Meiosis

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CHAPTER 45 Meiosis*

Meiosis (from the Greek, meaning “reduction”) is a specialized program of two coupled cell divisions used by eukaryotes to maintain the proper chromosome number for the species during sexual reproduction. The number of chromosomes is halved in meiosis; therefore, the subsequent fusion of male and female gametes restores the proper chromosome number for the species. The reduction in chromosome number is achieved by randomly separating homologous chromosomes, each pair of which is composed of one chromosome donated by the mother and one donated by the father. This pairing and subsequent separation of homologous chromosomes are typically made possible by genetic recombination, which occurs during the lengthy and complex prophase of the first meiotic division. The random segregation of homologous chromosomes and the genetic recombination that make this possible form the physical basis of the laws of classical genetics, first proposed by Gregor Mendel in 1866.

The unique events of meiosis occur in the first division, termed meiosis I (Figs. 45-1 and 45-2). Because the daughter cells have half the number of chromosomes, meiosis I is also known as the reductional division. The second division, meiosis II, is similar in most respects to mitosis: Sister chromatids segregate from each other, and the number of chromosomes remains the same (Box 45-1; see also Chapter 44). Meiosis II is called the equational division. Meiosis is an ancient process that occurs in virtually all higher eukaryotes, including the animal, fungal, and plant kingdoms.

BOX 45-1 Important Differences between Meiosis and Mitosis

Meiosis involves two cell divisions. The two meiotic divisions are preceded by a round of DNA replication. There is no DNA replication between meiosis I and meiosis II.

The products of meiosis are haploid. The products of mitosis are diploid.

The products of meiosis are genetically different. After recombination and random assortment of homologs in meiosis I, the sister chromatids that segregate in meiosis II are different from each other. In normal mitosis, sister chromatids are identical.

Prophase is longer in meiosis I. Proper orientation and segregation of homologous chromosomes is achieved thanks to the pairing, synapsis (synaptonemal complex formation), and recombination that occur in a lengthened prophase during the first meiotic division. In humans, mitotic prophase lasts well under an hour, while meiotic prophase lasts many days in males and many years in females.

Recombination is increased in meiosis. Recombination occurs in prophase I of meiosis at a rate 100-fold to 1000-fold higher than that in mitosis. The process has two main consequences: the formation of chiasmata and the introduction of genetic variation. Chiasmata are structures that physically link the homologous chromosomes after crossover and play an essential role in meiotic chromosome segregation.

Kinetochore behavior differs in meiosis. During meiosis I, kinetochores of sister chromatids attach to spindle microtubules emanating from the same pole. Homologous kinetochore pairs connect to opposite poles. In mitosis, sister kinetochores attach to spindle microtubules coming from opposite poles.

Chromatid cohesion differs in meiosis. Sister chromatid cohesion is essential for orientation of bivalents (paired homologous chromosomes) on the metaphase I spindle. During anaphase of meiosis I, cohesion is destroyed between sister chromatid arms, and chiasmata are released to allow segregation of homologs. Cohesion at sister centromeres persists until the onset of anaphase II, when it is lost to permit segregation of sisters. In prometaphase of meiosis II, sister chromatids are joined only by the centromeres, whereas at the beginning of mitotic prometaphase, sisters are joined all along the arms.

Each human somatic cell has 23 pairs of homologous chromosomes (46 in all). One of each pair is donated by each parent in the egg and sperm, respectively. The number of homologs, 23, is known as the haploid chromosome number. In animals, the only haploid cells are gametes (sperm and eggs). At fertilization, haploid gametes fuse to form a zygote, restoring the diploid chromosome number of 46. In plants, the haploid phase is represented by gametophytes, which produce ovules and pollen. In most fungi, such as yeasts, haploid and diploid forms are alternate phases of the life cycle.

Much of our knowledge of meiosis is based on studies from the budding yeast, Saccharomyces cerevisiae. The use of powerful yeast genetic analysis has enabled an extensive study of the role of particular gene products in meiosis in vivo. Furthermore, because yeast meiosis produces four equivalent spores, it is possible to examine all products of meiosis genetically and biochemically.

