Question

7. DNA synthesis involves the polymerization of nucleotides into polynucleotide chains. In this concern answer the following questions     
a. Define and indicate the significance of ;
i. Okazaki fragments,
ii. DNA ligase, and
iii. RNA primer during DNA replication.
b. Several temperature-sensitive mutant strains of E. coli display the following characteristics. Predict what enzyme or function is being affected by each mutation.
i. Newly synthesized DNA contains many mismatched base pairs.
ii. Okazaki fragments accumulate, and DNA synthesis is never completed.
iii. No initiation occurs.
iv. Synthesis is very slow.
v. Supercoiled strands remain after replication, which is never completed.

Answer 1

7a.
i. Okazaki fragments: Short (200-300 ntd) fragments produced on the lagging strand.
They are extended in a direction opposite to the replication fork in 5'-3' direction.
They are produced due to the inefficiency of DNA polymerase to extend a strand in 3'-5' direction.

ii. DNA ligase: It joins a nick i.e. a break in sugar-phosphate backbone by forming phoshphodiester bond
It joins Okazaki fragments on the lagging strand to produce a single continuous strand.

iii. RNA primer: DNA polymerase cnanot initiation starnd synthesis. It can only extend an existing 3'-OH group in 5'-3' direction. Hence, primase produces a short primer to initiate DNA replication.

7b.
i. 3'-5' exonuclease activity of DNA polymerase = Proofreading function
ii. DNA ligase
iii. Primase
iv. Processivity clamp
v. Topoisomerase/DNA gyrase