Meiosis II: the second division

All the hard work of crossing over, reduction division and the sorting out of chromosome homologues to opposite poles of the cell was completed in Meiosis I: the first division.

Meiosis II is almost an after thought in which the DNA pairs of each homologous chromosome are similarly separated.

After a brief, or nonexistent, Prophase II, the individual chromosomes reattach to kinetochore tubules and are once again pulled to the central region of the two, smaller spindles at the opposite poles of the cell. This is Metaphase II.

If there has been any detectable unpacking of the chromosomes between the First and Second Meiosis, this is when the chromosomes condense fully once more.

In Anaphase II the single DNA copies of each chromosome are finally dragged apart. Attached by their kinetochores to spindle tubules that point to opposite poles, the shortening of the tubules strips one almost identical DNA molecule from its partner, a relationship that started way back in S-phase of interphase, when these DNA molecules were first duplicated.

At the end of Anaphase II, and the beginning of Telophase II, a new set of nuclear membranes forms around each tiny package of separated biological information, still tightly condensed into visible chromosomes. This is probably the last time they will be together. The fate of each informational package now depends on the species of organism in which it is taking place.

In large, multicellular organisms like ourselves, our somatic cells (body cells) are all diploid. Meiosis, therefore, takes place in specialized reproductive organs, the gonads, to produce haploid cells that will eventually fuse in pairs to produce new diploid zygotes again. Meiosis in such organisms is said to be gametic.

The haploid products produced at the end of Meiosis II go on to differentiate in quite complex ways to produce the recognizable sexual gametes; the sperm and eggs. Since each of these gametes has a very different role, and method of accomplishing that role, the two gametes are often very, very different in physical form, and the path of differentiation they take after Meiosis II.

In other organisms, such as the brewer's yeast, the haploid products produced at the end of Meiosis II are packaged and then released as independent, free living forms in their own right. This is said to be a sporic outcome of meiosis. The haploid form of a plant or fungus may live for a long time in this genetic state before recombining its biological information with that of another individual and re-entering the diploid state for a time.

Many fungi, however, only exist as diploids for a tiny, brief part of their life cycle. In these species fertilization, the fusion of two haploid cells to produce a zygote, is followed almost immediately by a meiotic division of this very same cell to produce haploids once more.

This zygotic type of meiosis is only used, therefore, to mix up and randomize the biological information contributed by two parents into the huge possible alternate forms in the various offspring. It is pure variation, which, after all, is the main purpose of meiosis and sexual reproduction.