Question

PCNA (the eukaryotic clamp)

___ of identical subunits
attaches DNA pol to the ___
In E. coli, the clamp has just __ subunits, but the same overall helical structure as PCNA.

Answer 1

PCNA in eukaryotes is composed of three identical subunits, whereas the E. coli clamp has two subunits but a similar structure. For the E. coli RNA polymerase, the subunit that confers specificity to transcription is the sigma factor.

Step-by-step explanation:

PCNA (the eukaryotic clamp) consists of three identical subunits and serves the function of attaching DNA polymerase to the DNA strand during replication. This molecular structure increases the efficiency of DNA replication by making it more processive, meaning that it can synthesize a longer stretch of DNA without dissociating from the template. In E. coli, the clamp, also known as the beta clamp, has two subunits, but it preserves a similar overall helical structure as PCNA.

The fifth subunit of the E. coli RNA polymerase that confers specificity to transcription is the sigma factor (σ). This is an essential component as it allows the polymerase to bind to specific promoter regions of the DNA, ensuring that mRNA is synthesized from the correct initiation site. Without σ, the core enzyme would lack specificity and could initiate transcription at random locations, leading to non-functional mRNA sequences.