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Modern Genetic Analysis Chapter 4. The Inheritance of Genes Cell Division
Genetics In Process 4-3: Meiosis

Meiosis consists of two nuclear divisions distinguished as meiosis I and meiosis II. The events of meiosis I are quite different from those of meiosis II, and the events of both differ from those of mitosis. Each meiotic division is formally divided into prophase, metaphase, anaphase, and telophase. Of these stages, the most complex and lengthy is prophase I, which is divided into five stages. Once again, try to imagine those stages as merging dynamically into one another with no clear borders.

Prophase I.

For convenience prophase I is divided into several stages. Much of what happens during these different stages of prophase I is a continuous process involving the compaction of chromosomes and stabiliza-tion of homologous chromosome associations within the tetrad so that the tetrad can be segregated cleanly at anaphase.

Leptotene.

The chromosomes become visible at this stage as long, thin single threads (see part a of the figure). Although they appear single, they are in fact composed of pairs of replicated DNA molecules. The process of chromosome contraction continues during leptotene and throughout the entire prophase. During leptotene small areas of thickening called chromomeres develop along each chromosome, which give it the appearance of a necklace of beads.

Zygotene.

This is a time of active pairing of the chromosomes (part b), making it apparent that the chromosome complement of the meiocyte is in fact two complete chromosome sets. Thus, each chromosome has a pairing partner, and the two become progressively paired, or synapsed, along their lengths. It should be noted that pairing does not occur in mitosis in which all the chromosomes lie randomly scattered around the equatorial plate, without regard to homologs.

How can homologs (members of a chromosome pair) find each other during zygotene? The probable answer to this is that the ends of the chromosomes, the telomeres, are anchored in the nuclear membrane, and it is likely that homologous telomeres are close, so that the two homologs can pair precisely along their length. Although the mechanism for this pairing is not precisely under-stood, we know that it involves an elaborate structure composed of protein and DNA, called a synaptonemal complex, which is always found sandwiched between homologs during synapsis. The photomicrograph shows several views of synaptonemal complexes. Part a shows the 62 chromosomes of a silk moth synapsed into 31 complexes. In part b, on the left is an unpaired chromo-some (note the dark line through its core). This dark line is repeated twice in the synaptonemal complex shown on the right, representing the cores of two homologs lined up together.

Pachytene.

This stage is characterized by thick, fully synapsed chromosomes (part c). Thus, the number of homologous pairs of chromosomes in the nucleus is equal to the number n. Nucleoli are often pronounced during pachytene. The beadlike chromomeres align precisely in the paired homologs, producing a distinctive pattern for each pair.

Diplotene.

Although each homolog appeared to be a single thread during leptotene, the DNA had, in fact, already replicated during the premeiotic S phase. This becomes manifest during diplotene as a longitudinal doubleness of each paired homolog (part d). As in mitosis, the divided subunits are called sister chromatids. Since each member of a homologous pair produces two sister chromatids, the synapsed structure now consists of a bundle of four homologous chromatids, the tetrad. At diplotene, the pairing between homologs becomes less tight; in fact, they appear to repel each other, and as they separate slightly, cross-shaped structures called chiasmata (singular, chiasma) appear between nonsister chromatids. Each homologous group of four generally has one or more chiasmata. Chiasmata are the visible manifestations of events called crossovers that occurred earlier, probably during zygotene or pachytene. Crossovers represent one major way in which meiosis differs from mitosiscrossovers during mitosis are rare. A crossover is a precise breakage, swapping, and reunion between two nonsister chromatids. Studies performed on abnormal lines of organisms that undergo crossing-over very inefficiently, or not at all, show severe disruption of the orderly events that partition chromosomes into daughter cells at meiosis. Thus, crossing-over obviously helps determine how paired homologs behave, and the presence of at least one crossover per pair is usually essential for proper segregation. Crossovers have another interesting role, which, as we shall see in Chapter 5, is to make new allele combinations, an important source of genetic variation in populations.

Diakinesis.

This stage (part e) differs only slightly from diplotene, except for further chromosome contraction. By the end of diakinesis, the long, filamentous chromosome threads of interphase have been replaced by compact units that are far more maneuverable.

Metaphase I.

The nuclear membrane and nucleoli have disappeared by metaphase I, and each pair of homologs takes up a position in the equatorial plane (part f). At this stage of meiosis, although it is likely that DNA replication has proceeded right through the centromeres, they do not appear to have divided, so the sister centromeres act as one. This apparent lack of division represents a major difference from mitosis. The two nonsister centromeres attach to spindle fibers from opposite poles.

Anaphase I.

As in mitosis, anaphase begins when chromosomes move directionally to the poles. The pairs of sister chromatids move to opposite poles (parts g and h).

Telophase I.

This telophase (part i) and the ensuing "interphase," called interkinesis (part j), are not universal. In many organisms, these stages do not exist, no nuclear membrane re-forms, and the cells proceed directly to meiosis II. In other organisms, telophase I and the interki-nesis are brief in duration; the chromosomes elongate and become diffuse, and the nuclear membrane re-forms. In any case, there is never DNA synthesis at this time, and the genetic state of the chromosomes does not change.

Prophase II.

The presence of the haploid number of sister chromatid pairs in the contracted state characterizes prophase II (part k).

Metaphase II.

The pairs of sister chromatids arrange themselves on the equatorial plane during metaphase II (part l). Here the chromatids often partly dissociate from each other instead of being closely pressed together as they are in mitosis.

Anaphase II.

Centromeres split and sister chromatids are pulled to opposite poles by the spindle fibers (part m).

Telophase II.

The nuclei re-form around the chromosomes at the poles (part n).

The four products of meiosis are shown in part o. In the anthers of a flower, the four products of meiosis develop into pollen grains; these are shown in part p. In other organisms, differentiation produces other kinds of structures from the products of meiosis, such as sperm cells in animals.

© 1999 by W. H. Freeman and Company. All rights reserved.