Question

During cell division a complete copy of DNA in its nucleus is made in

• A. G1 phase
• B. G2 phase
• C. S phase
• D. M phase

Hint:

Cell Cycle Phases:
$\text{G}_1$ = Growth & RNA synthesis.
S phase = DNA Replication (Synthesis).
$\text{G}_2$ = Spindle protein synthesis & pre-mitotic check.
M phase = Mitosis.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The eukaryotic cell cycle partitions interphase into Gap 1 ($ ext{G}_1$), Synthesis (S), and Gap 2 ($ ext{G}_2$) phases prior to mitosis (M).
Key Formula or Approach:
\[ \text{Cell Cycle: } \text{G}_1 \longrightarrow \text{S (DNA Replication: } 2C \rightarrow 4C) \longrightarrow \text{G}_2 \longrightarrow \text{M (Mitosis)} \]

Step 2: Detailed Explanation:
During the eukaryotic cell cycle:
1. S Phase (Synthesis Phase): The entire nuclear genomic DNA is semi-conservatively replicated, doubling the nuclear DNA content from $2C$ to $4C$ while maintaining chromosome number ($2n$). Centrosome duplication also initiates in the cytoplasm during S phase.
2. G1 Phase: Biosynthesis of enzymes and RNA for cell growth.
3. G2 Phase: Synthesis of tubulin and proteins for mitotic spindle formation.
4. M Phase: Chromosome segregation and cytokinesis.

Step 3: Final Answer:
Hence, nuclear DNA replication occurs in S phase, corresponding to option (C).