Question

Select the incorrect statement regarding DNA replication.

• A. Leading strand is formed in $5^{\prime }\to 3^{\prime }$ direction
• B. Okazaki fragments are formed in $5^{\prime }\to 3^{\prime }$ direction
• C. DNA polymerase catalyses polymerisation in $5^{\prime }\to 3^{\prime }$ direction
• D. DNA polymerase catalyses polymerisation in $3^{\prime }\to 5^{\prime }$ direction

Answer 1

Answer: (D)

A. The leading strand is the DNA strand that DNA polymerase constructs in the 5' → 3' direction. This strand of DNA is made in a continuous manner, moving as the replication fork grows.DNA polymerase is then able to use the free 3'-OH group on the RNA primer to make DNA in the 5' → 3' direction.

B. On the lagging strand, DNA polymerase is moving away from the replication fork, meaning copying is discontinuous. As DNA polymerase is moving away from a helicase, it must constantly return to copy newly separated stretches of DNA. This means the lagging strand is copied as a series of short fragments (Okazaki fragments), each preceded by a primer. The primers are replaced with DNA bases and the fragments joined together by a combination of DNA pol I and DNA ligase

C. DNA polymerase adds nucleotides to the 3’ end of a primer, extending the new chain in a 5’ → 3’ direction.

D. When an incorrect base pair is recognized, DNA polymerase moves backward by one base pair of DNA. The 3'–5' exonuclease activity of the enzyme allows the incorrect base pair to be excised (this activity is known as proofreading).

So, the incorrect statement is "D".