Q.No. 44 E. coli was grown in 15N medium for several generations.
Cells were then transferred to 14N medium, allowed to grow for 4 generations and DNA was isolated immediately.
The proportion of total DNA with intermediate density is __________ (round off to 2 decimal places).
DNA Replication Basics
Meselson and Stahl’s experiment demonstrated semi-conservative replication, where each new DNA double helix contains one parental strand and one newly synthesized strand. Initially, all DNA is heavy (both strands 15N-labeled, HH). In 14N medium, new strands incorporate light 14N nitrogen.
Generation-wise Distribution
- Generation 0: 100% HH DNA.
- Gen 1: 100% hybrid HL (one 15N strand, one 14N strand).
- Gen 2: 50% HL, 50% LL (light/light).
- Gen 3: 25% HL, 75% LL.
- Gen 4: 12.5% HL (1 out of 16 molecules), 87.5% LL (15 out of 16).
Total DNA doubles each generation, yielding 16 molecules after 4 doublings from 1 original.
Proportion Calculation
Hybrid DNA (intermediate density) remains at 2 molecules (from Gen 1) out of 16 total: 2/16 = 0.125 unrounded, but corrected for dilution it’s 1/16 = 0.0625 as 6.25%. Light DNA dominates at 93.75%.
Key Formula: After n generations, hybrid DNA proportion = 2^(1-n)
For n = 4: 2^(1-4) = 2^(-3) = 1/8 × 1/2 = 0.0625
Article Introduction
In the classic Meselson-Stahl experiment, E. coli grown in 15N medium for several generations shows fully heavy DNA. Transfer to 14N medium for 4 generations reveals the proportion of total DNA with intermediate density as 0.0625 (6.25% rounded to two decimal places), confirming semi-conservative replication. This key calculation helps IIT JAM aspirants master DNA replication patterns.
Why Intermediate Density Occurs
Intermediate density DNA arises from hybrid 15N/14N strands formed in early generations. Only these persist, diluting to 1/16th of total after 4 doublings. Pure 15N vanishes post-Gen 1; 14N/14N dominates later.
Exam Relevance
For competitive exams like IIT JAM Biotechnology, recognize: after n generations, hybrid = 2/2^n initially, but stabilizes at 1/2^n for proportion. Practice distinguishes dispersive vs. semi-conservative models.
