DNA Polymerase III

Definition

DNA Polymerase III is a crucial enzyme involved in DNA replication, responsible for synthesizing new DNA strands by adding nucleotides to a growing chain during the process of replication. This enzyme works primarily on the leading strand in a continuous manner, while it synthesizes the lagging strand in short segments known as Okazaki fragments. Its high processivity and fidelity make it essential for accurate DNA replication.

5 Must Know Facts For Your Next Test

1. DNA Polymerase III has a high processivity rate, meaning it can add thousands of nucleotides without detaching from the DNA strand.
2. It requires a primer with a free 3'-OH group to initiate DNA synthesis, which is typically provided by RNA primers laid down by primase.
3. The enzyme has proofreading ability, using its 3' to 5' exonuclease activity to remove incorrectly paired nucleotides, ensuring high fidelity in DNA replication.
4. In prokaryotes, DNA Polymerase III is part of a larger complex called the clamp loader that helps in the assembly and function of the polymerase at the replication fork.
5. While primarily known for its role in elongating new DNA strands, DNA Polymerase III also plays a role in the repair of damaged DNA during cellular processes.

Review Questions

• How does DNA Polymerase III differ in its function on the leading strand compared to the lagging strand?
  • DNA Polymerase III synthesizes the leading strand continuously in the 5' to 3' direction, allowing for uninterrupted elongation. In contrast, on the lagging strand, it synthesizes short segments called Okazaki fragments due to the antiparallel nature of the DNA strands. This requires multiple primers and periodic restarting of synthesis, which leads to the fragmented nature of lagging strand synthesis.
• What mechanisms ensure that DNA Polymerase III maintains high fidelity during DNA replication?
  • DNA Polymerase III ensures high fidelity through its proofreading capabilities. It has a built-in 3' to 5' exonuclease activity that allows it to recognize and remove incorrectly paired nucleotides. This proofreading occurs immediately after nucleotide incorporation, allowing the enzyme to correct mistakes before continuing with further synthesis. Additionally, the enzyme's high processivity minimizes the chances of error over long stretches of DNA.
• Evaluate the importance of DNA Polymerase III in both DNA replication and repair processes within prokaryotic cells.
  • DNA Polymerase III is vital not only for accurate and efficient DNA replication but also plays a significant role in repair mechanisms within prokaryotic cells. Its ability to synthesize new strands quickly and with high fidelity allows cells to maintain genomic integrity during replication. Furthermore, when damaged DNA occurs, this polymerase can participate in repair pathways, ensuring that genetic information is accurately preserved across generations. Its multifunctionality highlights its essential role in cellular health and stability.

Related terms

Okazaki Fragments: Short segments of DNA synthesized on the lagging strand during DNA replication, which are later joined together by DNA ligase.

Leading Strand: The DNA strand that is synthesized continuously in the 5' to 3' direction during DNA replication.

DNA Ligase: An enzyme that joins Okazaki fragments together on the lagging strand by forming covalent bonds between adjacent nucleotides.