A thymine base in an incoming nucleotide spontaneously tautomerizes into the enol form just when it is being added to the growing nucleotide chain during DNA replication. This tautomeric change is not stable, and the T returns to the keto form before the next round of DNA replication. What nucleotide change will occur? For the answers below the nucleotide to the left is on top, i. e. A-T would be A on the top and T on the bottom.

A-T to C-G

A-T to G-C

O C-G to A-T

OC-G to T-A

The correct answer is A-T to G-C

Explanation:

Thymine remains stable in keto form so when it tautomerism in enol form it behaves like cytosine and does base pairing with guanine. As the tautomeric form is not stable the thymine converts back in its keto form.

So if thymine is present in enol form during replication it will base pair with the guanine nucleotide making G-T base pair. So guanine comes in place of adenine. Then in subsequent replication, this guanine will base pair with cytosine (G-C).

Therefore it is the substitution of AT base pair to GC base pair in case of thymine tautomerization therefore the correct answer is A-T to G-C.