Meiosis – Anatomy, Structure, Functions

Introduction to Meiosis

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

Key Points

Sexual reproduction is the production of haploid cells and the fusion of two of those cells to form a diploid cell.

Before sexual reproduction can occur, the number of chromosomes in a diploid cell must decrease by half.

Meiosis produces cells with half the number of chromosomes as the original cell.

Haploid cells used in sexual reproduction, gametes, are formed during meiosis, which consists of one round of chromosome replication and two rounds of nuclear division.

Meiosis I is the first round of meiotic division, while meiosis II is the second round.

Key Terms

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

Introduction: Meiosis and Sexual Reproduction

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

Offspring Closely Resemble Their Parents: In-kind means that the offspring of any organism closely resemble their parent or parents. The hippopotamus gives birth to hippopotamus calves

• (a). Joshua trees produce seeds from which Joshua tree seedlings emerge
• (b). Adult flamingos lay eggs that hatch into flamingo chicks
• (c). Sexual reproduction is the production of haploid cells (gametes) and the fusion (fertilization) of two gametes to form a single, unique diploid cell called a zygote. All animals and most plants produce these gametes, or eggs and sperm. In most plants and animals, through tens of rounds of mitotic cell division, this diploid cell will develop into an adult organism.

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

Comparing Meiosis and Mitosis

Mitosis and meiosis share some similarities, but also some differences, most of which are observed during meiosis I.

Key Points

For the most part, in mitosis, diploid cells are partitioned into two new diploid cells, while in meiosis, diploid cells are partitioned into four new haploid cells.

In mitosis, the daughter cells have the same number of chromosomes as the parent cell, while in meiosis, the daughter cells have half the number of chromosomes as the parent.

The daughter cells produced by mitosis are identical, whereas the daughter cells produced by meiosis are different because crossing over has occurred.

The events that occur in meiosis but not mitosis include homologous chromosomes pairing up, crossing over, and lining up along the metaphase plate in tetrads.

Meiosis II and mitosis are not reduction divisions like meiosis I because the number of chromosomes remains the same; therefore, meiosis II is referred to as equatorial division.

When the homologous chromosomes separate and move to opposite poles during meiosis I, the ploidy level is reduced from two to one, which is referred to as a reduction division.

Key Terms

• reduction division: the first of the two divisions of meiosis, a type of cell division
• ploidy: the number of homologous sets of chromosomes in a cell
• equatorial division: a process of nuclear division in which each chromosome divides equally such that the number of chromosomes remains the same from parent to daughter cells

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 distinct differences that lead to very different outcomes. The purpose of mitosis is cell regeneration, growth, and asexual reproduction, while the purpose of meiosis is the production of gametes for sexual reproduction. Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new daughter 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 most plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new haploid daughter cells. The nuclei resulting from 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.

Comparing Meiosis and Mitosis: Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell.

The main differences between mitosis and meiosis occur in meiosis I. In meiosis I, the homologous chromosome pairs become associated with each other and are bound together with the synaptonemal complex. Chiasmata develop and crossover occurs between homologous chromosomes, which then line up along the metaphase plate in tetrads with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I.

When the tetrad is broken up and the homologous chromosomes move to opposite poles, the ploidy level is reduced from two to one. For this reason, meiosis I am referred to as a reduction division. There is no such reduction in ploidy level during mitosis.

Meiosis II is much more similar to a mitotic division. In this case, the duplicated chromosomes (only one set, as the homologous pairs, have now been separated into two different cells) line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles. During anaphase II and mitotic anaphase, the kinetochores divide and sister chromatids, now referred to as chromosomes, are pulled to opposite poles. The two daughter cells of mitosis, however, are identical, unlike the daughter cells produced by meiosis. They are different because there has 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. Meiosis II is, therefore, referred to as equatorial division.

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Functions of Meiosis

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

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

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

Origin of Meiosis

There are two conflicting theories on how meiosis arose. One is that meiosis evolved from prokaryotic sex (bacterial recombination) as eukaryotes evolved from prokaryotes.^[rx][rx] The other is that meiosis arose from mitosis.^[rx]

From prokaryotic sex

In prokaryotic sex, DNA from one prokaryote is taken up by another prokaryote, and its information is integrated into the DNA of the recipient prokaryote. In extant prokaryotes, the donor DNA can be transferred either by transformation or conjugation.^[rx][rx] Transformation in which DNA from one prokaryote is released into the surrounding medium and then taken up by another prokaryotic cell may have been the earliest form of sexual interaction. One theory on how meiosis arose is that it evolved from transformation.^[rx] According to this view, the evolutionary transition from prokaryotic sex to eukaryotic sex was continuous.

