Question

A scientist randomly mutates the DNA of a bacterium. She then sequences the bacterium’s daughter cells, and finds that the daughters have many errors in their replicated DNA. The parent bacterium likely acquired a mutation in which enzyme?

1) DNA ligase

2) Primase

3) DNA pol II

4) DNA pol I

Answer 1

4) DNA pol I should be the answer to this question because we need to find the enzyme which is responsible for MANY errors in replication out of the 4 :-

DNA ligase plays a role as a glue for DNA fragments leading to catalysis of the formation of the phosphodiester bond therefore, improper functioning of this enzyme doesn't cause the mutation in multiple nucleotides during replication

Primase is responsible for synthesis of the RNA primer during DNA replication since DNA polymerases need an existing strand to attach to this. Thus, this cannot be responsible for many errors.

DNA polymerase I and II are both prokaryotic polymerase enzymes with low processivity. Though the function is still debated, DNA polymerase II is mostly used for repair and just as a backup enzyme in DNA replication. DNA polymerase I is used both in repair and replication as it is used to connect the okazaki fragments formed after RNA primer removal. Here there is a debate but we can solve this by knowing that DNA polymerase I also performs a proofreading function as it goes over the lagging strand in DNA replication. If mutated, this proofreading function will fail. DNA pol I has both 5' -> 3' and 3'-5' exonuclease activity but DNA pol II has 3' -> 5' exonuclease activity.

Therefore, by knowing the functions we can conclude that erroneous nucleotides in the daughter cells DNA is most probably caused by mutation of DNA polymerase I.