gene mutation occurs on two strands of DNA or just one strand? Suppose we have one strand with 3'-AAA-5' and the other strand is 5'-TTT-3'. So when point mutation occurs, does it happen to two strands at the same time? e.g. 3'-AAA-5' becomes 3'-AGA-5' and 5'-TTT-3' becomes 5'-TCT-3'? Or the mutation can occur with only one strand? e.g. only 3'-AAA-5' becomes 3'-AGA-5'? but in this case, A would pair up with G. When transcription starts, the 3'-5' strand is transcribed, if the mutation does no occur in this strand, does it mean this mutation wouldn't affect any downstream products? Is it same for indel?

A "mutation" is a permanent alteration of the genetic composition of an individual. To obey this definition, a base change in one strand must be accompanied by a corresponding change in the opposite strand. Only in this way a mutation can be transmitted to the offspring.

The situation where only one of the strands has undergone a base change constitutes a pre-mutational DNA damage, as the double strand will show a deformation at that site. The enzymes of DNA repair mechanisms recognize and correct many such "deformations", but sometimes they do not know which was the original base, and introduce a change that become fixed in the DNA sequence.