Transformation, like meiosis, is a complex process requiring the function of numerous gene products. A key similarity between prokaryotic sex and eukaryotic sex is that DNA originating from two different individuals (parents) joins up so that homologous sequences are aligned with each other, and this is followed by an exchange of genetic information (a process called genetic recombination). After the new recombinant chromosome is formed it is passed on to progeny.

When genetic recombination occurs between DNA molecules originating from different parents, the recombination process is catalyzed in prokaryotes and eukaryotes by enzymes that have similar functions and that are evolutionarily related. One of the most important enzymes catalyzing this process in bacteria is referred to as RecA, and this enzyme has two functionally similar counterparts that act in eukaryotic meiosis, RAD51, and DMC1.^[rx]

Support for the theory that meiosis arose from prokaryotic transformation comes from the increasing evidence that early diverging lineages of eukaryotes have the core genes for meiosis. This implies that the precursor to meiosis was already present in the prokaryotic ancestor of eukaryotes. For instance the common intestinal parasite Giardia intestinalis, a simple eukaryotic protozoan was, until recently, thought to be descended from an early diverging eukaryotic lineage that lacked sex. However, it has since been shown that G. intestinalis contains within its genome a core set of genes that function in meiosis, including five genes that function only in meiosis.^[rx] In addition, G. intestinalis was recently found to undergo a specialized, sex-like process involving meiosis gene homologs.^[rx] This evidence, and other similar examples, suggest that a primitive form of meiosis was present in the common ancestor of all eukaryotes, an ancestor that arose from an antecedent prokaryote.^[rx][rx]

From mitosis

Mitosis is the normal process in eukaryotes for cell division; duplicating chromosomes and segregating one of the two copies into each of the two daughter cells, in contrast with meiosis. The mitosis theory states that meiosis evolved from mitosis.^[rx] According to this theory, early eukaryotes evolved mitosis first, became established, and only then did meiosis and sexual reproduction arise.

Supporting this idea are observations of some features, such as the meiotic spindles that draw chromosome sets into separate daughter cells upon cell division, as well as processes regulating cell division that employ the same, or similar molecular machinery. Yet there is no compelling evidence for a period in the early evolution of eukaryotes, during which meiosis and accompanying sexual capability did not yet exist.

In addition, as noted by Wilkins and Holliday,^[rx] there are four novel steps needed in meiosis that are not present in mitosis. These are

• (1) pairing of homologous chromosomes,
• (2) extensive recombination between homologs;
• (3) suppression of sister chromatid separation in the first meiotic division; and
• (4) avoiding chromosome replication during the second meiotic division. Although the introduction of these steps seems to be complicated, Wilkins and Holliday argue that only one new step, homolog synapsis, was particularly initiated in the evolution of meiosis from mitosis. Meanwhile, two of the other novel features could have been simple modifications, and extensive recombination could have evolved later.^[rx]

Coevolution with mitosis

If meiosis arose from prokaryotic transformation, during the early evolution of eukaryotes, mitosis and meiosis could have evolved in parallel. Both processes use shared molecular components, where mitosis evolved from the molecular machinery used by prokaryotes for DNA replication and segregation, and meiosis evolved from the prokaryotic sexual process of transformation. However, meiosis also made use of the evolving molecular machinery for DNA replication and segregation.

Function

Stress-induced sex

Abundant evidence indicates that facultative sexual eukaryotes tend to undergo sexual reproduction under stressful conditions. For instance, the budding yeast Saccharomyces cerevisiae (a single-celled fungus) reproduces mitotically (asexually) as diploid cells when nutrients are abundant, but switches to meiosis (sexual reproduction) under starvation conditions.^[rx] The unicellular green alga, Chlamydomonas reinhardtii grows as vegetative cells in a nutrient-rich growth medium, but depletion of a source of nitrogen in the medium leads to gamete fusion, zygote formation, and meiosis.^[rx] The fission yeast Schizosaccharomyces pombe, treated with H2O2 to cause oxidative stress, substantially increases the proportion of cells that undergo meiosis.^[rx] The simple multicellular eukaryote Volvox carteri undergoes sex in response to oxidative stress^[rx] or stress from heat shock.^[rx] These examples, and others, suggest that, in simple single-celled and multicellular eukaryotes, meiosis is an adaptation to respond to stress.

