DNA replication machinery and repair mechanisms tested on USMLE Step 1.
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- Which enzyme unwinds the DNA double helix ahead of the replication fork?
- Helicase
- Which enzyme relieves positive supercoiling generated ahead of the replication fork?
- Topoisomerase (DNA gyrase in bacteria)
- Fluoroquinolone antibiotics inhibit which bacterial enzyme?
- DNA topoisomerase II (DNA gyrase)
- What protein coats single-stranded DNA at the replication fork to prevent it from reannealing?
- Single-strand binding (SSB) protein
- Which enzyme synthesizes the short RNA primer needed to begin DNA synthesis?
- Primase
- DNA polymerases synthesize new DNA strands only in the ____ direction.
- 5' to 3'
- Which strand of DNA is synthesized continuously during replication?
- The leading strand
- Which strand of DNA is synthesized discontinuously as short fragments during replication?
- The lagging strand (as Okazaki fragments)
- What is the main replicative DNA polymerase in bacteria?
- DNA polymerase III
- Which enzymatic activity of DNA polymerase III corrects mismatched nucleotides during synthesis?
- 3' to 5' exonuclease (proofreading) activity
- Which bacterial DNA polymerase removes RNA primers and fills the resulting gaps with DNA?
- DNA polymerase I
- Which enzymatic activity allows DNA polymerase I to remove RNA primers ahead of it?
- 5' to 3' exonuclease activity
- Which enzyme seals the nick between adjacent DNA fragments by forming a phosphodiester bond?
- DNA ligase
- What structure encircles DNA and holds DNA polymerase III onto the template, increasing its processivity?
- Beta sliding clamp
- What is the site of active DNA unwinding and synthesis called?
- The replication fork