Chromosomes in body cells

1. A chromosome is a long molecule of DNA in the nucleus that carries the genes controlling a cell's characteristics.
2. The nucleus of a human body cell contains $46$ chromosomes.
3. These chromosomes are arranged in $23$ pairs, and the two chromosomes of a pair carry the same genes in the same order.
4. One chromosome of each pair was inherited from the mother in the egg cell, and the other was inherited from the father in the sperm cell.
5. A cell that contains two copies of every chromosome is described as diploid.
6. The diploid number of a human body cell is written $2n=46$, where $n$ stands for one complete set of chromosomes.
7. Every body cell is diploid, including skin cells, muscle cells and nerve cells.

Haploid gametes

1. Gametes are the sex cells that fuse together in sexual reproduction.
2. In humans the gametes are the sperm cell and the egg cell.
3. In flowering plants the gametes are the nucleus inside a pollen grain and the egg cell inside an ovule.
4. A gamete contains one chromosome from each pair rather than both, so it carries a single set of chromosomes.
5. A cell that carries a single set of chromosomes is described as haploid.
6. A human gamete therefore contains $23$ chromosomes, one from each of the $23$ pairs, written $n=23$.

How meiosis works

1. Meiosis is the type of cell division that produces gametes.
2. In humans, meiosis takes place in the testes, where it makes sperm cells, and in the ovaries, where it makes egg cells.
3. In flowering plants, meiosis takes place in the anthers, which make pollen, and in the ovules, which make egg cells.
4. Meiosis starts with a single diploid parent cell in which every chromosome has been copied.
5. The cell then divides twice, so one parent cell gives rise to four daughter cells.
6. Each daughter cell receives half the number of chromosomes of the parent cell, so each one is haploid.
7. One human cell with $46$ chromosomes therefore produces four cells with $23$ chromosomes each.
8. Each daughter cell is genetically different from the other three and from the parent cell it came from.
9. The role of meiosis is therefore to produce genetically different haploid gametes from one diploid cell.

Fertilisation restores chromosome number

1. Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete.
2. Each gamete brings a single set of chromosomes, so the new cell receives $23$ chromosomes from the mother and $23$ from the father.
3. The new cell therefore contains $23+23=46$ chromosomes and is diploid, $2n=46$.
4. The halving in meiosis and the doubling at fertilisation cancel each other out, so the chromosome number of the species stays the same from one generation to the next.
5. If gametes were diploid instead, two gametes of $46$ chromosomes would fuse to give a cell with $92$ chromosomes.
6. That generation would then make gametes of $92$ chromosomes and offspring of $184$, so the number would double at every fertilisation.
7. Halving the chromosome number in meiosis is what prevents this doubling and keeps the chromosome number of the species constant.

Zygote growth by mitosis

1. The diploid cell formed at fertilisation is called a zygote.
2. The zygote divides repeatedly by mitosis to form a ball of cells that develops into an embryo.
3. Every chromosome is copied before each of these divisions, so each new cell receives a full set of $46$ chromosomes.
4. All of the body cells built in this way are diploid and genetically identical to the zygote they came from.
5. The chromosome number of an organism can therefore be tracked through the whole cycle: diploid body cell, haploid gamete, diploid zygote, diploid body cell.

Comparing mitosis and meiosis

1. Both divisions begin after the chromosomes have been copied, but they differ in the number of divisions, daughter cells, chromosome sets, genetic similarity and biological role.
2. Number of divisions: mitosis divides the parent cell once, while meiosis divides it twice.
3. Number of daughter cells: mitosis produces two daughter cells, while meiosis produces four.
4. Chromosome number: the daughter cells of mitosis are diploid with $46$ chromosomes, while the daughter cells of meiosis are haploid with $23$.
5. Genetic make-up: the daughter cells of mitosis are genetically identical to the parent cell and to each other, while the daughter cells of meiosis are genetically different.
6. Use in the organism: mitosis is used for growth, for the repair of damaged tissue and for asexual reproduction.
7. Meiosis is used only to make gametes for sexual reproduction, and it takes place only in the reproductive organs.

How meiosis creates variation

1. The four gametes made by one meiosis are all genetically different from one another.
2. Each gamete receives one chromosome from each pair, and which chromosome of a pair it receives varies from gamete to gamete.
3. Every gamete therefore ends up with a different combination of the chromosomes the parent inherited from its own mother and father.
4. Because a human cell has $23$ pairs to choose from, the number of different combinations a single person can produce is very large.
5. Fertilisation is also random, because any sperm cell can fertilise any egg cell.
6. Any one of a huge number of genetically different sperm cells can fuse with any one of a huge number of genetically different egg cells, so the number of possible zygotes is enormous.
7. This is why offspring produced by sexual reproduction differ from each other and from both of their parents, while offspring produced by mitosis alone do not.