What Mitosis Produces

1. One division by mitosis produces exactly two daughter cells from one parent cell.
2. Each daughter cell receives an identical set of chromosomes to the set in the parent cell nucleus.
3. Because the chromosomes carry the genes, the daughter cells also carry identical alleles, so they are genetically identical.
4. The daughter cells are body cells, not gametes, and Edexcel refers to them as diploid body cells.

Diploid Body Cells

1. A diploid cell contains two full sets of chromosomes, one set from each parent, so the chromosomes exist as matching pairs.
2. In humans the diploid number is 46, made up of 23 homologous pairs.
3. Mitosis keeps the diploid number constant, so a human body cell that divides gives two cells with 46 chromosomes each.
4. Gametes are the exception. They are haploid with 23 chromosomes, and they are made by meiosis rather than mitosis.

Why the Daughter Cells Are Identical

1. The DNA is replicated during interphase, before division begins, so every chromosome exists as two identical copies.
2. At metaphase the copied chromosomes line up along the equator in a single row.
3. At anaphase the two copies of each chromosome are separated and one copy is pulled to each pole.
4. This means each pole ends up with one complete copy of every chromosome, which is why the two nuclei are identical to each other and to the original.
5. Nothing is swapped between chromosomes during mitosis, so no new combinations of alleles are produced and there is no genetic variation between the daughter cells.

Mitosis in Growth and Repair

1. An organism grows by producing more cells, and in animals those extra cells come from mitosis.
2. Identical cells are exactly what growth and repair need, because a replacement skin cell must do the same job as the cell it replaced.
3. Damaged tissue is repaired the same way, so the new cells fit into the existing tissue and function normally.
4. Cells that divide often, such as those lining the small intestine, are replaced every few days by mitosis.

Mitosis in Asexual Reproduction

1. In asexual reproduction a single parent produces offspring by mitosis, so the offspring are clones of the parent.
2. There are no gametes and no fertilisation, so no genetic material is combined from two parents.
3. Spider plants produce plantlets on long stems, and each plantlet carries the same alleles as the parent plant.
4. A hydra reproduces by growing a bud on its side, which detaches as a genetically identical individual.

The Advantage and the Cost of Identical Offspring

1. Identical offspring are produced quickly and without the need to find a mate, so a well-adapted organism can colonise a favourable habitat rapidly.
2. Every offspring inherits the parent's useful characteristics, which is why growers propagate fruit trees and potatoes from cuttings and tubers rather than seed.
3. The cost is a complete lack of variation within the population.
4. If the environment changes or a new disease appears, no individual has an allele that gives resistance, so the whole population is at risk.