Question

DNA polymerase III can only add nucleotides to an existing chain, so _________________ is required.

1) an RNA primer
2) DNA polymerase I
3) helicase
4) a DNA primer

Answer 1

DNA polymerase III requires an RNA primer synthesized by RNA primase to initiate DNA strand elongation, as it cannot add nucleotides without a free 3'-OH group on an existing nucleic acid strand.

Step-by-step explanation:

DNA polymerase III can only add nucleotides to an existing chain, so an RNA primer is required. During DNA replication, DNA polymerase III requires a free 3'-OH group to add nucleotides by forming a phosphodiester bond between the 3'-OH end and the 5' phosphate of the next nucleotide. Due to its inability to synthesize a new strand from scratch, replication necessitates the synthesis of a short RNA primer by RNA primase. This primer provides the necessary 3'-OH group, allowing DNA polymerase III to extend the new strand.

Note that while DNA polymerase I is involved in DNA replication as well, it plays a role later in the process by removing RNA primers and replacing them with DNA nucleotides.