Question

DNA polymerases have 3'-->5' exonuclease activity.

1. Explain what 3'-->5' exonuclease activity is.

2. What triggers this exonuclease activity?

Answer 1

1. DNA is composed of two strands running in antiparallel orientation, with directionality 5′ to 3′ and other with directionality 3′ to 5′. DNA polymerase adds nucleotides to the 3′ end of growing DNA strand. If by chance an incorrect base is added to the growing chain, the DNA polymerase recognizes the incorrect base pair and moves backward by one base pair. The 3'-->5' exonuclease activity of the polymerase enables the excision of the incorrect base pair. this maintains the fidelity of DNA polymerase.

2. Addition of incorrect base by the DNA polymerase leads to alteration in shape of the DNA due to incorrect base pairing. this loss of interaction triggers the exonuclease activity of polymerase. The template primer pair leaves the polymerase domain of the enzyme , migrating to the exonuclease domain and the incorrect base is excised.