Prophase I.

Prophase I of meiosis begins very much as prophase of mitosis but there are several things that happen in prophase I of meiosis which do not typically happen in mitosis. In these diagrams the paternal chromosomes, chromosomes that come from the mother are shown in black and the maternal chromosomes, chromosomes that come from the mother are in red.

The key new events in prophase I are:

Tetrad formation

Pairs of homologous chromosomes line up to form tetrads(T) C represents a pair of centrioles that will migrate to the one end of the cell.

Tetrad: The term tetrad refers to the four chromatids of the paired homologous chromosomes. Remember that during prophase, each chromosome has two strands of DNA or chromatids.

Crossing over

Non sister chromatids of the homologous chromosomes swap pieces of DNA. This is called crossing over. Crossing over is important because it results in new combinations of genetic material that were not found in the parent DNA.

Recombination: The process of producing new genetic combinations not present in the original parent DNA is called recombination. It is an important process in genetics because it leads to offspring that have different genetic make up from each other as well as different from either parent.

Tetrads migrate to cell equator

Prophase I concludes when the tetrads line up the equator of the cell. This is different than what happens at the end of prophase of mitosis because in mitosis the individual chromosomes line up at the equator of the cell.

The Special Case of Sex Chromosomes.

The sex chromosomes provide a complication in meiosis because they are not homologous, yet they do pair up during prophase I of meiosis. The are able to pair up because they have a region called the pseudoautosomal region which is homologous between the X an Y chromosomes. The rest of the X and Y chromosomes, including the region around the centromere are not homologous. This is illustrated in the accompanying figure, where the pseudoautosomal region is shown in yellow. So when The X and Y chromosomes pair up they pair up only at this region, and this region is the only region where crossing over happens.

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Metaphase I

The key events to remember are:

• Chromosomes are at the equator of the cell.
• The tetrads line up as a unit.
• There is no separation into unduplicated chromosomes as happened in Mitosis.
• Which chromosome faces which direction in a tetrad is independent of what's happening in the other tetrads. This is called independent assortment.

Independent assortment

Independent assortment says that when the paired homologous chromosomes line up at the equator of the cell, they orient themselves independently of what the other pairs are doing. Thus during metaphase I the cell could end up looking either like figure A or figure B. Notice the different orientations of the lower tetrad indicated by the arrow.

In figure A the paternal chromosomes(black) are going to go in opposite directions since the paternal chromosome in the lower tetrad will be pulled to the left by the spindle apparatus while the paternal chromosome is going to be pulled to the right.

If the chromosomes end up arranged as in figure B, the paternal chromosomes are both going to be pulled in the same direction.

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Anaphase I

The key event to remember here is the chromosomes in the tetrads separate from each other. The result is two sets of chromosomes, each of which has one member of each pair of the original homologous chromosomes. This figure shows anaphase I for resulting from the metaphase I arrangement for metaphase I arrangement shown in figure A.

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Telophase I

The result is two daughter cells.

• Each daughter cell is haploid.
• The chromosomes are double stranded.
• The daughter cells are not genetically identical.
• Each daughter cell typically undergoes meiosis II

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Prophase II / Metaphase II. These are is just like prophase and metaphase in mitosis except that the cells are haploid and the sister chromatids are not identical due to crossing over from prophase I.

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Anaphase II. Just like anaphase of mitosis,except the cells are haploid.

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Telophase II. The result is 4 haploid cells, two from each of the daughter cells of meiosis I. Notice that the four daughter cells are not identical.

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Chromatids and Meiosis

One way to view meiosis is as a process to take the four homologous chromosomes and divide them up so that each daughter cell gets one of the four chromatids. The figure first shows four chromatids after crossing over is finished and the cell is in metaphase I. The black arrows arrows indicate each distribution of chromatids into separate cells. The first distribution in meiosis I divides the tetrad and results in daughter haploid cells with two chromatids per chromosome. The second distribution happens when each daughter cell from meiosis I undergoes meiosis II. The result is two new cells from each of the cells from meiosis one. Each of these four final daughter cells, gametes in animals, now has just one of the original chromatids that resulted from crossing over way back in prophase I. So each gamete gets one member of each pair of homologous chromosomes but not both. It is this fact that provides the link between Mendel's principle of segregation and chromosome behavior. 1/11/00 pgd

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