Question

(a) Name the stage in the cell cycle where DNA replication occurs.

(b) Explain the mechanism of DNA replication. Highlight the role of enzymes in the process.

(c) Why is DNA replication said to be semi-conservative? [5]

Answer 1

(a) During S-Phase (Synthesis phase) of Interphase, DNA replicates in cell cycle. [0.5]

(b) Mechanism of DNA replication : The DNA strands start separating from a particular point called origin of replication (single in prokaryotes and many in eukaryotes).

This unwinding is catalysed by enzyme called helicases.

When the double stranded DNA is unwound upto replication fork, enzyme DNA dependent DNA polymerase catalyses the joining of deoxyribonucleotide (A, G, C and 1) in the 5'-3' direction.

The enzyme forms one new strand in a continuous stretch (leading strand) in the 5'-3' direction on one of the template strands.

On the other template strand, the enzyme forms short stretches (discontinuous) strand of DNA also in the 5'-3'.

The discontinuous fragments which are called as okazaki fragments are later joined by DNA-ligase.

The two strands are held together by hydrogen bonds between nucleotides. [0.5x7=3.5]

(c) DNA replication is semi-conservative because during DNA replication in the two newly synthesised daughter DNA one strand is parental (conserved) and the other is newly synthesised. [1]