You are carrying out experiments to study DNA replication in a cell line derived from a breast cancer tumor. You discover that the DNA polymerase in these cells incorporates the incorrect nucleotide into the elongating strand approximately once every thousand bases. These errors suggest that the "proof-reading" capacity of the polymerase is defective. This would be indicative of the following processes not being correctly carried out by the defective polymerase?
A) 3′ to 5′ exonuclease activity
B) 5′ to 3′ exonuclease activity
C) insertion of nucleotides in 5′ to 3′ orientation
D) synthesis of ribonucleotide polymers
E) topoisomerase-mediated cleavage of the DNA backbone

Question

Quizplus · Accepted Answer

The 3′ to 5′ exonuclease activity is responsible for the proofreading function of DNA polymerase, allowing it to remove incorrectly incorporated nucleotides. A defect in this activity would lead to increased errors during DNA replication. The main enzymatic activity of DNA polymerases is the 5′->3′ synthetic activity. However, DNA polymerases possess two additional activities of importance for both replication and repair. These additional activities include a 5′->3′ exonuclease function and a 3′->5′ exonuclease function. The 3′->5′ exonuclease function is utilized during replication to allow DNA polymerase to remove mismatched bases and is referred to as the proof-reading activity of DNA polymerase. It is possible (but rare) for DNA polymerases to incorporate an incorrect base during replication. These mismatched bases are recognized by the polymerase immediately due to the lack of Watson-Crick base-pairing. The mismatched base is then removed by the 3′→5′ exonuclease activity and the correct base inserted prior to progression of replication.

You Are Carrying Out Experiments to Study DNA Replication in a Cell