Question

How does DNA polymerase discriminate between the insertion of correct and incorrect base pairs as DNA replication proceeds?

a. Incorrect nucleotides repel each other because of their charges.
b. Distances between atoms and bond angles give incorrect base pairs the same geometry as correct base pairs.
c. Incorrect base pairs are attracted to tightly to each other and the enzyme.
d. If the new base pair exhibits improper geometry (size and shape), the polymerase active site cannot adopt the conformation required for catalysis and the incorrect nucleotide is not incorporated.

Answer 1

DNA polymerase discriminates between correct and incorrect base pairs through the proofreading function where it checks the geometry of the inserted base pair and uses a 3' exonuclease activity to remove incorrect nucleotides. The mismatch repair and nucleotide excision repair systems also play key roles in correcting replication errors that DNA polymerase fails to proofread. The correct option is c. Incorrect base pairs are attracted to tightly to each other and the enzyme.

Step-by-step explanation:

DNA polymerase is responsible for adding new nucleotides during DNA replication and also for proofreading to ensure accuracy in the process. The enzyme discriminates between correct and incorrect base pairs by checking the geometry of the newly added base pair. If the base pair exhibits improper geometry, which refers to the size and shape of the nucleotides, the active site of the polymerase cannot properly fit and perform the catalysis required for the incorporation of the nucleotide. Therefore, the incorrect nucleotide is not added to the growing DNA strand. This incorrect base pairing triggers the DNA polymerase to perform a proofreading function, where the 3' exonuclease activity of the enzyme removes the incorrectly paired base. After the excision, the correct nucleotide can be inserted.

There are other repairing mechanisms such as mismatch repair and nucleotide excision repair, which function to correct errors that DNA polymerase does not catch during replication. These repair systems are essential to prevent mutations and ensure genomic stability. The correct option is c. Incorrect base pairs are attracted to tightly to each other and the enzyme.