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Cell Division: Eukaryotes
Meiosis I: completing first division
Meta- Ana- Telophase I

Completing Meiosis I:

By this point all the chromosomes have been paired up, exchanged parts and condensed down to their ultimate degree of packing. The nucleoli have vanished and the nuclear membrane has gone. It is time to start separating some of the biological information.

Metaphase I:

As the end of Prophase I approaches, a spindle of fibers begins to form. In animal cells producing male gametes (sperm) the centrioles are replicated and move into position, but in female gametes and all plants, there are no centrioles and the spindle forms without them.

Disruption of the nuclear membrane spreads out the condensed chromosome sets, which become attached to the spindle fibers, which in turn pull the chromosomes in opposite directions. This "tug of war" has the effect of pulling all chromosome sets into the middle of the cell and the center of the spindle.

Each pair of chromosomes (homologues) has two kinetochores (points of attachment for the spindle fibers), and separate sets of kinetochore microtubules attach to these points and extent to the poles. Thus, on each set of doubled chromosomes in the center of the cell, there are four individual kinetochores, two pointing to one pole and two pointing towards the opposite pole.

Anaphase I:

In Anaphase I, the kinetochore microtubules shorten dramatically, pulling the homologous pairs of the chromosome sets apart and in opposite directions. This is the reduction division where all of the complementary homologues are systematically pulled apart from each other and delivered to the opposite ends of the cell.

Within each compound set of chromosomes, each homologue is pulled in a different, opposite, direction. So two similar homologues from the same set never end up at the same pole (under normal circumstances). This is very important, since the purpose of meiosis is to divide up the paired sets of homologous biological information into single sets (reduction division). This is the point where, from an informational point of view, the cell goes from being diploid (2n) to haploid (n).

Biological information carried by these separating homologues has already been altered and rearranged during the crossing over of parts of the chromosomes in Prophase I. So none of the homologues are carrying exactly the same set of genes they were carrying at the start of the process. But a further 'randomization' of biological information is about to take place.

As the sets of homologues are aligned in Metaphase I, and kinetochore tubules attached to the four sets of kinetochores, the orientation of each chromosome complex, and the direction in which it is facing, is arranged completely by chance.

In any one meiotic event, involving more than one chromosome set, the orientation and thus the direction in which the homologues will be pulled in Anaphase I, is determined completely randomly, with the same outcome as if it's fate were to be decided by the toss of a coin.

This means that the combinations of homologues that end up at each pole is different in every meiosis, and the combination of biological information in every haploid produced is different (and unique). Always assuming that there are a reasonable number of chromosomes in the sets to begin with. This is certainly the case in the production of human sperm and eggs - no two have ever been the same since the start of the human race!

Huge randomization of biological information and a huge number of possible combinations of genes are produced in meiosis. In complex creatures like ourselves, which use meiosis to produce sex gametes, the number of ways in which one sperm or one egg can differ from the next is astronomical! Consequently there is a LOT of variation seen in individuals produced as a result of sexual reproduction. This is the raw material for evolutionary change.

Telophase I and later:

In most organisms this period between the First division and the Second division of meiosis is indistinct. In a lot of species, this is a short, vague period that does not last long and during which the spindle and the various spindle tubules rearrange themselves from a single spindle, into two smaller spindles at opposite poles.

Where they occur, centrioles divide and reorganize themselves and the cell generally prepares for yet another round of information separation.

© 2002, Professor John Blamire