Prokaryotic sex also appears to be an adaptation to stress. For instance, transformation occurs near the end of logarithmic growth, when amino acids become limiting in Bacillus subtilis,^[rx] or in Haemophilus influenzae when cells are grown to the end of a logarithmic phase.^[rx] In Streptococcus mutants and other streptococci, transformation is associated with high cell density and biofilm formation.^[rx] In Streptococcus pneumoniae, transformation is induced by the DNA damaging agent mitomycin C.^[rx] These, and other, examples indicate that prokaryotic sex, like meiosis in simple eukaryotes, is an adaptation to stressful conditions. This observation suggests that the natural selection pressures maintaining meiosis in eukaryotes are similar to the selective pressures maintaining prokaryotic sex. This similarity suggests continuity, rather than a gap, in the evolution of sex from prokaryotes to eukaryotes.

Stress is, however, a general concept. What is it specifically about the stress that needs to be overcome by meiosis? And what is the specific benefit provided by meiosis that enhances survival under stressful conditions?

DNA repair

In one theory, meiosis is primarily an adaptation for repairing DNA damage. Environmental stresses often lead to oxidative stress within the cell, which is well known to cause DNA damage through the production of reactive forms of oxygen, known as reactive oxygen species (ROS). DNA damages, if not repaired, can kill a cell by blocking DNA replication, or transcription of essential genes.

When only one strand of the DNA is damaged, the lost information (nucleotide sequence) can ordinarily be recovered by repair processes that remove the damaged sequence and fill the resulting gap by copying from the opposite intact strand of the double helix. However, ROS also causes a type of damage that is difficult to repair, referred to as double-strand damage. One common example of double-strand damage is the double-strand break. In this case, genetic information (nucleotide sequence) is lost from both strands in the damaged region, and proper information can only be obtained from another intact chromosome homologous to the damaged chromosome. The process that the cell uses to accurately accomplish this type of repair is called recombinational repair.

Meiosis is distinct from mitosis in that a central feature of meiosis is the alignment of homologous chromosomes followed by recombination between them. The two chromosomes that pair are referred to as non-sister chromosomes since they did not arise simply from the replication of a parental chromosome. Recombination between non-sister chromosomes at meiosis is known to be a recombinational repair process that can repair double-strand breaks and other types of double-strand damage.^[rx] In contrast, recombination between sister chromosomes cannot repair double-strand damages arising prior to the replication which produced them. Thus on this view, the adaptive advantage of meiosis is that it facilitates recombinational repair of DNA damage that is otherwise difficult to repair, and that occurs as a result of stress, particularly oxidative stress.^[rx][rx] If left unrepaired, this damage would likely be lethal to gametes and inhibit the production of viable progeny.

Even in multicellular eukaryotes, such as humans, oxidative stress is a problem for cell survival. In this case, oxidative stress is a byproduct of oxidative cellular respiration occurring during metabolism in all cells. In humans, on average, about 50 DNA double-strand breaks occur per cell in each cell generation.^[rx] Meiosis, which facilitates recombinational repair between non-sister chromosomes, can efficiently repair these prevalent damages in the DNA passed on to germ cells, and consequently prevent loss of fertility in humans. Thus with the theory that meiosis arose from prokaryotic sex, the recombinational repair is the selective advantage of meiosis in both single-celled eukaryotes and multicellular eukaryotes, such as humans.

An argument against this hypothesis is that adequate repair mechanisms including those involving recombination already exist in prokaryotes.^[rx] Prokaryotes do have a DNA repair mechanism enriched with recombinational repair,^[rx] and the existence of prokaryotic life in severe environments indicates the extreme efficiency of this mechanism to help them survive many DNA damages related to the environment. This implies that an extra costly repair in the form of meiosis would be unnecessary. However, most of these mechanisms cannot be as accurate as meiosis and are possibly more mutagenic than the repair mechanism provided by meiosis. They primarily do not require a second homologous chromosome for the recombination that promotes a more extensive repair. Thus, despite the efficiency of recombinational repair involving sister chromatids, the repair still needs to be improved, and another type of repair is required.^[rx] Moreover, due to the more extensive homologous recombinational repair in meiosis in comparison to the repair in mitosis, meiosis as a repair mechanism can accurately remove any damage that arises at any stage of the cell cycle more than mitotic repair mechanism can do ^[rx] and was, therefore, naturally selected. In contrast, the sister chromatid in mitotic recombination could have been exposed to a similar amount of stress, and, thus, this type of recombination, instead of eliminating the damage, could actually spread the damage^[rx] and decrease fitness.

