–58] but it appears to be less sensitive, because it can be silenced even in the presence of misaligned or misattached kinetochores, which can lead to aneuploidy [59–62]. Recent studies suggest that components of the SAC might not be sufficiently concentrated in the huge volume of the oocyte to arrest cell cycle progression in response to one or very few misattached kinetochores [63].

The SAC is also present in meiosis II [55], but its functionality remains elusive, because meiotic resumption primarily depends on the so-called cytostatic factor (CSF) [64]. CSF activity raises and stabilizes CDK1 activity after resumption of meiosis I and maintains an arrest in metaphase II until fertilization, while the SAC is silent and the kinetochores are bioriented [28]. Early mitotic inhibitor 2 (Emi2) is a key player in CSF activity and is thought to function as an inhibitor of the APC/C because it also binds to Cdc20 [65]. At fertilization, Emi2 becomes degraded, which is thought to contribute to the activation of the APC/C and consequently to degradation of cyclin B and securin, which allows progression into anaphase of meiosis II [66].

The zygote therefore inherits a cell cycle regulatory system from two meiotic divisions that either overrides the SAC or has an additional downstream mechanism to halt M-phase exit until fertilization. Zygotic cell cycle progression relies solely on stored maternal factors, as shown by CDK1 activation that is independent from the nuclei [67]. Also similar to meiosis, the first embryonic M-phase is unusually long compared to somatic mitoses, in line with the observed stabilization of CDK1 activity [32]. A recent study indicates that APC/C activation is delayed in the first M-phase compared to M-phase in the two-cell division and that this delay seems to be independent of Emi2 [33]. The increased duration of mitosis in the zygote is not caused by the dual spindle assembly process, because M-phase is not shortened in parthenogenic haploid “zygotes” [68]. Whether the prolonged zygotic M-phase is required for other events of the first embryonic cleavage, or whether it is a remnant of factors (other than Emi2) mediating the arrest in meiosis II, remains to be elucidated.

We still do not understand in detail how the cell cycle is controlled during early embryogenesis. It is clear that it changes massively from early to late preimplantation development: At the beginning, during the first lengthy cell cycles, checkpoints are most likely active. However, at blastocyst stage, the gap phases and some checkpoints of the cell cycle are skipped to allow for a massive burst in cell division [69]. But comparative studies of the cell cycle control machinery between the different cleavage divisions are still to be conducted. It is also unclear how stable microtubule kinetochore attachments and biorientation are established and controlled during the oocyte to embryo transition. From the available studies of mouse and human embryos, it seems that the SAC components are present and in principle functional during preimplantation cleavages [70,71]. Moreover, the SAC effector proteins, Bub3, Mad2, and BubR1, are essential for early embryogenesis as null mutants of these components die at very early stages of embryogenesis [72–74]. One study claims that Mad2 displaces from kinetochores already before a metaphase plate is established, indicating that the SAC might not be responsible for the prolonged M-phase in the first embryonic division [75]. But so far, we do not understand in detail how M-phase prolongation is achieved in the very first division. Systematic studies that analyze SAC activity and mitotic exit regulation and their potential impact on the fidelity of chromosome segregation in the first and subsequent embryonic mitoses are missing.


7.2 Meiosis

Sexual reproduction requires fertilization , a union of two cells from two individual organisms. If those two cells each contain one set of chromosomes, then the resulting cell contains two sets of chromosomes. The number of sets of chromosomes in a cell is called its ploidy level. Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid. If the reproductive cycle is to continue, 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. So, in addition to fertilization, sexual reproduction includes a nuclear division, known as meiosis, that reduces the number of chromosome sets.

Most animals and plants are diploid, containing two sets of chromosomes in each somatic cell (the nonreproductive cells of a multicellular organism), the nucleus contains two copies of each chromosome that are referred to as homologous chromosomes. Somatic cells are sometimes referred to as “body” cells. Homologous chromosomes are matched pairs containing genes for the same traits in identical locations along their length. Diploid organisms inherit one copy of each homologous chromosome from each parent all together, they are considered a full set of chromosomes. In animals, haploid cells containing a single copy of each homologous chromosome are found only within gametes. Gametes fuse with another haploid gamete to produce a diploid cell.

The nuclear division that forms haploid cells, which is called meiosis, is related to mitosis. As you have learned, mitosis is part of a cell reproduction cycle that results in identical daughter nuclei that are also genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei contain the same number of chromosome sets—diploid for most plants and animals. 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. To achieve the reduction in chromosome number, 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 stages are designated with a “I” or “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis I reduces the number of chromosome sets from two to one. The genetic information is also mixed during this division to create unique recombinant chromosomes. Meiosis II , in which the second round of meiotic division takes place in a way that is similar to mitosis, includes prophase II, prometaphase II, and so on.

Interphase

Meiosis is preceded by an interphase consisting of the G1, S, and G2 phases, which are nearly identical to the phases preceding mitosis. The G1 phase is the first phase of interphase and is focused on cell growth. In the S phase, the DNA of the chromosomes is replicated. Finally, in the G2 phase, the cell undergoes the final preparations for meiosis.

During DNA duplication of the S phase, each chromosome becomes composed of two identical copies (called sister chromatids) that are held together at the centromere until they are pulled apart during meiosis II. In an animal cell, the centrosomes that organize the microtubules of the meiotic spindle also replicate. This prepares the cell for the first meiotic phase.

Meiosis I

Early in prophase I, the chromosomes can be seen clearly microscopically. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. The tight pairing of the homologous chromosomes is called synapsis . In synapsis, the genes on the chromatids of the homologous chromosomes are precisely aligned with each other. An exchange of chromosome segments between non-sister homologous chromatids occurs and is called crossing over . This process is revealed visually after the exchange as chiasmata (singular = chiasma) (Figure 7.3).

