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Your cells copy three billion letters of DNA and proofread the work as they go along.

Biology Facts, sourced to NCBI Bookshelf: Molecular Biology of the Cell.

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The enzyme that builds a new strand checks the letter it just added and, if the pairing is wrong, backs up and removes it before continuing. That single step improves the accuracy a hundredfold, and a separate repair system sweeps the finished strand afterward for what slipped through. The final rate is roughly one error per billion letters copied. Without the proofreading it would be closer to one in ten thousand, which nothing alive could carry.

The claim comes from NCBI Bookshelf: Molecular Biology of the Cell.

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