Cell division (CCEA)

All cells have a cell cycle, where they grow, copy their DNA, and spilt into new cells during cell division.

There are two types of cell division – mitosis and meiosis.

Mitosis produces two new cells (daughter cells) that are genetically identical (clones) to the parent cell and each other.

Mitosis allows organisms to:

• grow

• replace worn out cells and

• repair damaged tissue

Meiosis only produces gametes (sex cells) so occurs in the testes and ovaries. All sex cells contain half the number of chromosomes.

During meiosis, a diploid (2n) cell undergoes two divisions to reduce its chromosome number, resulting in four haploid (n) gametes with half the number of chromosomes.

Humans have 23 chromosomes in their sex cells.

The four gametes produced in meiosis are genetically different.

The process of independent assortment leads to all gametes being different.

Independent assortment and the random nature of fertilisation lead to variation in living organisms – no two organisms are the same (apart from identical twins).

During fertilisation the haploid nucleus of a sperm cell and the haploid nucleus of an egg cell fuse together to form a diploid zygote.