Enzymes used in DNA replication - Practice Questions & MCQ

Nucleases:

• The polynucleotide is held together by phosphodiester bonds. 
• The nucleases hydrolyse the polynucleotide chain into the nucleotides. 
• It attacks either at 3′ OH end or 5′ phosphate end of the chain. 
•  The nucleases are of two types:
  • Exonucleases: The nuclease that attacks on outer free end of the polynucleotide chain is called exonuclease.
  • Endonucleases: The endonucleases attack within the inner portion of one or the double strands.

Polymerases:

• These enzymes carry out the process of polymerization of nucleotides and formation of polynucleotide chain.
• All DNA polymerases require a template strand, which is copied. 
• DNA polymerases also require a primer, which is complementary to the template. 
• Hence, for the catalysis of polymerization, DNA polymerase requires the four deoxyribonucleotide triphosphates e.g. dATP, dGTP, dTTP and dCTP, a DNA template, a primer for initiation of catalytic activity and Mg++
• The best‐studied bacterium, E. coli, has three DNA polymerase types
  • DNA polymerase I (Pol I):
    • It is primarily a repair enzyme.
    • DNA polymerase I only makes an average of 20 phosphodiester bonds before dissociating from the template. These properties make good sense for an enzyme that is going to replace damaged DNA. 
    • The 5′ to 3′ exonuclease activity removes base‐paired sequences ahead of the polymerizing activity. 
    • During replication, this can remove primers ahead of the polymerizing function of the polymerase. 
  • DNA polymerase II:
    • It is a specialized repair enzyme.
    • Pol II has the same editing (3′ to 5′) activity as Pol I, but not the 5′ to 3′ exonuclease activity.
  •  DNA polymerase III:
    • It is the actual replication enzyme.
    • Pol III is a multisubunit enzyme.
    • It lacks a 5′ to 3′ exonucleolytic activity, although a subunit of the enzyme carries out the editing (3′ to 5′) function during replication
    • Pol III synthesizes DNA at least a hundred times more rapidly than the other polymerases.

DNA Helicase:

• This enzyme is responsible for unwinding the double helix by disrupting the hydrogen bonds between the anti-parallel strands of DNA.

DNA Ligase:

• This enzyme joins the two strands of the polynucleotide chain by forming the phosphodiester bond.

DNA Topoisomerase and DNA Gyrase:

• DNA topoisomerase relaxes the negative and positive supercoils in the DNA of eukaryotes.
• DNA Gyrase is a subtype of topoisomerase found only in bacteria and plants. It relieves any form of tension produced after DNA is unwound.

Single-Stranded Binding Proteins (SSBs):

• The proteins that bind to the single DNA template strand and prevent the single stranded DNA templates from reattaching.
• They prevent the formation of H-bonds.

RNA Primase:

• It forms the RNA primer complementary to the DNA template strands at the site of origin of replication.