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#primer

3 public questions tagged with this topic.

Transcription Basics

RNA polymerase does not require primer because

RNA polymerase active site geometry allows de novo initiation without oligonucleotide primer. Structural studies reveal holoenzyme accommodates initiating NTPs at positions +1 and +2 base-paired to template DNA within catalytic cleft, stabilized by Watson-Crick interactions and base stacking with sigma factor contacts. Formation of first phosphodiester bond does not require pre-existing 3' hydroxyl unlike DNA polymerases. Consequently polymerase can start RNA chains at any promoter-defined site. This fundamental difference between replicative and transcriptional enzymes enables transcription to initiate spontaneously at regulatory signals without primase activity, simplifying gene expression control.

Ref: Watson et al. Chapter 13: RNA polymerase de novo initiation mechanism no primer required; Alberts Chapter 6 Active site NTP stabilization

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Replication Basics

DNA polymerase cannot start synthesis de novo because it lacks

DNA polymerases catalyze attack of 3' hydroxyl on alpha-phosphate of incoming dNTP forming phosphodiester linkage. They cannot create that hydroxyl de novo nor initiate phosphodiester bond between two free dNTPs alone. Initiation therefore demands preformed primer-template junction with free 3' OH precisely positioned in active site opposite template. In cells primase, specialized RNA polymerase, synthesizes short RNA primers complementary to template providing required hydroxyl. This obligate primer dependence ensures replication starts only at regulated origins and prevents random DNA synthesis elsewhere.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 27: Requirement of primer 3'OH for DNA polymerase activity

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Replication Basics

Primer used in DNA replication is usually

DNA polymerases are unable to initiate strand synthesis de novo, requiring pre-existing hydroxyl to elongate. Primase, specialized RNA polymerase subunit of Pol α-primase complex in eukaryotes and DnaG in bacteria, synthesizes short 8-12 nucleotide RNA primer complementary to ssDNA template, providing 3' OH substrate. RNA primer is later removed by RNase H and FEN1 and replaced by DNA via Pol δ during lagging strand maturation. Use of RNA allows marking of nascent fragments and distinction from bulk DNA for removal, prevents errors from ab initio DNA synthesis and ensures strand identity. Leading strand also requires priming at origin.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 5: Primer Used in DNA Replication is Usually RNA

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