Meiosis: An Essential Process for Sexual Reproduction

Without meiosis, there would be no sex because every fusion of gametes would increase the number of chromosomes in the progeny. Sexual reproduction is an important survival strategy that offers organisms a mechanism for altering the genetic makeup of offspring. This strategy has been conserved throughout higher eukaryotes and is inextricably linked with the mechanism of meiosis.

Homologous (maternal and paternal) chromosomes separate from each other in meiosis I. For each pair of homologs, the choice of spindle orientation in meiosis I is random (i.e., each homolog has two equivalent options for the direction to migrate). Thus, for humans (with 23 pairs of homologous chromosomes), each gamete has 223 (more than 8 million) possible chromosome complements as a result of the independent as-sortment of subtly different (polymarphic) chromosomes alone. This process does not create new versions of genes, but it guarantees the production of offspring with novel combinations of chromosomes.

Meiosis I also produces novel versions of chromosomes by exchange of DNA segments between homologs. This occurs because each chromosome must typically undergo at least one genetic recombination (crossover) event to segregate properly at anaphase of meiosis I. If the chromosomes of all the individuals of a species were identical, meiosis and sexual reproduction would only provide different combinations of the same chromosomes. However, human chromosomes vary between individuals, averaging about one difference (polymorphism) per 1000 base pairs, and it is estimated that, overall, at least 106 sites across the genome have variant versions. Recombination involves exchange of chromosomal segments, producing new chromosomes that are a patchwork of segments from the maternal and paternal homologs. The combined effects of recombination and random assortment of homologs in meiosis I yields a vast number of different gametes and provides an important source of genetic diversity that permits eukaryotic populations to adapt to changing environmental conditions.

The Language of Meiosis

Meiosis can be confusing because it has a language of its own, characterized by a number of unusual terms. The best way to understand meiosis is in terms of the essential biological processes that are involved. This reduces the process to only three essential key terms: pairing, recombination, and segregation. Each is discussed in detail later in this chapter, so they are defined only briefly here.

Pairing is the alignment of homologous chromosomes with one another within the cell nucleus. There are two stages of pairing. In alignment, DNA sequences on one chromosome find the corresponding DNA sequences on the homologous chromosome in the presence of the billions of base pairs of DNA in the cell nucleus. Recombination drives the pairing process but is completed later. In the second stage, synapsis, the paired homologous chromosomes become intimately associated with one another. A specialized scaffolding structure called the synaptonemal complex mediates this process.

Recombination, the physical exchange of DNA between homologous chromosomes, is the key event governing chromosome behavior during meiotic prophase. Recombination drives the pairing process and can occur without synapsis under specialized circumstances. Specialized chromatin structures called chiasmata (from the Greek, meaning “X-shaped cross”) form at sites where recombination has been completed. These chiasmata keep homologous chromosomes paired with one another until anaphase of meiosis I.

Meiosis is all about the segregation of the paired homologous chromosomes. This process shows some key differences from mitosis (Box 45-1). When the homologs are balanced at the metaphase plate of the meiosis I spindle, it is the chiasmata that hold them together and counteract the pulling force of the spindle on the kinetochores (Fig. 45-2). Cohesion between the chromatid arms holds chiasmata in place until it is released at anaphase of meiosis I. Centromeres of the sister chromatids remain associated with one another throughout meiosis I until anaphase of meiosis II. This means that at anaphase, when the chiasmata are released, each pair of sister chromatids migrates to the same spindle pole. As a result, the progeny of meiosis I have the haploid number of chromosomes each paired with a sister chromatid. Box 45-2 reviews some genetics terms that are helpful in understanding meiosis.

BOX 45-2 Brief Overview of Genetic Terminology

A comprehensive introduction to the field of genetics is beyond the scope of this text. However, here are a number of terms used by geneticists that will assist in the understanding of the discussion of genetic recombination and its role in meiosis (also see Box 6-2).

