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Biology · 2025 · 2 marks
CBSE 2025 · Region 5 · Set 1 · Q18
Given below are the diagrammatic representations of the replicating fork of DNA in E. coli. Study the diagrams and answer the questions that follow.…
Given below are the diagrammatic representations of the replicating fork of DNA in E. coli. Study the diagrams and answer the questions that follow. 
(a) Which one of the three diagrams (i), (ii) or (iii) is the correct representation of the replicating fork of DNA replication ? Explain your answer. (b) Name the enzyme used in E. coli to join the newly synthesised fragments of DNA.
(a) Which one of the three diagrams (i), (ii) or (iii) is the correct representation of the replicating fork of DNA replication ? Explain your answer. (b) Name the enzyme used in E. coli to join the newly synthesised fragments of DNA.Marking-scheme solution
(a)
Diagram (ii) is the correct replicating fork
The DNA – dependent DNA polymerase catalyse polymerisation only in one direction,
that is $\displaystyle 5$’→$\displaystyle 3$’
On one template strand with polarity $\displaystyle 3$’→ $\displaystyle 5$’ DNA synthesis is continuous and on other
template strand with polarity $\displaystyle 5$’→$\displaystyle 3$’ DNA replication is discontinuous.
(b)
DNA ligase
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CBSE Class 12 Biology past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.