How do cells grow and divide?The stages in a cell's life are called the cell cycle. This works a bit like a washing machine cycle - each stage must be finished completely before the next one is allowed to start. Instead of wash, rinse, spin and drain, the cell cycle is made up of stages in which the cell grows and rests, copies its DNA, and divides into two new cells.
How long does it take?
How is DNA copied?
How do cells divide?
What happens during mitosis?
What is meiosis?
The 46 chromosomes in a human cell consist of 23 pairs - one set inherited from each parent. When cells divide to make egg or sperm cells they receive only one chromosome from each of the pairs. The process of meiosis means the new cells usually end up with 23 chromosomes. When an egg and sperm join together at fertilisation, the new cell they make will have the normal number of 46 chromosomes again - the complete set needed for a making a new person.
How does meiosis work?
What happens when meiosis goes wrong?
Occasionally meiosis goes wrong. For example, Down's syndrome is usually the result of an error at meiosis when an egg or sperm cell gets an extra chromosome 21. So people with Down's syndrome have one too many chromosomes. Errors at meiosis occur more often in the egg cell as women get older, so the chances of a woman having a baby with Down's syndrome rises from about one in 1300 for a woman aged 25, to one in 30 by age 40.
Why is reshuffling DNA important?
When a cell divides to make eggs or sperm (meiosis), the two chromosomes in each pair become closely entwined around each other. During this time, they swap parts of themselves. This process, called recombination or 'crossing over', shuffles the genetic information - two stretches of DNA that were near each other on one chromosome may end up in different egg or sperm cells. Recombination ensures that every individual contains a unique set of genetic information.