Meiosis

Meiosis probably evolved from a mitosis-like process (van Heemst and Heyting 2000). Meiosis is responsible for two essential aspects of the sexual life cycle in eukaryotes: the transition from the diploid to the haploid state, and the generation of new combinations of alleles. Meiosis occurs only in the germ line (ovaries or testes).

During meiosis cells are produced that have a reduced number of chromosomes (the haploid or n number). This means that when the germ cells (eggs and sperm) fuse, the diploid (2n) number of chromosomes is restored (Figure 3.4). If meiosis did not reduce the number of chromosomes to n, the number of chromosomes in a sexually reproducing organism would double each generation. Both divisions in meiosis have prophase, metaphase, anaphase, and telophase stages, but their details are different (Figure 3.4). Meiosis may require days or weeks to complete. The essence of meiosis is that only one duplication of the chromosomes occurs, but two cell divisions occur, producing four haploid gametes from the original diploid cell. Meiosis requires two cell divisions (I and II) to produce daughter cells with the haploid set of chromosomes.

Meiosis I is the reductional division, in which the number of chromosomes is reduced from 2n to n. Prophase of meiosis I is a long stage and has been divided into substages (Figure 3.4). During prophase I, the chromosomes condense and become visible. Homologous chromosomes pair and become closely associated along their length. Each homologous chromosome consists of two sister chromatids joined at the centromere; thus the pairing of homologous chromosomes produces a four-stranded structure. During prophase I, the paired chromosomes are able to exchange genetic information by crossing over, which results in a shuffling of the genetic information in the gametes. The number of locations where genetic information was exchanged by crossing over often is indicated by the formation of chiasmata, which are visible under the microscope during prophase I. Chiasmata result from the physical exchange of nucleotides between chromatids of the homologous chromosomes.

During metaphase I, the two homologous chromosomes are located on opposite sides of the metaphase plate (Figure 3.4). The orientation of each chromosome pair relative to the two poles is random and thus which member of each pair of chromosomes (one set was originally derived from the mother and the other set was originally derived from the father) will move to a particular pole is random. This random alignment of chromosomes on the metaphase plate is the basis of Mendel's Law of Independent Assortment. Thus, genes originally derived from the individual's mother and father will end up assigned to daughter cells in a random fashion.

During anaphase I, the homologous chromosomes separate from each other and move to opposite poles. This physical separation of homologous chromosomes during anaphase I is the physical basis of Mendel's Law of Segregation. After anaphase I, a haploid set of chromosomes consisting of one homolog from each pair is located near each pole of the spindle. During telophase I, the spindle breaks down (Figure 3.4). Chromosomes may pass directly from telophase I to prophase II of meiosis II. Alternatively, there may be a pause between the two meiotic divisions. Chromosome duplication does not occur between meiosis I and II, however.

Meiosis II is similar to a mitotic division, with each daughter cell from meiosis I being replicated, resulting in the production of four haploid cells from the original diploid cell (Figure 3.4). Meiosis II is different from mitosis, however, because the chromatids of a chromosome are usually not identical along their entire length. This is due to the fact that

Single Germ Line Cell

Figure 3.4. Meiosis takes place in the germ line tissues and is a two-step process that results in the production ^^^^^^^^ of four haploid cells from a single precursor cell. Meiosis I reduces the chromosome number to the haploid state and involves a lengthy prophase, brief metaphase, anaphase, and telophase. The cells may immediately enter meiosis II. During meiosis II the cells divide to yield four haploid cells.

Figure 3.4. Meiosis takes place in the germ line tissues and is a two-step process that results in the production ^^^^^^^^ of four haploid cells from a single precursor cell. Meiosis I reduces the chromosome number to the haploid state and involves a lengthy prophase, brief metaphase, anaphase, and telophase. The cells may immediately enter meiosis II. During meiosis II the cells divide to yield four haploid cells.

crossing over could have occurred during prophase of meiosis I and resulted in an exchange of genetic information between the chromatids.

Meiosis has two unusual aspects. One of the most extraordinary aspects of meiosis I is that the two homologous chromosomes that are destined to pair and undergo recombination (crossing over) are able to find each other in a vast set of nonhomologous sequences. How this is achieved is a matter of considerable interest (Roeder 1997, Haber 1998). In Drosophila, pairing of homologous chromosomes may be facilitated through specialized pairing sites on the chromosomes. Heterochromatin, especially in the centromeres and telomeres, has been implicated as a mechanism that facilitates chromosomal pairing (Walker and Hawley 2000). During the pairing of homologous chromosomes, an elaborate ladder of protein called the synaptonemal complex is formed that helps to hold them together (Haber 1998).

A second extraordinary aspect of meiosis is the pairing of sister chromatids until their disjunction. This cohesion also is facilitated by protein complexes (van Heemst and Heyting 2000). During pairing, recombination by crossing over occurs at a 100- to 1000-fold higher frequency in meiosis than in mitosis. Recombination tends to occur at certain chromosomal loci called "hotspots" and occurs more often between homologous chromosomes rather than between the sister chromatids.

Crossing over occurs about twice per paired set of chromosomes and serves two roles: the resulting recombination yields new combinations of alleles and plays a mechanical role in separation (disjunction) of the homologous chromosomes at meiosis I (van Heemst and Heyting 2000). Appropriate separation of homologous chromosomes in meiotic anaphase I requires that paired homologous chromosomes, rather than individual chromosomes, lineup on the metaphase I spindle. At anaphase I, the homologous chromosomes move to opposite poles (which results in meiosis I being the reductional division).

Each metaphase chromosome has a distinct morphology that is identifiable by staining with lactic-acetic orcein or other stains (Figure 3.5). The location of the centromere allows cytogeneticists to distinguish particular chromosomes. The arms of the chromosome take up stains in a banding pattern that is characteristic of an individual chromosome.

0 0

Responses

  • Semolina
    When a meiosis occurs in a diploid cell with 30 chromosomes, what is the result?
    5 years ago
  • anna
    Are drosophila melanogaster haploid or diploid?
    4 years ago
  • theresa
    Where Does Mitosis Take Place?
    4 years ago

Post a comment