In DNA repair, how is it determined which strand contains the error? 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?

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.