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DNA replication is more accurate than transcription (or RNA replication) because mechanisms exist for proof-reading and repair of DNA, but not for RNA. Consider a segment of a DNA strand, AGTC. Its complement is GACT. Now suppose its complement is mutated to TACT — the DNA repair system will replace the wrong T by G. Why isn’t A in the original strand replaced by C? How does the system determine that the first strand is correct and its complement is incorrect?

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I've edited this question, removing anthropomorphic "know" and the implication that the DNA, rather than enzymes, did the repair. Although the question was answered some time ago, this makes it more useful for indexing and reference. – David Jun 25 at 12:07
up vote 3 down vote accepted

The reason the fact it isn't realistic is important. DNA repair machinery works by repairing common errors that occur due to common mutational pathways. The enzymes are specific for this, for example one particular enzyme targets mutations caused by UV and itself is activated by sunlight thus it's specificity makes it repair the correct chain. Also during replication, newly synthesised DNA lacks methylation. The repair enzymes thus know which chain is correct.

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Your specific scenario is not realistic, thymine is a pyrimidine base while guanine is a purine base. They are very different chemically and can't spontaneously convert into each other. So this is a type of damage that only occurs during the replication of DNA.

The typical damage that occurs and can be repaired is oxidation, methylation and hydrolysis of DNA. This kind of damage results in modified or missing bases and can be detected due to that.

You might also be interested in why thymine is used in DNA instead of uracil, as uracil can be easily damaged in a way that would not be detectable.

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Perhaps it is not realistic, but talk theoretically, if this kind of error happened, can it get auto repaired? – Popopo Jul 5 '13 at 17:12

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