bio4 - shawn3387

[ArchivalUser]

Which of the following descriptions of DNA replication is not common
to the synthesis of both leading and lagging strands?
a. RNA primer is synthesized
b. DNA polymerase III synthesizes DNA
c. Helicase (rep protein) continuously unwinds duplex DNA at the replication
fork during synthesis
d. Nucleoside monophosphates are added in a 5′to 3′direction along the growing
DNA chain
e. DNA ligase repeatedly joins the ends of DNA along the growing strand

[ArchivalUser]

D???

[ArchivalUser]

d?

[ArchivalUser]

The answer is e. (Murray, pp 412“434. Scriver, pp 3“45. Sack, pp
3“29. Wilson, pp 99“121.) In the leading strand, DNA is synthesized continuously
in the 5′to 3′direction by DNA polymerase. In contrast, in the
lagging strand, which is in the 3′to 5′direction, DNA polymerase III synthesizes
small (approximately 1000 nucleotides) Okazaki fragments. For
the synthesis of these small fragments, all the same roles and steps apply
except that additional enzymes are needed to fill the gap between the fragments
and join the fragments. Consequently, DNA ligase is repeatedly
needed to join the ends of the DNA fragments along the growing lagging
strand. DNA ligase catalyzes the formation of a phosphodiester bond
between the 3′hydroxyl group at the end of one DNA chain and the 5′
DNA phosphate group at the end of the other. DNA ligase is only functional
when double-helical DNA molecules are the substrate. It does not
work on single-stranded DNA. DNA ligase effects the joining of strands of
DNA not only during the normal synthesis of DNA, but during the splicing
of DNA chains in genetic recombination as well as the repair of damaged
DNA.