Question 7 of 25intermediate🔧 ApplyNumerical2 marks

If E. coli was allowed to grow for 80 minutes then what would be the proportions of light and hybrid densities DNA molecule?

Correct Answer

After 80 minutes (4 generations), the proportions are:

Light DNA (14^{14}N-14^{14}N): 75% or 3/4

Hybrid DNA (15^{15}N-14^{14}N): 25% or 1/4

Exercise: In-text Question (Page 12) | Q: 7 | (Page 12)
For More Understanding

Explanation

The question refers to Meselson and Stahl's experiment on semi-conservative DNA replication. E. coli divides every 20 minutes. Starting with heavy DNA (15^{15}N-15^{15}N) and transferring to 14^{14}N medium: After 20 min (1st generation) - 100% hybrid DNA. After 40 min (2nd generation) - 50% hybrid and 50% light. After 60 min (3rd generation) - 25% hybrid and 75% light. After 80 min (4th generation) - the proportion becomes 25% hybrid and 75% light. Each generation, the hybrid DNA halves while light DNA proportion increases.

Solution Steps

  1. Step 1: Identify that E. coli divides every 20 minutes, so 80 minutes = 4 generations

  2. Step 2: At Generation I (20 min): 100% hybrid DNA forms

  3. Step 3: At Generation II (40 min): 50% hybrid + 50% light DNA

  4. Step 4: At Generation III (60 min): 25% hybrid + 75% light DNA

  5. Step 5: At Generation IV (80 min): Proportion is 25% hybrid and 75% light DNA

PreviousNext