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The Problem

Mitosis produces two cells with the same number of chromosomes as the parent cell. Mitosis of a diploid cell (2n) produces two diploid daughter cells. If two diploid cells went on to participate in sexual reproduction, their fusion would produce a tetraploid (4n) zygote.

The Solution: Meiosis

Meiosis is a process of cell division in eukaryotes characterized by: Fusion of two such cells produces a 2n zygote.

Meiosis in Animals

Used to produced the gametes: sperm and eggs

Meiosis in Plants

Used to produce spores. Spores are the start of the gametophyte generation which, in time, will produce gametes (by mitosis because the starting cells are already haploid).

Meiosis I

Prophase of meiosis I (prophase I) is a more elaborate process than prophase of mitosis (and usually takes much longer).

Here is a brief overview of the process. A more detailed view is provided below.

At metaphase I, microtubules of the spindle fibers attach to the

Result: one homologue is pulled above the metaphase plate, the other below. The chiasmata keep the homologues attached to each other, and the cohesin keeps the sister chromatids together.

At anaphase I,

Link to discussion of the mechanism of cohesin breakdown.

Meiosis II

Chromosome behavior in meiosis II is like that of mitosis.

Genetic Recombination

Meiosis not only preserves the genome size of sexually reproducing eukaryotes but also provides three mechanisms to diversify the genomes of the offspring.

1. Crossing Over

Chiasmata represent points where earlier (and unseen) nonsister chromatids had swapped sections. The process is called crossing over. It is reciprocal; the segments exchanged by each nonsister chromatid are identical (but may carry different alleles).

Each chromatid contains a single molecule of DNA. So the problem of crossing over is really a problem of swapping portions of adjacent DNA molecules. It must be done with great precision so that neither chromatid gains or loses any genes. In fact, crossing over has to be sufficiently precise that not a single nucleotide is lost or added at the crossover point if it occurs within a gene. Otherwise a frameshift would result and the resulting gene would produce a defective product or, more likely, no product at all.

Link to a model of how two DNA molecules can cross over.
Crossing over between two DNA molecules carrying different alleles enables the order of genes to be mapped. Follow these links to see examples.

In the diagram above, only a single chiasma is shown. However, multiple chiasmata are commonly found (in humans the average number of chiasmata per tetrad is just over two). In this photomicrograph (courtesy of Prof. Bernard John), a tetrad of the grasshopper Chorthippus parallelus shows 5 chiasmata.

2. Random Assortment

In meiosis I, the orientation of paternal and maternal homologues at the metaphase plate is random. Therefore, although each cell produced by meiosis contains only one of each homologue, the number of possible combinations of maternal and paternal homologues is 2n, where n = the haploid number of chromosomes. In this diagram, the haploid number is 3, and 8 (23) different combinations are produced.

Random assortment of homologues in humans produces 223 (8,388,608) different combinations of chromosomes.

Furthermore, because of crossing over, none of these chromosomes is "pure" maternal or paternal. The distribution of recombinant and non-recombinant sister chromatids [View] into the daughter cells at anaphase II is also random.

So I think it is safe to conclude that of all the billions of sperm produced by a man during his lifetime (and the hundreds of eggs that mature over the life of a woman), no two have exactly the same gene content.

3. Fertilization

By reducing the number of chromosomes from 2n to n,the stage is set for the union of two genomes. If the parents differ genetically, new combinations of genes can occur in their offspring.

Taking these three mechanisms together, I think that it is safe to conclude that no two human beings have ever shared an identical genome unless they had an identical sibling; that is a sibling produced from the same fertilized egg.

The behavior of chromosomes during meiosis (2nn) and fertilization (n + n2n) provide the structural basis for Mendel's rules of inheritance. Link to discussions of Mendel's monohybrid and dihybrid crosses.

Prophase I — a detailed view

The lengthy and complex events of prophase I can be broken down into 5 stages.

1. Leptotene

2. Zygotene

3. Pachytene

Link to a model of how two DNA molecules can recombine segments.

4. Diplotene

5. Diakinesis

In some organisms, the chromosomes decondense and begin to be transcribed for a time. This is followed by the chromosomes recondensing in preparation for metaphase I.

In creatures where this does not occur, the chromosomes condense further in preparation for metaphase I.

Checkpoints: Quality Control of Meiosis

It shouldn't be surprising that things can go wrong in such a complicated process. However, cells going through meiosis have checkpoints that monitor each pair of homologues for

Any failure that is detected stops the process and usually causes the cell to self-destruct by apoptosis.

However, despite these checkpoints, errors occasionally do go uncorrected.

Errors in Meiosis

It is estimated that from 10–25% of all human fertilized eggs contain chromosome abnormalities, and these are the most common cause of pregnancy failure (35% of the cases).

These structural abnormalities Aneuploidy — the gain or loss of whole chromosomes — is the most common chromosome abnormality. It is caused by nondisjunction, the failure of chromosomes to correctly separate: Examples:
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30 May 2024