Q.43 Both strands of a DNA molecule are labeled with radioactive thymidine and are allowed to
duplicate in an environment containing non-radioactive thymidine. The number of DNA
molecules that will contain radioactive thymidine after three duplications is _________.
The number of DNA molecules containing radioactive thymidine after three duplications is 2.
This result follows from the semi-conservative model of DNA replication, where each new molecule retains one parental strand. Starting with one fully radioactive molecule (both strands labeled), non-radioactive thymidine produces hybrids that dilute the label over generations.
Step-by-Step Replication
DNA replication doubles molecules each generation while conserving one original strand per daughter.
-
Generation 0: 1 molecule (RR: both strands radioactive).
-
Generation 1: Original unwinds; each R strand pairs with new NR strand → 2 hybrids (R-NR). Radioactive molecules: 2/2.
-
Generation 2: Each hybrid replicates → from each R-NR: one R-NR (hybrid) + one NR-NR (light). Total: 2 hybrids + 2 light. Radioactive molecules: 2/4.
-
Generation 3: 2 hybrids → 2 R-NR + 2 NR-NR; 2 light → 4 NR-NR. Total: 2 hybrids + 6 light (8 molecules). Radioactive molecules: 2/8.
Only the two hybrids retain radioactive strands after three rounds.
Replication Outcomes Table
| Generation | Total Molecules | Hybrids (R-NR) | Light (NR-NR) | Radioactive Molecules |
|---|---|---|---|---|
| 0 | 1 | 0 | 0 | 1 |
| 1 | 2 | 2 | 0 | 2 |
| 2 | 4 | 2 | 2 | 2 |
| 3 | 8 | 2 | 6 | 2 |
Both strands of a DNA molecule labeled with radioactive thymidine undergo semi-conservative replication in non-radioactive medium, a classic CSIR NET Life Sciences question testing the Meselson-Stahl model. After three duplications, exactly 2 DNA molecules retain radioactive thymidine, as original strands persist only in specific hybrids.
Understanding Semi-Conservative Replication
DNA molecules radioactive thymidine three duplications hinges on each replication preserving one parental strand. The two original radioactive strands generate hybrids that halve in proportion (1/2, 1/4, 1/8). This mirrors density-shift experiments proving Watson-Crick’s model.
Detailed Generational Breakdown
Follow the fate of radioactive strands:
-
1st duplication: 2 hybrids (100% radioactive molecules).
-
2nd: 2 hybrids + 2 non-radioactive (50%).
-
3rd: 2 hybrids + 6 non-radioactive (25%, or 2/8).
Common Options Explained
CSIR NET often lists 1, 2, 4, 8:
-
1: Wrong; ignores both original strands propagating separately.
-
2: Correct; only hybrids carry label post-3 rounds.
-
4: Matches generation 2, not 3.
-
8: Assumes all molecules labeled, violating semi-conservative rule.
For CSIR NET preparation, master this via tables and simulations. The pattern generalizes: radioactive molecules = 2^(n-1) hybrids after n>0 generations from fully labeled start, but total 2^n.
