(3.3) Shown is a representation of an origin of replication. Synthesis of new DN
ID: 143050 • Letter: #
Question
(3.3) Shown is a representation of an origin of replication. Synthesis of new DNA occurs on both strands and in both directions. template 1 template 3 3" 5' 3' CAAGG AC ORI a) For the following, use sites A and B with respect to fork 1 and sites C and D with respect to fork 2 i) On which strand(s) will replication be continuous? template 1 template 2 template 3 template 4 ii) To which site or sites (A, B, C, or D) can the primer 5-GUUCC-3' bind to initiate replication? DNA Replication ii When DNA ligase is inhibited, it differentially affects the synthesis from the leading and the lagging strands. Explain which strand (leading or lagging) is more affected by the lack of DNA ligase and whyExplanation / Answer
a i) Replication will be continuous in template 1 and template 4. The helicase unwinds the double-stranded DNA for replication, making a forked structure. Now DNA polymerase work for replication, but this enzyme can work only in the 5' to 3' direction, so it replicates the leading strand continuously. We can see that replication can happen continuously in 5' to 3' direction only in templates 1 and 4.
ii) Primer 5'-GUUCC-3' can bind to B and C sites to initiate replication, because this primar can only bind to 3'-CAAGG-5' in order to initiate replication and only sites b and C are having those sequences at the said direction.
iii) The lagging strand is actually more affected by the lack of DNA ligase. DNA replication on the lagging strand happens in small stretches called Okazaki fragments. For replication of the lagging strand to be continued and complete, a phosphodiester bond must be formed between the 3'OH group on one Okazaki fragment and the 5' phosphate group on the other. DNA ligase makes this bond, it's absence would therefore affect the lagging strand.
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