Q.92 A mature rat sperm cell has 2.5 μg of genomic DNA that is equivalent of a
haploid genome. Compared to this sperm cell, the amount of genomic DNA
(in μg) in a somatic cell, which is in the G2 phase of cell cycle, will
be _____ (in integer).
The mature rat sperm cell contains 2.5 μg of genomic DNA, representing one haploid (1N, or “C”) genome equivalent. A somatic cell in G2 phase has replicated its diploid (2N) DNA, resulting in 4 times the haploid amount, or 10 μg. This is a standard cell cycle concept tested in competitive exams like GATE Life Sciences.
Cell Cycle DNA Content
Somatic cells are diploid (2C DNA in G1 phase), with DNA replication during S phase doubling it to 4C by G2 phase. Mature sperm are haploid (1C) after meiosis II, retaining unreplicated DNA. Thus, G2 somatic DNA = 4 × sperm haploid DNA = 4 × 2.5 μg = 10 μg.
Detailed Solution
- G1 somatic: 2C (diploid, unreplicated) = 2 × 2.5 μg = 5 μg
- S phase: DNA replicates to 4C
- G2 somatic: 4C (diploid, replicated) = 10 μg
- Sperm remains 1C post-meiosis, no replication occurs.
The integer answer is 10.
No Options Provided
This numerical answer type (NAT) question from GATE XL 2023 Q92 has no multiple-choice options to evaluate. Common distractors in similar problems include 5 μg (G1 amount) or 20 μg (misinterpreting as tetraploid), but G2 is definitively 4C.
Introduction to Rat Sperm Cell DNA Content
In cell biology, understanding rat sperm cell DNA content is crucial for exams like CSIR NET and GATE Life Sciences. A mature rat sperm carries 2.5 μg of genomic DNA, equivalent to one haploid genome (1C or 1N). Somatic cells, being diploid, double this baseline, with G2 phase specifics key to solving quantitative problems on G2 phase somatic cell genomic DNA amount.
Cell Cycle Phases and DNA Amounts
The cell cycle includes G1, S, G2, and M phases, where DNA content changes predictably.
- G1 phase: Somatic cell has 2C DNA (diploid, unreplicated) = 5 μg (2 × 2.5 μg haploid).
- S phase: DNA replication doubles content to 4C without chromosome division.
- G2 phase: Cell retains 4C DNA (two sister chromatids per chromosome), preparing for mitosis = 10 μg.
- M phase: Divides to two G1 daughters (2C each).
Sperm, post-meiosis II, stays at 1C (2.5 μg), highly condensed for fertilization.
| Phase | Ploidy | DNA Content (relative to sperm) | Absolute (μg) |
|---|---|---|---|
| Sperm (haploid) | 1N | 1C | 2.5 |
| Somatic G1 | 2N | 2C | 5 |
| Somatic G2 | 2N | 4C | 10 |
Why G2 Somatic Cell Has 10 μg DNA
Rat somatic cells start diploid (two haploid sets). Post-S phase replication in G2, each chromosome has two identical chromatids, yielding 4C total—four times the sperm’s 1C. Calculation: 4 × 2.5 μg = 10 μg (integer). This ignores minor cytoplasmic DNA, focusing on genomic nuclear DNA as per question phrasing.
GATE Life Sciences Context
This mirrors GATE XL 2023 Q92, testing cell cycle mastery for CSIR NET aspirants. Missteps like confusing G1 (5 μg) or post-mitotic amounts are common; remember G2 precedes mitosis with replicated DNA.
Key Takeaways for Exam Prep
- Haploid sperm: Always 1C baseline.
- G2 somatic: 4C = 4 × haploid.
- Practice conversions: pg to μg scales similarly (rat genome ~2.5-3 pg/cell, scaled here).
