Meiosis

Gametes, the sex cells, are made by meiosis in the ovaries and testes. The DNA is copied, then the cell divides twice, making four gametes with one chromosome from each pair: half the normal number. Each gamete gets a different mix of chromosomes, so every gamete is genetically different. At fertilisation two gametes join and the full number of chromosomes is restored.

Key facts

  • 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.
  • Meiosis makes variation: each pair of chromosomes is shared out independently, so the gametes all get different mixes.

Worked examples

Example 1

Which of these is meiosis used for?

  1. Meiosis makes gametes: sperm, eggs and pollen.
  2. So the answer is: making sperm cells.

Answer: making sperm cells

Example 2

Which kind of cell division is this: it makes four daughter cells?

  1. Meiosis: it makes four daughter cells.

Answer: meiosis

Example 3

A body cell of a dog has 78 chromosomes. How many chromosomes are in each of its gametes?

  1. Meiosis halves the number of chromosomes.
  2. 78 ÷ 2 = 39.

Answer: 39

Example 4

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 5

Why must gametes have half the normal number of chromosomes?

  1. Two gametes join at fertilisation.
  2. Each brings half, so the new cell has the full number.

Answer: so that the fertilised egg has the full number of chromosomes

Example 6

Why does meiosis make genetically different gametes?

  1. During meiosis, each gamete gets one chromosome from each pair, in a random mix.
  2. So every gamete is different, which causes variation.

Answer: the chromosomes are shuffled, so each gamete gets a different mix

Example 7

A fruit fly has 8 chromosomes (4 pairs) in each body cell. Ignoring crossing over, how many different combinations of chromosomes can its gametes have?

  1. Independent assortment: each pair can go either way.
  2. 2^4 = 16 combinations.

Answer: 16

Example 8

A pea plant has 14 chromosomes (7 pairs) in each body cell. Ignoring crossing over, how many different combinations of chromosomes can its gametes have?

  1. Independent assortment: each pair can go either way.
  2. 2^7 = 128 combinations.

Answer: 128