The genotype of an organism is the combination of genes present on the chromosomes of that organism. The phenotype is the physical manifestation of the action of these gene products (i.e., the appearance and macromolecular composition of the organism). In discussing recombination, scientists typically refer to the presence or absence of specific genetic markers. Each genetic marker is a particular DNA sequence in or around a gene that can be monitored by examining the phenotypes of the cells that carry it. A genetic marker might be the presence of a functional gene, a mutation with altered activity, or simply a polymorphism of DNA sequence that has no known functional consequence.

A haploid organism has one copy of each chromosome. A diploid organism has two homologous copies of each chromosome. A diploid organism that is homozygous for a particular genetic marker has the same sequence of that particular region of the DNA on both the maternal and paternal homologous chromosomes. A heterozygous organism has different forms of the genetic markers on the two homologous chromosomes. Although the physical events of genetic recombination occur in both homozygotes and heterozygotes, they are most readily detected in the latter.

Two genetic markers located on different chromo-somes will separate from one another in the anaphase of meiosis I 50% of the time as a result of the random distribution of chromosomes to the two spindle poles. If they are on the same chromosome, they will be linked to one another unless the chromosome undergoes a genetic recombination event between them. The greater the separation of two markers on one chromosome, the more likely it is for such an intervening recombination event to occur.

Two types of recombination events occur during meiosis (Fig. 45-3). The first of these—noncrossover events (frequently referred to as gene conversion)—may involve the loss of one or more genetic markers. Noncrossover events are the most common outcome of the programmed double-strand DNA breaks that occur during leptotene. They are thought to involve the invasion of a double helix by a region of single-stranded DNA with complementary sequence but then ejection of this sequence before assembly of a Holliday junction and completion of recom-bination.

The second type of recombination event—crossing over—involves the physical breakage and reunion of DNA strands on two different chromosomes, typically producing a balanced exchange of DNA sequences. This is what most people think of as recombination. In recombination by crossing over, the makeup of genetic markers remains constant; it is the linkage between different markers that changes.

Recombination

Because recombination is the key to the behavior of chromosomes in meiosis I, this process is discussed in detail herein to provide a mechanistic underpinning for understanding later events. Meiotic recombination is very similar to the process of homologous recombinational repair of double-strand DNA breaks in somatic cells (review Box 43-1 and Fig. 43-15 as a prelude to studying meiotic recombination).

Two key differences distinguish meiotic recombination from the repair process in somatic cells. First, meiotic cells create double-strand DNA breaks on purpose, using a specialized enzyme called Spo11. Second, somatic cells repair DNA breaks using the corresponding DNA sequence on their sister chromatid as a guide. Meiotic cells use the homologous chromosome instead. The mechanism for this switch in selectivity is not known.

Spo11 generates programmed double-strand DNA breaks very early during meiotic prophase (Fig. 45-3). Spo11 is a type II DNA topoisomerase (see Chapter 13, under the section titled “Proteins of the Mitotic Chromosome and Chromosome Scaffold”) that cleaves both DNA strands in a reaction that produces a covalent linkage between a tyrosine on the enzyme and the cleaved phosphodiester backbone. Where it has been measured, Spo11 creates about threefold to fivefold more DNA breaks than ultimately complete the recombination pathway to produce reciprocal exchanges of DNA between homologous chromosomes, or crossovers. An alternative pathway is thought to process the excess breaks, producing noncrossover events (Box 45-2 and Fig. 45-3I-J). Each pair of homologous chromosomes thus undergoes many noncrossover events and a very few crossover events (often only one) during meiosis I prophase.

