Question

How does DNA polymerase III function as the primary replication polymerase, elongating a new nucleotide strand from the 3′-OH of the primer, and what role does DNA polymerase I play in removing and replacing the RNA nucleotides of the primers with DNA nucleotides?

Answer 1

DNA polymerase III is the primary enzyme that synthesizes new DNA strands by adding nucleotides to the RNA primer on the leading and lagging strands. DNA polymerase I removes RNA primers and replaces them with DNA nucleotides. DNA ligase seals the gaps, forming a continuous DNA molecule.

Step-by-step explanation:

Function of DNA Polymerase III and DNA Polymerase I in DNA Replication

DNA polymerase III is the primary enzyme responsible for the elongation of new DNA strands during replication. It adds nucleotides to the 3′-OH end of the primer, which is a short RNA sequence synthesized by primase, allowing for the construction of a new DNA strand in the 5' to 3' direction.

On the leading strand, DNA polymerase III synthesizes DNA continuously, while on the lagging strand, Okazaki fragments are formed and subsequently joined. DNA polymerase I plays a critical role in removing the RNA primers and replacing them with DNA nucleotides. After the RNA nucleotides of the primers are removed, DNA polymerase I fills in the gaps with DNA nucleotides, and DNA ligase seals the gaps between Okazaki fragments, thereby creating a continuous DNA strand. This process ensures the replication fork can progress smoothly and that the DNA is accurately replicated.

Answer 2

DNA polymerase III is essential for DNA strand elongation, adding nucleotides in the 5' to 3' direction, using an RNA primer. DNA polymerase I removes the RNA primers and replaces them with DNA nucleotides. DNA ligase then seals the nicks, finalizing the replication process.

Step-by-step explanation:

Understanding DNA Replication: The Roles of DNA Polymerase III and I

DNA polymerase III is a crucial enzyme in DNA replication, primarily responsible for the elongation of a new DNA strand. It functions by adding nucleotide triphosphates (dNTPs) to the 3'-OH end of a previously laid down RNA primer. This primer is necessary because DNA polymerase III is unable to initiate synthesis on its own; it can only add to an existing nucleotide chain. In essence, DNA polymerase III reads the template DNA strand and adds complementary nucleotides in the direction of 5' to 3' to synthesize the new strand.

On the leading strand, synthesis is continuous, as the direction of elongation aligns with the unwinding of the DNA double helix. In contrast, the lagging strand is synthesized in short segments known as Okazaki fragments, requiring multiple primers and thus more complex orchestration. Once a fragment is synthesized, DNA polymerase I plays a pivotal role in primer removal and replacement. It has exonuclease activity, which allows it to remove the RNA nucleotides of the primer and fill the resulting gap with DNA nucleotides.

The replication process is finely tuned and remarkably quick, with DNA polymerase III adding around 1000 nucleotides per second. While DNA polymerase I eliminates RNA primers and filling gaps, DNA ligase works to seal nicks in the DNA backbone, joining Okazaki fragments together to form a continuous DNA strand. Without the concerted actions of these enzymes, the replication process would not be as accurate or efficient, leading to potential genetic errors and instability.