Hi! I'm trying to make sense of this illustration (from the textbook Biological Science by Scott Freeman).
The general question is: How do point mutations arise from mistakes in DNA replication?
If you don't mind, however, I'd like to explain how I interpret the illustration so you can see the confusion. As the original molecule of DNA is replicated (in gray), a mistake occurs in the synthesis of the bottom strand in the new molecule (the noncomplimentary bases G and T have been paired together). Now, it seems like a second replication is required for the mutation to arise: the defective molecule is replicated resulting in two new molecules, one free of mistakes (because it takes as its template the top strand) and one "wrong" where the mutation is present.
But my doubt is, wouldn't the middle molecule already constitute a mutation? If a mRNA were to transcribe that sequence, the codon is already different from the original molecule. Must a DNA molecule be replicated two times for a mutation to arise (where the first time a mistake is made and a second where such mistake is, let's say, "consolidated")?
Thank you very much in advance.