Question

Nuclear DNA replicates in which phase of the cell cycle

• A. G2 phase
• B. M phase
• C. S phase
• D. D phase

Hint:

Remember: S stands for Synthesis.
Therefore, the S phase is the dedicated phase of the cell cycle where the cell synthesizes a copy of its nuclear genome.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The cell cycle is an orderly sequence of events by which a cell duplicates its contents and divides into two.
It is divided into Interphase (comprising G1, S, and G2 phases) and Mitosis (M phase).

Step 2: Detailed Explanation:
Let us analyze the events occurring in each phase of the cell cycle:
- G1 phase (Gap 1): This is a period of active cell growth and metabolic activity. The cell prepares the enzymes and substrates required for DNA replication.
- S phase (Synthesis phase): This is the specific window during which nuclear DNA replication occurs.
The cell synthesizes an exact copy of its entire genomic DNA, doubling the DNA content of the cell (from $2\text{C}$ to $4\text{C}$), while the chromosome number remains diploid ($2\text{N}$).
Histone proteins are also synthesized during this phase to package the newly replicated DNA.
- G2 phase (Gap 2): The cell continues to grow and synthesizes proteins (like tubulin) necessary for spindle fibers in preparation for mitosis.
- M phase (Mitosis): The duplicated chromosomes are segregated equally into two daughter nuclei.
There is no standard "D phase" in the cell cycle.

Step 3: Final Answer:
Therefore, nuclear DNA replication takes place exclusively during the S phase, making option (C) the correct choice.