Q.50 In a typical sexually reproducing angiospermic plant, if an endosperm cell contains 4.8 × 108 nucleotide
pairs of DNA, then a microsporocyte of this plant will have ____ × 108 nucleotide pairs of DNA.
This value is derived by relating the triploid (3n) DNA content of the endosperm
cell to the diploid (2n) DNA content of the microsporocyte in a typical angiosperm.
Both values are assumed to be measured at the G1 phase, where DNA is unreplicated.
Ploidy Levels in Angiosperms
Endosperm formation occurs through double fertilization:
- One haploid sperm (n) fertilizes the egg → 2n zygote
- The second sperm (n) fuses with two polar nuclei (2n) → 3n endosperm
Microsporocytes (pollen mother cells) are diploid (2n) sporophytic cells
that undergo meiosis to produce haploid microspores. DNA content scales directly with
ploidy when genome size and cell-cycle stage are constant.
Step-by-Step Calculation
Given:
- Endosperm DNA (3n, G1) = 4.8 × 108 bp
First, calculate haploid (1n) genome size:
1n DNA = (4.8 × 108) ÷ 3 = 1.6 × 108 bp
Now calculate microsporocyte DNA (2n, G1):
2n DNA = 2 × 1.6 × 108 = 3.2 × 108 bp
Therefore, the correct answer is 3.2 × 108.
Common Options Explained
- 2.4 × 108: Assumes half of endosperm DNA, ignoring the 3n vs. 2n relationship.
- 4.8 × 108: Equals endosperm DNA, fails to adjust for ploidy difference.
- 9.6 × 108: Assumes G2 phase (4n), which is not implied in the question.
- 1.6 × 108: Represents haploid genome size; microsporocyte is diploid.
- 3.2 × 108: Correct application of 2n/3n scaling in G1 phase.
Double Fertilization Basics
In angiosperms, one sperm fertilizes the egg to form the diploid embryo,
while the second sperm fuses with the diploid central cell to form the
triploid endosperm. The endosperm provides nutrition to the developing embryo.
Why 3.2 × 108?
The haploid genome size is 1.6 × 108 bp. Since the microsporocyte is diploid (2n),
its DNA content in the G1 phase is:
2 × 1.6 × 108 = 3.2 × 108 bp
If the cell were in G2, the DNA content would double to 6.4 × 108 bp,
but the problem context clearly implies G1.
Summary Table
| Cell Type | Ploidy | Relative DNA Content | DNA Amount (×108 bp) |
|---|---|---|---|
| Endosperm | 3n | 3C | 4.8 |
| Microsporocyte (G1) | 2n | 2C | 3.2 |
| Sperm | n | 1C | 1.6 |
Exam Tip
CSIR NET and GATE Life Sciences frequently test DNA content questions.
Always confirm ploidy level and cell-cycle stage
before calculating DNA amounts to avoid common traps.
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