Question

If a base is misread, and in consequence, the wrong nucleotide is written in the nascent DNA strand, which of the following mechanisms could correct the error? Select all that apply Select one or more: a. Excision repair by DNA polymerase III b. DNA proofreading by polymerase III c. Excision repair by DNA polymerase d. Mismatch repair by DNA polymerase e. Mismatch repair by DNA polymerase III

Answer 1

If a base is misread and wrong base is inserted in new strand, it can repaired by different processes.

First is the proofreading activity of DNA polymerase III, which is 3'-5' exonuclease activity of this DNA polymerase. If any mistake made during polymerization by DNA polymerase and wrong base is inserted by it. It will be repaired by 3'-5' proofreading activity of DNA polymerase III. Hence , option b) is the correct answer.

Excision repair is used to repair damaged nucleotide bases. Base excision repair is used to repair abnormal bases like uracil, hypoxanthine, xanthine resulting from deamination or alkylation processes. Nucleotide excision repair is used to repair pyrimidine dimers. So excision repair doesn't repair wrong base in the nacent DNA, it only repairs abnormal bases or modified bases. Hence, options a) and c) are incorrect answers.

Mismatch repair is used to repair the mismatched bases or nucleotides in the nacent DNA molecule. It distinguish parental DNA strand from daughter/ new strand and repair it. This process of repair involve many proteins and enzymes which include MutH, MutS, MutL, Single strand binding proteins (SSB), exonuclease I, exonuclease VII, exonuclease X, RecJ nuclease, DNA helicase II, DNA ligase and DNA polymerase. The DNA polymerase which is involved in mismatch repair system is DNA polymerase III, not DNA polymerase I. Hence option d) is the incorrect answer and option e) is the correct answer.

So conclusion is that only options b) and e) are correct answers.