Meiosis (Cambridge (CIE) IGCSE Biology): Revision Note

Meiosis: Extended

Extended Tier Only

• Meiosis is a type of nuclear division that gives rise to cells that are genetically different

• Meiosis is involved in the production of gametes (sex cells)

• The number of chromosomes must be halved when the gametes are formed

• Otherwise there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilised egg)

• This halving occurs during meiosis, and so it is described as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells

• Meiosis starts with chromosomes doubling themselves (as in mitosis (opens in a new tab)) and lining up in the centre of the cell

• After this has happened the cells divide twice so that only one copy of each chromosome passes to each daughter cell

• These daughter cells form the gametes

  • Gametes are described as being haploid - having half the normal number of chromosomes

• Because there is a double division, each cell division by meiosis produces four haploid cells 

What is the process of meiosis?

• Each chromosome makes identical copies of itself (forming X-shaped chromosomes)

• First division: chromosomes pair up along the centre of the cell, recombination occurs and then cell fibres will pull the pairs apart, each new cell will have one of each recombinant chromosome pair

• Second division: chromosomes will line up along the centre of the cell, cell fibres will pull them apart (as with mitosis)

• A total of four haploid daughter cells will be produced

Why is meiosis important?

• Production of gametes e.g. sperm cells and egg cells, pollen grains and ovum

• Increases genetic variation of offspring

• Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation, each offspring will be different from any others

Differences between mitosis and meiosis