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Figure 45-3 the events of recombination. Recombination occurs between homologs rather than sisters. A, Paired homologous chromosomes. Sister chromatids are held tightly together by cohesin, shown here schematically as hoops. B, Spo11 makes a double-strand break. C, Resection of the break. D, First strand invasion. At this point, the pathway splits in two, one outcome leading to a crossover and the other to a noncrossover. Crossover pathway: E, The second resected strand invades its homologous partner. New DNA synthesis fills the gaps. F, The resulting molecule contains a double Holliday junction (see Fig. 43-15B). If the resolvase (nuclease) cuts the double Holliday junction asymmetrically as shown (i.e., one vertical and one horizontal cut), the result is a crossover (G). If the cuts are symmetrical, a noncrossover molecule is produced. Noncrossover pathway: H, In most cases, the invading DNA strand is ejected prior to stabilization and formation of a double Holliday junction. I, DNA gap-filling and ligation yield a noncrossover chromosome (J).

In both mice and yeast, double-strand breaks generated by Spo11 are required for normal segregation of homologous chromosomes. In Spo11-null mice, recombination is not initiated, and synapsis, if it occurs at all, is aberrant, often involving nonhomologous chromosomes (Fig. 45-4). In these mutant mice, spermatocytes die by apoptosis early in meiotic prophase, and oocytes die somewhat later. In contrast, the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster do not require Spo11-induced double-strand breaks for synapsis of homologous chromosomes.

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Figure 45-4 Pairing of homologous chromosomes is severely disrupted in the Spo11 mutant. Pachytene chromosomes from wild-type mice (A) and mice in which the Spo11 gene has been disrupted (B).

(From Baudat F, Manova K, Yuen JP, et al: Chromosome synapsis defects and sexually dimorphic meiotic progression in mice lacking Spo11. Mol Cell 6:989–998, 2000.)

Once the DNA double-strand breaks have been produced, they are processed by the 5′ → 3′ exonuclease MRN (Mre11/Rad50/Nbs1), which chews back one strand of the double helix (a process called resection), leaving single-stranded tails at the 3′ end of the DNA molecules (Fig. 45-3C; see also Fig. 43-15). The same exonuclease functions in somatic DNA repair and in meiotic recombination.

Next, the single-stranded tails “invade” the other chromosomes, looking for complementary DNA se-quences. This process is driven by Rad51 and Dmc1, two proteins that are related to the E. coli RecA protein, which is essential for DNA recombination in bacteria. These proteins polymerize into nucleoprotein filaments on DNA and use ATP hydrolysis to catalyze homologous pairing and strand exchange reactions. The process inserts a single-stranded region of DNA into a double helix, displacing one of the two paired strands. Dmc1 functions only in meiosis, but Rad51 has other essential functions as well. Dmc1 may promote the search for homologous chromosomes, rather than sister chromatids as occurs in somatic DNA repair. Mutants that lack Dmc1 are defective in homologous chromosome pairing. Rad51p and Dmc1p are found in structures called early recombination nodules that are distributed along the chromosome axes early in meiosis (Fig. 45-9).

It is now believed that if only one single strand successfully invades the homologous chromosome, the outcome is a noncrossover event, whereas invasion of both single-strand tails leads to crossovers. The double invasion produces branched intermediates known as double Holliday junctions (Fig. 45-3F-G; see also Fig. 43-15B). These are then cleaved by as yet unknown nucleases and converted to mature crossover recombination products.

A second system for segregating homologs in meiosis I has been found in fruit flies and yeast. This process of achiasmate segregation functions on chromosomes that have not undergone genetic recombination. Flies have two types of achiasmate segregation, depending on whether homologous or nonhomologous chromosomes are involved. One model for homologous achiasmate segregation proposes that nonrecombined chromosomes remain paired owing to stickiness of heterochromatin at the end of pachytene and, as a result, segregate properly in anaphase I of meiosis. Heterologous achiasmate segregation uses an entirely distinct but unknown mechanism that does not require previous physical pairing of the chromosomes that segregate from one another.

D. melanogaster males do not bother with any of this, do not recombine, and yet still segregate their chromosomes happily in meiosis. So all the complication of meiotic recombination is not the only way to produce haploid gametes. This might be regarded as a cruel joke of evolution by those students who find all the Greek terms of meiotic nomenclature to be daunting.