Who am I?

There are many different sorts of cells but all have roughly the same cell cycle. However, the time taken to complete it can vary enormously. The cell cycle of a fly embryo cell takes just eight minutes while a human liver cell cycle can last longer than a year. Some cells can withdraw from the cell cycle and rest before re-entering it. Many cells in growing embryos often skip the resting stages altogether. Cells that have stopped dividing, like brain cells, never re-enter the cell cycle.

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Almost every one of your cells has a set of genes, made of DNA, which are instructions for making proteins. The 6000 million letters of DNA code in your cells is coiled up as 46 bundles - chromosomes. Before a cell can divide, it must unravel its chromosomes and copy all its DNA, so that each new cell will get a complete copy. On average, this whole process takes just seven hours.

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Once it has copied all its DNA, a cell normally divides into two new cells. This process is called mitosis. Each new cell gets a complete copy of all the DNA, bundled up as 46 chromosomes. Cells that are making egg or sperm cells must divide in a different way. Each egg and sperm cell ends up with only half the amount of DNA present in the original cell, bundled up as 23 chromosomes. This special way of dividing is called meiosis.

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Once the cell has copied its DNA, just before it divides, the 46 chromosomes - usually long strings in the nucleus - coil up into compact bundles. The edge of the nucleus then dissolves, and temporary protein scaffolding appears in the cell: the spindle apparatus. The chromosomes use this to line up in the centre of the cell. The chromosomes then split and one of each duplicate chromosome moves to the opposite end of the cell. Finally, the cell narrows in the middle and divides.

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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.

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Cells that divide by meiosis to make the egg and the sperm actually divide twice. First, the cell copies the DNA in all 46 chromosomes, and the copies of the same chromosome pair up. One chromosome from each pair moves to each end of the cell. The cell then splits into two: each new cell with 23 chromosomes. These new cells divide again, but this time each replicated chromosome splits down the middle, as in mitosis. The result: four cells, each with 23 chromosomes.

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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.

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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.

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