Mitosis, meiosis and the cell cycle

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.

Worked examples

Example 1

A bacterium-like cell divides every 20 minutes. Starting from one cell, how many cells are there after 2 hours?

  1. 2 hours = 120 minutes. 120 ÷ 20 = 6 divisions
  2. The number doubles at every division. 2^6 = 64
  3. So there are 64 cells.

Answer: 64

Example 2

A horse has 64 chromosomes in each body cell. How many chromosomes are in one of its sperm cells, and in a fertilised egg?

  1. Sperm cells are made by meiosis, which halves the number. 64 ÷ 2 = 32
  2. The egg also has 32. At fertilisation they join. 32 + 32 = 64
  3. The fertilised egg has the full number again.

Answer: 32 in a sperm cell, 64 in a fertilised egg

Example 3

The diagram shows a cell dividing by mitosis. Which stage is it, and what happens next?

  1. The chromosomes are lined up across the middle of the cell.
  2. So this is metaphase.
  3. Next the spindle fibres pull the two copies of each chromosome apart (anaphase).
  4. Then a nucleus forms at each end and the cell divides into two identical cells.

Answer: metaphase; next, in anaphase, the copies of each chromosome are pulled to opposite ends

Example 4

In a root tip, 200 cells were counted and 10 were in metaphase. The cell cycle lasts 24 hours. Estimate how long metaphase lasts.

  1. Fraction of cells in metaphase: 10 ÷ 200 = 0.05
  2. The cycle in minutes: 24 × 60 = 1440
  3. Multiply. 0.05 × 1440 = 72
  4. So metaphase lasts about 72 minutes.

Answer: 72 minutes