Question

In which stage DNA duplication occurs?
A. G1
B. G2
C. S

Answer 1

Hint: DNA replication or duplication is the biological process of producing two identical replicas of DNA from one original DNA molecule.
There are four phases in a cell cycle- G1, G2, S, and M phase.
G1, G2, and S phase together are known as interphase.

Complete answer: S phase is also known as Synthesis Phase.
1. It is the phase of the cell cycle in which DNA is replicated. S phase occurs between the G1 phase and the G2 phase. Throughout the M phase and G1 phase, cells assemble inactive pre-replication complexes (pre-RC) on replication origins distributed throughout the genome.
2. During S-phase, the cell converts pre-RCs into active replication forks to initiate DNA replication. This process depends on the kinase activity of Cdc7 and various S-phase CDKs, both of which are upregulated upon S-phase entry.
3. Activation of the pre-RC is a closely regulated and highly sequential process.
After Cdc7 and S-phase CDKs phosphorylate their respective substrates, the second set of replicative factors associate with the pre-RC.
4. The stable association encourages minichromosome maintenance protein complex (MCM) helicase to unwind a small stretch of parental DNA into two strands of ssDNA, which in turn gives work to replication protein A (RPA), an ssDNA binding protein.
5. RPA recruitment primes the replication fork for loading of replicative DNA polymerases and proliferating cell nuclear antigen (PCNA) sliding clamps.
6. Loading of these factors completes the active replication fork and initiates the synthesis of new DNA.

So, the correct answer is C. S phase

Note: DNA must be packaged into nucleosomes to function properly, synthesis of canonical (non-variant) histone proteins occurs alongside DNA replication.
Free histones produced by the cell during S-phase are rapidly incorporated into new nucleosomes.