Every time one of these cells divides, its DNA must be copied and, despite the gargantuan task of copying 3 billion letters, this process is highly resistant to copying errors. Led by the enzyme DNA polymerase, the chemical machine that does the copying is incredibly accurate, on average making only one mistake in a billion letters. To put that into context, it is like copying out each of the 775,000 words in the Bible around 280 times, and making just one mistake.
