Meiosis: Function and Stages of Meiosis

Meiosis is a form of nuclear division that is of funda­mental importance among sexually reproducing organisms.

An in-depth discussion of meiosis on a cel­lular as well as a genetic basis is beyond the scope of this book; such discussions are normally treated at length in textbooks of genetics.

However, for the sake of completeness we will consider some of the ma­jor meiotic events and their implications. Meiosis occurs in eukaryotic organisms whose cells contain the diploid number (2n) of chromosomes.

Dip­loid implies “double” in the sense that the genetic in­formation present in any one chromosome can also be found in an identical (or somewhat modified) form in a second chromosome in the nucleus. The two chromo­somes forming such pairs are said to be homologous.

Human cells contain 46 chromosomes or 23 homologous pairs (i.e., in hu­mans w = 23). The 46 chromosomes of the zygote formed at fertilization are derived equally from the sperm cell and egg cell of the male and female par­ents.

Each of these gametes contributes one member of each pair of homologues. Once the zygote is formed, mitosis produces the billions of cells that ultimately make up the whole organism. Because sperm cells and egg cells contain only one member of each pair of homologues, they are said to be haploid. It is meiosis that produces haploid cells, the process being re­stricted to the reproductive tissues (i.e., ovaries and testes).

During meiosis, the replicated chromosomes of the nucleus are apportioned among four daughter nuclei, each nucleus acquiring half the number of chromo­somes of a diploid cell. Although the resulting cell nu­clei contain only half the diploid number of chromo­somes, the chromosome set is genetically complete, because each nucleus acquires one member of each pair of homologous chromosomes.

The homologous chromosomes are assorted randomly at anaphase, and this accounts in part for the genetic variation that characterizes sexually reproducing organisms. Addi­tional genetic variation occurs during the prophase of the first nuclear division by a process called crossing- over. The genetic implications of random assortment and crossing-over are principal subjects of genetic courses.

The various stages of meiosis may be summarized as follows:

Meiotic Division I

Prophase I

1. Leptotene stage (leptonema):

The chromosomes become visible as condensation of the chromatin be­gins; each chromosome can be seen to consist of two chromatids.

2. Zygotene stage (zygonema):

Homologous chromosomes are aligned side-by-side so that allelic genes (i.e., those encoding products of similar or identical function) are situated adjacent to one another. This phenomenon is called synapsis. The unit consisting of two synapsed and duplicated homologous chromo­somes is called a bivalent. As synapsis progresses, a protein framework joining adjacent, non-sister chro­matids of each tetrad is formed at one or more points in the narrow space separating the homologues.

It is in the region of these synaptonemal complexes that crossing-over occurs. Crossing-over or chiasma for­mation results from the cleavage by endonucleases of the DNA in corresponding positions of two non-sister chromatids, followed by the transposition and rejoin­ing of the free ends of homologous strands (see Fig. 20-23 for details). As a result of crossing-over, new combinations of genes are created in the homologous chromosomes.

3. Pachytene stage (pachynema):

During this stage the chromatids become increasingly distinct as con­densation continues.

4. Diplotene stage (diplonema):

The diplotene stage is characterized by the separation of the paired homologous chromosomes except at points where chi- asmata are formed.

5. Diakinesis:

Diakinesis brings prophase I to an end. During this stage chromosome condensation is completed.

Metaphase I:

In this phase, the spindle apparatus forms, much as it does in mitosis, and the bivalents align on the equato­rial plate. The centromeres of homologous chromo­somes attach to spindle fibers arising from opposite poles of the cell.

Anaphase I:

Homologous chromosomes (but not sister chromatids) of each tetrad separate from each other and move to opposite poles of the spindle.

Telophase I:

Telophase I brings the first meiotic division to a con­clusion as the separated homologues aggregate at their respective poles so that two nuclear areas are distinguishable. In most organisms, a new nuclear en­velope is formed and some de-condensation of the chro­mosomes occurs.

Interkinesis (or Interphase):

Interkinesis is the period between the end of telo­phase I and the onset of prophase II. This period is usually quite short. The DNA of the two nuclei pro­duced by the first meiotic division does not engage in replication during interkinesis.

Meiotic Division II:

Prophase II:

The events characterizing this phase are similar to mi­totic prophase, although each cell nucleus has only half the number of chromosomes as a cell in prophase I, that is, the nucleus is already haploid. Each chromo­some remains composed of the two sister chromatids formed prior to prophase I, except for segments that were interchanged during crossing-over.

Metaphase II:

The events occurring in this phase are similar to those in mitotic metaphase. The paired chromatids migrate to the center of the spindle and are attached there to the spindle’s microtubules.

Anaphase II:

The events occurring in this phase are similar to those in mitotic anaphase, but differ from those of anaphase I of meiosis. In anaphase II, sister chromatids sepa­rate from one another and are drawn to opposite poles of the spindle. (Recall that sister chromatids do not separate in anaphase I.)

Telophase II:

The events occurring in this phase are similar to those in mitotic telophase. The separated chromosome groups are enclosed in a newly developing nuclear en­velope and begin to undergo decondensation. Meiosis produces four cells, each with the haploid number of chromosomes. In many higher animals and some plants, meiosis in the female reproductive tis­sues is accompanied by an uneven division of the cyto­plasm, in which case one of the two cells formed dur­ing telophase I is a nonfunctional polar body and may not enter prophase II (Fig. 20-24).

In some organisms (such as humans) the polar body completes meiosis, but the two smaller polar bodies produced during telo­phase II are similarly nonfunctional. The second mei­otic division of the larger cell produced during telo­phase I is also unequal and produces an additional polar body. During the production of spermatozoa in the male reproductive tissues, division of the cytoplasm is equal, but remarkable cytoplasmic differentiation of the four spherical haploid spermatids pro­duced by meiosis is required (Fig. 20-24) before functional, flagellated spermatozoa are produced.