Meiosis · Grade 9
In Grade 9 you learn: meiosis; chromosome numbers in gametes and at fertilization.
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 fertilization 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 fertilization a sperm and an egg join, so the fertilized egg has the full number of chromosomes again. It then divides by mitosis to form an embryo.
Worked examples
Example 1
Which of these is meiosis used for?
Answer: making sperm cells
Example 2
Which kind of cell division is this: 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?
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 fertilized egg?
Answer: 32 in a sperm cell, 64 in a fertilized egg
Example 5
Why must gametes have half the normal number of chromosomes?
Answer: so that the fertilized egg has the full number of chromosomes
Example 6
Why does meiosis make genetically different gametes?
Answer: the chromosomes are shuffled, so each gamete gets a different mix