AQA GCSE Meiosis(Biology)

Meiosis

Meiosis is a type of cell division that produces gametes (sex cells).

Meiosis occurs in the reproductive organs:

–Testes in men produce sperm

–Ovaries in females produce eggs

Meiosis is known as reduction division, it will halve the chromosome number.

Cell type	Chromosome number
Human body cell	46
Human gamete	23

Process of meiosis

Stage 1 DNA is copied

Stage 2 Pairs of chromosomes line up along centre

Stage 3 cell divides in the first division, with one chromosome of each pair going into a different daughter cell.

Stage 4 cell divides in the second division, with each daughter cell receiving one chromatid out of each chromosome.

Overall there are two divisions and four gametes produced. Each with half the normal number of chromosomes.

In the example shown above the cell just before the cell 1st division has 4 chromosomes, but the gamete daughter cells produced at the end of the 2nd division only have 2 chromosomes each. So, the chromosome number has halved.

Overall: Meiosis produces four genetically different haploid gametes.

Haploid means half the normal number of chromosomes.

Genetic Variation in Gametes

Gametes produced by meiosis are genetically different.This leads to variation in offspring.

During meiosis, chromosomes are shared out randomly into gametes. This means each gamete gets a different mixture of chromosomes.

Imagine a pack of cards being shuffled. Each time you deal cards, you get a different hand. In the same way, chromosomes are shuffled when gametes form, so each gamete is different.

Fertilisation Restores Chromosome Number

Fertilisation is the fusion of the gametes. Each gamete is haploid (23 chromosomes). When they fuse to form a zygote, the diploid number of 46 chromosomes is restored.

Which gametes fuse is random during fertilisation, which increases variation.