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Key Concepts
Meiosis; the outcomes

The specialized cell division called meiosis serves two purposes in the life cycle of most organisms; it reduces the higher genetic number (the diploid number in most cases), down to the lower genetic number (the haploid number) - this is usually in preparation for the production of gametes - and; it randomizes the genetic information carried by the chromosomes - this introduces a considerable amount of diversity into the gametes, and hence the future zygotes.

    Key concepts:
  • In Prophase I of Meiosis I, by an elaborate mechanism not yet fully understood, homologous chromosomes recognize one another and pair up.
  • Each pair of homologues are arranged in the nucleus in a linear, side by side fashion so that the equivalent genes are directly opposite.
  • Since there are actually 4 copies of the DNA present at this time, this is called a tetrad. [Note; don't confuse this term with the same word in yeast genetics].
  • Homologues are held in physical contact by chiasmata, and it is at these points that parts of one chromosome are exchanged with parts of the homologous chromosome. This is the first kind of "randomization" of biological information that takes place in meiosis, and is called crossing over.
  • As a result of crossing over, all four DNA molecules of a tetrad of chromatids are subtly different from each other.
  • The location of the chiasmata and the amount of information exchange is different in every meiotic cell, so all the gametes will be subtly different to each other.
  • In Metaphase I of Meiosis I the multiply paired chromosomes line up and are attached to the spindle fibers. The orientation of this alignment process is random, and there is no mechanism for ensuring that one homologue will always be pulled in one particular direction in Anaphase I.
  • As a result, in Anaphase I, each homologue is pulled apart from each other homologue, but the direction in which it moves, and the pole to which it is dragged, is determined randomly. The same is true for the separation of chromatids in Anaphase II.
  • Each part of each chromosome ends up in a different potential gamete in a purely random fashion, and when there are a large number of chromosomes involved, the total number of possible outcomes is astronomic.
  • This is called independent assortment.
  • Because of the combined effects of both crossing over and independent assortment, there probably never has been a case of two gametes, male or female, with exactly the same combination of genes and chromosome parts.

Science@a Distance
© 2001, Professor John Blamire