As prophase I progresses, the close association between homologous chromosomes begins to break down, and the chromosomes continue to condense, although the homologous chromosomes remain attached to each other at chiasmata. The number of chiasmata varies with the species and the length of the chromosome. At the end of prophase I, the pairs are held together only at chiasmata (Figure 7.3) and 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 produced by meiosis. A single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete, it will carry some DNA from one parent of the individual and some DNA from the other parent. The recombinant sister chromatid has a combination of maternal and paternal genes that did not exist before the crossover.

The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. The microtubules assembled from centrosomes at opposite poles of the cell grow toward the middle of the cell. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome attached at one pole and the other homologous chromosome attached to the other pole. The homologous chromosomes are still held together at chiasmata. In addition, the nuclear membrane has broken down entirely.

During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles. The orientation of each pair of homologous chromosomes at the center of the cell is random.

This randomness, called independent assortment, is the physical basis for the generation of the second form of genetic variation in offspring. Consider 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. In metaphase I, these pairs line up at the midway point between the two poles of the cell. 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. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.

In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations depends on the number of chromosomes making up a set. There are two possibilities for orientation (for each tetrad) thus, the possible number of alignments equals 2 n where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million (2 23 ) possibilities. This number does not include the variability previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition (Figure 7.4).

To summarize the genetic consequences of meiosis I: the maternal and paternal genes are recombined by crossover events occurring on each homologous pair during prophase I in addition, the random assortment of tetrads at metaphase produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.

In anaphase I, the spindle fibers pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. It is the chiasma connections that are broken in anaphase I as the fibers attached to the fused kinetochores pull the homologous chromosomes apart (Figure 7.5).

In telophase I, 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 chromatids in telophase I.

Cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei in other organisms. In nearly all species, cytokinesis separates the cell contents by either a cleavage furrow (in animals and some fungi), or a cell plate that will ultimately lead to formation of cell walls that separate the two daughter cells (in plants). At each pole, there is just one member of each pair of the homologous chromosomes, so 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 there are duplicate copies of the set because each homolog still consists of two sister chromatids that are still attached to each other. However, although the sister chromatids were once duplicates of the same chromosome, they are no longer identical at this stage because of crossovers.

Concepts in Action

Review the process of meiosis, observing how chromosomes align and migrate, at this site.

Meiosis II

In meiosis II, the connected sister chromatids remaining in the haploid cells from meiosis I will be split to form four haploid cells. In some species, cells enter a brief interphase, or interkinesis , that lacks an S phase, before entering meiosis II. Chromosomes are not duplicated during interkinesis. The two cells produced in meiosis I go through the events of meiosis II in synchrony. Overall, meiosis II resembles the mitotic division of a haploid cell.

In prophase II, if the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed. In 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. In metaphase II, the sister chromatids are maximally condensed and aligned at the center of the cell. In anaphase II, the sister chromatids are pulled apart by the spindle fibers and move toward opposite poles.

In telophase II, the chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four genetically unique haploid cells. At this point, the nuclei in the newly produced cells are both haploid and have only one copy of the single set of chromosomes. 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.

Comparing Meiosis and Mitosis

Mitosis and meiosis, which are both forms of division of the nucleus in eukaryotic cells, share some similarities, but also exhibit distinct differences that lead to their very different outcomes. Mitosis is a single nuclear division that results in two nuclei, usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original. They have the same number of sets of chromosomes: one in the case of haploid cells, and two in the case of diploid cells. On the other hand, meiosis is two nuclear divisions that result in four nuclei, usually partitioned into four new cells. The nuclei resulting from meiosis are never genetically identical, and they contain one chromosome set only—this is half the number of the original cell, which was diploid (Figure 7.6).

The differences in the outcomes of meiosis and mitosis occur because of differences in the behavior of the chromosomes during each process. Most of these differences in the processes occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs become associated with each other, are bound together, experience chiasmata and crossover between sister chromatids, and line up along the metaphase plate in tetrads with spindle fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I, never in mitosis.

Homologous chromosomes move to opposite poles during meiosis I so the number of sets of chromosomes in each nucleus-to-be is reduced from two to one. For this reason, meiosis I is referred to as a reduction division . There is no such reduction in ploidy level in mitosis.

Meiosis II is much more analogous to a mitotic division. In this case, duplicated chromosomes (only one set of them) line up at the center of the cell with divided kinetochores attached to spindle fibers from opposite poles. During anaphase II, as in mitotic anaphase, the kinetochores divide and one sister chromatid is pulled to one pole and the other sister chromatid is pulled to the other pole. If it were not for the fact that there had been crossovers, the two products of each 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.

Cells produced by mitosis will function in different parts of the body as a part of growth or replacing dead or damaged cells. They may even be involved in asexual reproduction in some organisms. Cells produced by meiosis in a diploid-dominant organism such as an animal will only participate in sexual reproduction.


Functions of Crossing Over

Organisms that divide only asexually without the chance of such recombination suffer from a condition called Muller’s Ratchet. That is, each generation of that species contains at least as many genetic mutations as the previous generation, if not more. In other words, when all the progeny are genetically identical to one another, there is no scope for genetic errors to be corrected, or for new and beneficial combinations to arise.

Crossing over increases the variability of a population and prevents the accumulation of deleterious combinations of alleles, while also allowing some parental combinations to be passed on to the offspring. This way, there is a balance between maintaining potentially useful allelic combinations as well as providing the opportunity for variation and change.



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