Mitosis, meiosis and the cell cycle · Year 10
The nucleus of every cell holds chromosomes made of DNA. Body cells divide by mitosis to make new, identical cells for growth and repair. Sex cells (gametes) are made by a different kind of division, meiosis, which halves the number of chromosomes, so that fertilisation can bring the number back to normal.
Key facts
- A chromosome is a long molecule of DNA carrying many genes. In body cells chromosomes come in pairs: humans have 23 pairs, 46 chromosomes in all.
- The cell cycle: the cell grows and makes more ribosomes and mitochondria; the DNA replicates, so each chromosome has two copies; in mitosis one set of chromosomes is pulled to each end of the cell; then the cytoplasm and cell membrane divide, making two identical cells.
- Mitosis is used for growth, repair and asexual reproduction. The new cells are genetically identical to the parent cell.
- The stages of mitosis: prophase (the chromosomes condense), metaphase (they line up across the middle), anaphase (the two copies of each chromosome are pulled to opposite ends) and telophase (a new nucleus forms at each end). Then the cell splits in two.
- Meiosis happens in the ovaries and testes. The DNA is copied, then the cell divides twice, making four gametes, each with one chromosome from each pair (23 in humans). Every gamete is genetically different.
- At fertilisation a sperm and an egg join, so the fertilised egg has the full number of chromosomes again. It then divides by mitosis to form an embryo.
- The time a cell spends in a stage is proportional to how many cells are seen in it: time = (cells in the stage ÷ total cells) × length of the cycle. Mitotic index = cells in mitosis ÷ total cells.