How cancer starts: how a normal cell turns into cancer
DNA copying mistakes: why every new cell carries a few typos
Each time one of your cells splits, it copies 6 billion letters of DNA and checks its work twice. It still gets a few letters wrong, every time, in everyone. Why is that usually fine?
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Fifty years ago, inside one of Rono's pigment cells, one letter changed. The letters are DNA: a very long string written with only four letters, A, C, G and T, kept in the middle of the cell.
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The string holds instructions called genes. Each gene tells the cell how to make one tool it needs, such as a tool for making the dark pigment of the skin.
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Before a cell splits, it copies all of its DNA, so each new cell gets a full set. That is about 6 billion letters, every time.
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The copying is careful. Each new letter is checked as it goes, and a wrong one is removed and replaced. Then a second check goes over the copy and fixes most of what was missed.
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Even so, a few mistakes still get through, about three every time a cell divides. Out of 6 billion letters, that is a very good copy, but never a perfect one.
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A changed letter is called a mutation. Once it is there, every cell copied from that cell carries it too.
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Most mutations land where they change nothing, like a typo in a page that nobody reads. Everyone's cells carry many of them, and the body works just fine.
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For cancer, a mutation matters most when it lands in the wrong place: in one of the few genes that decide when a cell copies. Fifty years ago, in Rono's pigment cell, one landed right there.
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Quick check
Twins at 60.
Show the answer
No. Every copy adds a few new mistakes, and they land in different places in each body. So each twin's cells collect their own mutations.
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Go further
The real names of the two checks.