Meiosis and gametes

1. Meiosis is the form of cell division that produces gametes, and in humans it takes place only in the testes and the ovaries.
2. In flowering plants the same division occurs in the anthers, which make pollen, and in the ovules, which make egg cells.
3. One diploid parent cell that enters meiosis gives rise to four haploid daughter cells.
4. A diploid human body cell holds $46$ chromosomes arranged as $23$ homologous pairs, one chromosome of each pair having come from each parent.
5. Each gamete formed from that cell holds $23$ chromosomes, exactly one taken from every pair.
6. The change is written $2n\to n$, where $n$ stands for one complete set of chromosomes.
7. In humans $n=23$, so a body cell is $2n=46$ and a sperm cell or egg cell is $n=23$.
8. Meiosis is the only division in the body that changes the chromosome number, which is why it is confined to the reproductive organs.

Why the number is halved

1. Fertilisation is the fusion of a male gamete nucleus with a female gamete nucleus.
2. Two haploid nuclei fuse, so the zygote that forms carries $23+23=46$ chromosomes.
3. The diploid number is restored exactly, and the zygote then divides repeatedly by mitosis to build the whole organism.
4. Every body cell of the new individual therefore carries the same $46$ chromosomes as the body cells of its parents.
5. Because one full set arrives from each parent, every homologous pair holds one maternal chromosome and one paternal chromosome.
6. If gametes were diploid, the chromosome number would double at every fertilisation, running $46\to 92\to 184$ in three generations.
7. A cell with the wrong chromosome number carries the wrong number of copies of every gene on the extra chromosomes, so it cannot develop normally.
8. Meiosis is described as a reduction division because it is the step that takes one chromosome away from every pair.

The two divisions of meiosis

1. Before meiosis starts, every chromosome is copied during interphase, so it becomes two identical sister chromatids joined at a centromere.
2. Meiosis I separates the homologous pairs, and it is this division that halves the chromosome number.
   1. Prophase I: the chromosomes condense and each one pairs up alongside its homologous partner.
   2. Metaphase I: the pairs line up along the equator of the cell, each pair positioned independently of the others.
   3. Anaphase I: spindle fibres pull one whole chromosome of each pair to each pole, so the sister chromatids stay joined together.
   4. Telophase I: two nuclei form and the cell divides, producing two cells that are already haploid.
3. Meiosis II separates the sister chromatids and runs in the same way as mitosis.
   1. Prophase II: the chromosomes condense again inside each of the two new cells.
   2. Metaphase II: the chromosomes line up singly along the equator.
   3. Anaphase II: each centromere divides and the sister chromatids are pulled to opposite poles.
   4. Telophase II: four nuclei form and both cells divide, giving four haploid cells in total.
4. The overall result is one diploid cell becoming four haploid cells, each carrying one chromosome from every original pair.

How meiosis creates variation

1. Crossing over happens in prophase I, when the chromatids of a homologous pair lie against each other and swap matching sections of DNA.
2. The point at which they join and exchange material is called a chiasma, and every chromatid that leaves it carries a new combination of alleles.
3. Independent assortment happens at metaphase I, because the way one pair lines up at the equator has no effect on the way any other pair lines up.
4. Either chromosome of a pair can end up facing either pole, so each pair contributes one of two possible outcomes to the gamete.
5. For $n$ homologous pairs the number of possible chromosome combinations in a gamete is $2^n$.
6. With $n=23$ in humans this gives $2^{23}=8388608$ combinations from independent assortment alone, before crossing over is counted.
7. Random fertilisation adds a further step, because any one of those sperm cells can fuse with any one of those egg cells.
8. The four gametes produced by one meiosis are therefore genetically different from one another, and offspring differ from their parents and from each other.

Meiosis compared with mitosis

1. Mitosis produces two daughter cells, whereas meiosis produces four.
2. Mitosis keeps the chromosome number at $2n$, whereas meiosis reduces it to $n$.
3. Mitosis involves a single division, whereas meiosis involves two divisions one straight after the other.
4. The cells made by mitosis are genetically identical to the parent cell, whereas the cells made by meiosis are all genetically different.
5. Mitosis occurs throughout the body for growth, repair and asexual reproduction, whereas meiosis occurs only in the reproductive organs.
6. Neither crossing over nor independent assortment occurs in mitosis, which is why mitosis produces no genetic variation.