Phases

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

• Growth 1 (G₁) phase: In this very active phase, the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In G₁, each of the chromosomes consists of a single linear molecule of DNA.
• Synthesis (S) phase: The genetic material is replicated; each of the cell’s chromosomes duplicates to become two identical sister chromatids attached at a centromere. This replication does not change the ploidy of the cell since the centromere number remains the same. The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the light microscope. This will take place during prophase I in meiosis.
• Growth 2 (G₂) phase: G₂ phase as seen before mitosis is not present in meiosis. Meiotic prophase corresponds most closely to the G₂ phase of the mitotic cell cycle.

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

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

Diagram of the meiotic phases

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

Meiosis I

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

Schematic of the synaptonemal complex at different stages of prophase I and the chromosomes arranged as a linear array of loops.

Prophase I

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

Leptotene

The first stage of prophase I am the leptotene stage, also known as leptonema, from Greek words meaning “thin threads”.^[rx] In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—become “individualized” to form visible strands within the nucleus.^{[rx] [rx] .}The chromosomes each form a linear array of loops mediated by cohesin, and the lateral elements of the synaptonemal complex assemble forming an “axial element” from which the loops emanate.^[rx] Recombination is initiated in this stage by the enzyme SPO11 which creates programmed double-strand breaks (around 300 per meiosis in mice).^[rx] This process generates single-stranded DNA filaments coated by RAD51 and DMC1 which invade the homologous chromosomes, forming inter-axis bridges, and resulting in the pairing/co-alignment of homologs (to a distance of ~400 nm in mice).^[rx][rx]

Zygotene

Leptotene is followed by the zygotene stage, also known as zygonema, from Greek words meaning “paired threads”,^[rx]  which in some organisms is also called the bouquet stage because of the way the telomeres cluster at one end of the nucleus.^[rx] In this stage the homologous chromosomes become much more closely (~100 nm) and stably paired (a process called synapsis) mediated by the installation of the transverse and central elements of the synaptonemal complex.^[rx] Synapsis is thought to occur in a zipper-like fashion starting from a recombination nodule. The paired chromosomes are called bivalent or tetrad chromosomes.

Pachytene

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

Diplotene

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

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

Diakinesis

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

Meiotic spindle formation

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

Metaphase I

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

Anaphase I

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

Telophase I

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

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

Meiosis II

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

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

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

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

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

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

Prophase I arrest

Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of meiosis.^[rx] In humans, as an example, oocytes are formed between three and four months of gestation within the fetus and are therefore present at birth. During this prophase I arrested stage (dictyate), which may last for many years, four copies of the genome are present in the oocytes. The arrest of oocytes at the four genome copy stage was proposed to provide the informational redundancy needed to repair damage in the DNA of the germline.^[rx] The repair process used likely involves homologous recombinational repair.^[rx][rx] Prophase arrested oocytes have a high capability for efficient repair of DNA damages.^[rx] The adaptive function of the DNA repair capability during meiosis appears to be a key quality control mechanism in the female germline and a critical determinant of fertility.^[rx]

Genetic diversity

Another hypothesis to explain the function of meiosis is that stress is a signal to the cell that the environment is becoming adverse. Under this new condition, it may be beneficial to produce progeny that differs from the parent in their genetic makeup. Among these varied progeny, some may be more adapted to the changed condition than their parents. Meiosis generates genetic variation in the diploid cell, in part by the exchange of genetic information between the pairs of chromosomes after they align (recombination). Thus, on this view,^[rx] an advantage of meiosis is that it facilitates the generation of genomic diversity among progeny, allowing adaptation to adverse changes in the environment.

However, in the presence of a fairly stable environment, individuals surviving to reproductive age have genomes that function well in their current environment. This raises the question of why such individuals should risk shuffling their genes with those of another individual, as occurs during meiotic recombination? Considerations such as this have led many investigators to question whether genetic diversity is a major adaptive advantage of sex.

References