RNA Polymerase - Practice Questions & MCQ

Prokaryotic RNA Polymerase:

• Prokaryotes use the same RNA polymerase to transcribe all of their genes.
• RNA polymerase is composed of five polypeptide subunits:
  • Four of these subunits, denoted α, α, β, and β' comprise the polymerase core enzyme. 
    • The two α-subunits are necessary to assemble the polymerase on the DNA. 
    • the β-subunit binds to the ribonucleoside triphosphate that will become part of the nascent “recently born” mRNA molecule.
    • the β' binds the DNA template strand.
  • The fifth subunit, σ, is involved only in transcription initiation. It confers transcriptional specificity such that the polymerase begins to synthesize mRNA from an appropriate initiation site. 
  • The RNA polymerase comprised of all five subunits is called the holoenzyme.

Eukaryotic RNA Polymerase:

• Instead of a single polymerase comprising five subunits, the eukaryotes have three polymerases.
• The RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S).
• The RNA polymerase II transcribes precursor of mRNA, the heterogeneous nuclear RNA (hnRNA), that is, all protein-coding nuclear pre-mRNAs.
• The  RNA polymerase III is responsible for transcription of tRNA, 5srRNA, and snRNAs (small nuclear RNAs).