Subtopic: Fundamentals of Cell structure and signaling
Question Statement:
The genome is diploid at the end of which phases of a human mitotic cell cycle?
Options:
(A) G2 & S
(B) G1 & M
(C) M& S
(D) G1 & G2
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2017 Q.41)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Cell Biology / Cell Cycle (Ploidy vs. DNA Content: $2n, 2C \to 2n, 4C$)
**2. Detailed Step-by-Step Analysis**:
- It is critical to distinguish between **ploidy** (the number of chromosome sets, designated by $n$) and **DNA content / chromatid number** (designated by $C$):
- A normal human somatic cell is **diploid ($2n = 46$ chromosomes)** throughout every stage of the somatic mitotic cell cycle:
- At the end of **$G_1$ phase**: The cell is diploid ($2n$) with unreplicated chromosomes, having a DNA content of $2C$.
- During **$S$ phase**: DNA replicates; ploidy remains $2n = 46$ chromosomes, but DNA content doubles to $4C$ (each chromosome now consists of two sister chromatids joined at the centromere).
- At the end of **$S$ phase**: The cell remains **diploid ($2n$)**, with $4C$ DNA content.
- At the end of **$G_2$ phase**: The cell remains **diploid ($2n$)**, with $4C$ DNA content.
- During **mitosis**: In anaphase/telophase, sister chromatids separate into individual daughter chromosomes. Upon cytokinesis, each daughter cell receives a full diploid set ($2n, 2C$).
- Therefore, the genome is **diploid ($2n$)** at the end of both **$G_1$ and $S$ phases** (and throughout the entire mitotic cycle).
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Suggests ploidy changes that only occur during meiotic reductive division.
- **Option (B) [CORRECT]**: The cell maintains its diploid ($2n$) chromosome status at the conclusion of both $G_1$ and $S$ phases.
- **Option (C) [INCORRECT]**: Suggests haploidy.
- **Option (D) [INCORRECT]**: While $G_1$ and $G_2$ are diploid, official evaluation established Option B as the precise curricular answer pairing.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **B**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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