Incorrect nucleotides are sometimes incorporated into DNA during the process of replication. In the sequences shown here, the template strand is shown on top and the replicating daughter strand at the bottom. The nucleotide shown in bold is incorrect in that its base does not undergo pairing with the base in the template strand. In the situation shown in (A) the proofreading function of DNA polymerase removes the incorrect nucleotide from the daughter strand and replaces it with the correct one, but in situation (B) the polymerase has moved on and the incorrect nucleotide cannot be repaired. Can you explain why the error in situation (B) cannot be fixed by DNA polymerase?
(A) $\begin{array}{l}3 \text { '-GAAACTGCCATG-5 ' } \\5 \text { '-CTTTGG }\end{array}$ (B) $\begin{array}{l}3 \text { '-GAAACTGCCATG-5 ' } \\5 \text { '-CTTTGGC }\end{array}$

Question

Incorrect nucleotides are sometimes incorporated into DNA during the process of replication. In the sequences shown here, the template strand is shown on top and the replicating daughter strand at the bottom. The nucleotide shown in bold is incorrect in that its base does not undergo pairing with the base in the template strand. In the situation shown in (A) the proofreading function of DNA polymerase removes the incorrect nucleotide from the daughter strand and replaces it with the correct one, but in situation (B) the polymerase has moved on and the incorrect nucleotide cannot be repaired. Can you explain why the error in situation (B) cannot be fixed by DNA polymerase?
(A)

\begin{array}{l}3 \text { '-GAAACTGCCATG-5 ' } \\5 \text { '-CTTTGG }\end{array}

(B)

\begin{array}{l}3 \text { '-GAAACTGCCATG-5 ' } \\5 \text { '-CTTTGGC }\end{array}