Meiosis (AQA GCSE Biology: Combined Science)

• Cells in reproductive organs divide by meiosis to form 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 (fertilized 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
• It starts with chromosomes doubling themselves as in mitosis 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 gamete
• We describe gametes as being haploid – having half the normal number of chromosomes
• Because of this double division, meiosis produces four haploid cells

The process of cell division by meiosis to produce haploid gamete cells

Process

• Each chromosome is duplicated (makes identical copies of itself), forming X-shaped chromosomes
• First division: the chromosome pairs line up along the centre of the cell and are then pulled apart so that each new cell only has one copy of each chromosome
• Second division: the chromosomes line up along the centre of the cell and the arms of the chromosomes are pulled apart
• A total of four haploid daughter cells will be produced

Importance

• Produces gametes eg. sperm cells and egg cells in animals, pollen grains and ovum cells in plants
• 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

• Gametes join at fertilisation to restore the normal number of chromosomes
• When the male and female gametes fuse, they become a zygote (fertilised egg cell)
• This contains the full number of chromosomes, half of which came from the male gamete and half from the female gamete
• The zygote divides by mitosis to form two new cells, which then continue to divide and after a few days form an embryo
• Cell division continues and eventually many of the new cells produced become specialised (the cells differentiate) to perform particular functions and form all the body tissues of the offspring
• The process of cells becoming specialised is known as cell differentiation