Meiosis – sexual reproduction (Edexcel GCSE Biology Combined Science): Revision Notes
Meiosis – sexual reproduction
Meiosis is the formation of four non-identical cells from one cell. Cells in the reproductive organs divide by meiosis to form gametes. Gametes only have one copy of each chromosome.
- The cell makes copies of its chromosomes, so it has double the amount of genetic information.
- The cell divides into two cells, each with half the amount of chromosomes (46).
- The cell divides again producing four cells, each with a quarter the amount of chromosomes (23).
- These cells are called gametes and they are all genetically different from each other because the chromosomes are shuffled during the process, resulting in random chromosomes ending up in each of the four cells. These gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes, the normal number.
- This cell divides by mitosis to produce many copies.
- More and more cells are produced, and an embryo forms.
- The cells begin to take on different roles after this stage (differentiation).
Flowering plants the male gametes are found in the pollen and female gametes found in ovaries at bottom of the stigma.
The process:
Before the cell divides, the DNA replicates.
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First division: Chromosomes line up in pairs in the centre of the cell.
The pairs are pulled apart so each new cell has one copy of each chromosome, so there is genetic variation.
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Second division: Chromosomes line up in the centre of the cell, and the arms of the chromosomes are pulled apart.
You get 4 haploid daughter cells (gametes), which have a single set of each chromosome and are genetically different.