Meiosis


Meiosis is the process that generates gametes (or sometimes spores) from regular, somatic cells. Meiosis halves the number of chromosomes in a somatic cell, such that the resultant gametes are haploid in their chromosome number. Meiosis first requires DNA replication, during the S phase of the cell cycle, and involves two rounds of division, termed meiosis I and meiosis II. A full meiotic division in one diploid, somatic cell produces four gametes, each containing one full set of chromosomes.

Meiosis is essential for sexual reproduction and occurs in all eukaryotes including single-celled protists. Prokaryotes divide asexually by alternative processes such as binary fission.

Meiosis is distinct from mitosis in that the products of meiosis are genetically unique, while the products of mitosis are genetically identical to (clones of) their parents.

THE PHASES OF MEIOSIS:
As an overview, meiosis I involves the separation of pairs of homologous chromosomes, each chromosome containing two sister chromatids, to create two haploid daughter cells. Because meiosis I reduces the ploidy of the cells, it is called a reductional division. Meiosis II then involves the separation of the sister chromatids of each chromosome, so that each chromatid from each homologous chromosome has been isolated in its own daughter cell. Because there is no reduction in ploidy, only a reduction in chromatid number, this is called an equational division. Meiosis II is essentially an equivalent process to mitosis. The final product of meiosis I & II is four haploid daughter cells, or gametes.

Meiosis I:

Before meiosis I, the cell undergoes interphase in much the same way as it does before mitosis. DNA is replicated to form two sister chromatids on each chromosome; this occurs during the synthesis (S) phase of interphase.
  1. Prophase I: Homologous chromosomes pair up in a tetrad, or bivalent. DNA may be exchanged between the two homologous chromosomes in a phenomenon called homologous recombination (or 'crossing over'). Recombination may occur between non-sister chromatids (i.e. the chromatid of one chromosome and its homologous partner) at specific locations called chaismata (singular: chaisma). Recombination is a source of genetic diversity in gametes because it produces new combinations of alleles. Prophase I is itself subdivided into the following phases:
    1. Leptotene: Chromosomes condense from their original form as loosely-bound chromatin into long threads that are visible under a microscope. However sister chromatids are still tightly bound to one another and cannot be distinguished from one another.
    2. Zygotene: The synapsis (pairing) of homologous chromosomes into bivalents occurs
    3. Pachytene: Synaptonemal complexes form between non-sister chromatids, across which DNA can be exchanged in 'crossing over'
    4. Diplotene: Synaptonemal complexes break down and the homologous chromosomes are allowed to separate a little, although chaismata still hold them in place together
    5. Diakinesis: The chromosomes condense further, such that all four chromatids of the bivalent are visible under a microscope. The nucleolus and nuclear membrane break down, and spindles form. Two centrosomes (microtubule organising centres) migrate to the poles of the cells, releasing kinetochore microtubules to the nucleus after its membrane is disintegrated. Each bivalent of homologous chromosomes has four structures called kinetochores which may bind to the kinetochore microtubules. In meiosis I, the two kinetochores of sister chromatids fuse and act as a single unit. This is to ensure that the entire chromosome is carried by the microtubule, rather than only one chromatid as will be the case in meiosis II. Non-kinetochore microtubules form links between the two centrosomes.
  2. Metaphase I: Bivalents are attached to the kinetochore microtubules by their kinetochores. The bivalents are then oriented so that one chromosome is on one side of the metaphase plate, an imaginary equatorial line that is equidistant from the poles of the cell, while its homologous partner is on the other side. This is called independent assortment of homologous chromosomes because the orientation of one bivalent on the plate is independent to the orientation of every other bivalent on the plate.
  3. Anaphase I: The kinetochore microtubules shorten, causing the homologous chromosomes to be pulled apart a little. The chromosomes are then separated all the way to the poles by the polymerisation and elongation of non-kinetochore microtubules, which puts distance between the two centrosomes and forces them apart, carrying the chromosomes with them.
  4. Telophase I: The separated chromosomes congregate at either pole. Microtubule spindles disintegrate, two nuclear membranes reform around the chromosomes at each pole. Cytokinesis causes the daughter nuclei to separate into new cells. The cell may enter a period of rest, called Interphase II before entering meiosis II; however no DNA replication occurs this time.

Meiosis II:

  1. Prophase II: Chromosomes condense, nuclear membranes break down, centrioles migrate to poles, and kinetochore and non-kinetochore microtubules form. Non-kinetochore microtubules, as before, connect the two centrosomes.
  2. Metaphase II: Kinetochore microtubules attach to the two kinetochores on each sister chromatid. Previously these kinetochores acted as a single functional unit to keep the chromosome intact; now they act as separate units so that each sister chromatid can be separated. The metaphase plate is perpendicular (rotated 90 degrees) with respect to the metaphase plate of metaphase I
  3. Anaphase II: As the kinetochore microtubules contract and pull on the kinetochores, sister chromatids are pulled apart to opposite poles of the cell (this also involves the same action of non-kinetochore microtubules as in Anaphase I). Because they are no longer attached, the sister chromatids can be considered sister chromosomes.
  4. Telophase II: Similarly to telophase I, the microtubule fibres now disintegrate, nuclear membranes reform around the two groups of chromosomes and cytokinesis ensues to create new cells for the daughter nuclei of this division. Once this round of cytokinesis is done, the meiotic division is complete and four haploid gametes, each genetically unique, have been produced.