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Cell Division: Eukaryotes
Meiosis I: first division
Prophase I

Mechanism:

All cells undergoing a meiotic division pass through two distinct periods called First division, in which the homologous chromosomes are separated, and the Second division, in which the duplicated DNA molecules are separated.

Prophase I

This is the most complex of the various stages of meiosis. Once upon a time the only tool for scientists studying meiosis was the light microscope, so the various stages of Prophase I were given names based only on what could be seen this way. Since no one at that time understood exactly what was going on, the names are more descriptive of what was seen rather than what was happening. (These are the names given in brown, below).

These names have lasted a long time in the history of science, and are given here for comparison. However, we now have a much better understanding of the function that is taking place at all stages of Prophase I, and newer names have been devised which better described the mechanisms that are taking place. These names will also be used here for clarity. (In red, below).


Stages

  1. First Stage
    • Condensation
    • Leptotene

  2. Second Stage
    • Pairing
    • Zygotene

  3. Third Stage
    • Recombination
    • Pachytene

  4. Fourth Stage
    • Coiling
    • Diplotene

  5. Fifth Stage
    • Recondensation
    • Diakinesis


First Stage:

Condensation: threads of DNA wrapped in nuclear proteins and histones gradually become visible. These threads often have "bead-like" swellings along their length, but their significance is unknown.

Close examination of these threads, which continue to shorten and thicken, shows that they are already doubled. Each half was once called a chromatid, and is now known to be one partner of the doubled DNA molecules of each chromosome made back in S-phase.


Second Stage:

At some point in the thickening process it is possible to make out that each thread represents a double-DNA chromosome. During the second stage, these chromosomes start to pair up with their complementary partner (the other chromosome that is carrying the same set of genes).

Such chromosome pairs are said to be homologous and the pair is said to be a pair of homologous chromosomes.

As this process of pairing continues (also called synapsis), the homologous chromosomes come into a tighter and tighter arrangement. It is difficult to see what the paired chromosomes are doing if a light microscope is the only instrument used, but if these complexes are viewed using an electron microscope, the close association between them can be seen.

There appears to be a thin space between the two chromosomes which contains a multiply-threaded structure called a synaptonemal complex. This complex extends the length of the chromosome pair and is attached to the nuclear envelope.


Third Stage:

This is one of the longest stages of Prophase I, and it is during this stage that biological information is exchanged between chromosome pairs.

Homologous chromosomes are in very close contact and the physical association between the DNA molecules of the pairs of chromosomes (4 DNA molecules in all) is very strong at certain points.

It is believed that the act of crossing over, or the physical exchange of parts of the chromosomes, takes place at these points of close contact during this third stage of Prophase I.


Fourth Stage:

The dissolving and break down of the synaptonemal complex, and the separation of the individual components of the two sets of chromosomes marks the beginning of the fourth stage, the coiling stage.

The homologous chromosomes move apart in such a way as to suggest that they might even be repelling one another. But they do not separate entirely. At scattered points along the structure the points of crossing over still remain, and these act as "spot welds" or chiasmata, which hold all four parts of the DNA and chromosomes together.

Human female egg cells reach this stage in the unborn fetus, about 4 months before delivery. Then the process is frozen in time, and nothing more happens, often for 30 years, until the monthly ovulation when one egg cell breaks free of the ovary and completes the meiotic sequence.


Fifth Stage:

More and more packing of DNA with histones and proteins makes the chromosomes even shorter and more densely packed. This is the recondensation, or final stage of Prophase I.

All of the chromosomes are now at their maximum density and degree of packing, and the chiasmata move along the length of each structure until they reach the ends of the chromosomes, a process called terminalization.

In its most extreme form the four parts of a recondensed chromosome set may only be held together by a few remaining chiasmata at each end.

It is during this last stage of Prophase I that the nucleoli vanishes and the nuclear membrane breaks down so that the cell can enter the next stages of meiosis I.

events of Prophase

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© 2002, Professor John Blamire