primase
Summary
Primase is an RNA polymerase enzyme that synthesizes short RNA primers to initiate DNA replication, since DNA polymerases cannot start synthesis de novo. It works with helicase as part of the primosome complex and is essential for both leading and lagging strand synthesis.
Detail
Primase (DNA primase) is an RNA polymerase that lays down short RNA primers (~5-10 nucleotides) complementary to the DNA template, providing a free 3'-OH group for DNA polymerase III (in prokaryotes) or DNA polymerase alpha (in eukaryotes) to extend. This is necessary because all DNA polymerases require a primer to begin synthesis—they can only add nucleotides to an existing 3'-OH end, not initiate a new strand.
On the leading strand, only one primer is needed since synthesis is continuous. On the lagging strand, multiple primers are required because synthesis occurs discontinuously in Okazaki fragments, with each fragment requiring its own RNA primer laid down by primase.
In eukaryotes, primase is physically associated with DNA polymerase alpha, forming the Pol α-primase complex, which synthesizes the initial RNA-DNA primer before replication is handed off to Pol epsilon (leading strand) or Pol delta (lagging strand). After primer synthesis, the RNA primers are later removed and replaced with DNA by DNA polymerase I (prokaryotes, via 5'->3' exonuclease activity) or FEN1/RNase H (eukaryotes), and the nicks are sealed by DNA ligase.
Clinical/board relevance: Understanding primase is key to grasping the mechanism of DNA replication, particularly why Okazaki fragments form and why RNA primers must be removed and replaced. This ties into questions about replication fork machinery, antibiotics/antivirals targeting nucleic acid synthesis, and genetic diseases involving replication defects. Primase itself is not a common drug target, but understanding the replication machinery helps differentiate mechanisms of drugs like fluoroquinolones (topoisomerase inhibitors) from other replication-targeting therapies.
Sources
- First Aid for the USMLE Step 1
- Molecular Biology of the Cell (Alberts et al.)
- Lippincott Illustrated Reviews: Biochemistry
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