Amber Dance writes:

You began when egg and sperm met, and the DNA from your biological parents teamed up. Your first cell began copying its newly melded genome and dividing to build a body.

And almost immediately, genetic mistakes started to accrue.

“That process of accumulating errors across your genome goes on throughout life,” says Phil H. Jones, a cancer biologist at the Wellcome Sanger Institute in Hinxton, England.

Scientists have long known that DNA-copying systems make the occasional blunder — that’s how cancers often start — but only in recent years has technology been sensitive enough to catalog every genetic booboo. And it’s revealed we’re riddled with errors. Every human being is a vast mosaic of cells that are mostly identical, but different here or there, from one cell or group of cells to the next.

Cellular genomes might differ by a single genetic letter in one spot, by a larger lost chromosome chunk in another. By middle age, each body cell probably has about a thousand genetic typos, estimates Michael Lodato, a molecular biologist at the University of Massachusetts Chan Medical School in Worcester.

These mutations — whether in blood, skin or brain — rack up even though the cell’s DNA-copying machinery is exceptionally accurate, and even though cells possess excellent repair mechanisms. Since the adult body contains around 30 trillion cells, with some 4 million of them dividing every second, even rare mistakes build up over time. (Errors are far fewer in cells that give rise to eggs and sperm; the body appears to expend more effort and energy in keeping mutations out of reproductive tissues so that pristine DNA is passed to future generations.)

“The minor miracle is, we all keep going so well,” Jones says. [Continue reading…]