Mitosis as cell division producing two genetically identical cells for growth and repair, meiosis as division producing four genetically different gametes with half the chromosome number, and why meiosis creates variation.

What this dot point is asking

CCEA Double Award wants the two types of cell division: mitosis for growth and repair (identical cells) and meiosis for making gametes (genetically different cells with half the chromosome number). You must be able to compare them and explain why meiosis creates variation.

Mitosis

Mitosis is used for growth, repair of damaged tissue, and replacing worn-out cells. Before a cell divides, it copies its DNA so each new cell gets a complete identical set. This is also how some organisms reproduce asexually, producing identical offspring (clones).

Because the cells made by mitosis are genetically identical, asexual reproduction produces no variation. This is useful when an organism is well suited to a stable environment, but it is a weakness if the environment changes, because there is no variation for natural selection to act on. Strawberry plants spreading by runners and bacteria dividing in two are everyday examples of mitosis used for asexual reproduction.

Meiosis

Halving the chromosome number is essential: when two gametes join at fertilisation, the full number is restored. In humans, body cells have 46 chromosomes, gametes have 23, and fertilisation makes a cell with 46 again. If meiosis did not halve the number, the chromosome count would double every generation, which would not work.

Meiosis only happens in the reproductive organs - the ovaries and testes in animals - because that is where gametes are made. Everywhere else in the body, growth and repair use mitosis. So the two types of division have separate jobs and separate places.

Why meiosis creates variation

Meiosis shuffles the genes so each gamete carries a different combination of alleles. Then at fertilisation, a gamete from the mother joins a gamete from the father, combining two different sets at random. The result is offspring that are genetically different from each other and from their parents. This is the main source of variation in sexually reproducing organisms.

Examples in context

Example 1. Healing a cut. When you cut your skin, the cells around the wound divide by mitosis to make identical new skin cells, closing the gap. No variation is needed - the body just needs more of the same cells, which is exactly what mitosis provides.

Example 2. Why siblings differ. Brothers and sisters are not identical because each was made from a different combination of gametes. Meiosis produced gametes with different allele mixes, and fertilisation combined a different pair each time, giving each child a unique genetic make-up.

Try this

Q1. How many cells does meiosis produce? [1 mark]

• Cue. Four.

Q2. What process is used for growth and repair? [1 mark]

• Cue. Mitosis.