If E. coli having heavy DNA was allowed to grow for 80 minutes on a medium containing ¹⁴NH₄Cl then what would be the proportions of light and hybrid densities DNA molecule?

80minutes= 80/20= 4 generations

Initially DNA contain ¹⁵N¹⁵N

Then it is placed in a medium containing ¹⁴N for 3 generation.

After 1st generation:

¹⁵N¹⁵N→ ¹⁵N¹⁴N    ¹⁵N¹⁴N (2 Hybrid DNA molecules)

After 2^nd generation

¹⁵N¹⁴N    ¹⁵N¹⁴N → ¹⁵N¹⁴N    ¹⁴N¹⁴N   ¹⁵N¹⁴N    ¹⁴N¹⁴N (2 light DNA and 2 Hybrid DNA molecules)

After 3^rd generation:

¹⁵N¹⁴N    ¹⁴N¹⁴N   ¹⁵N¹⁴N    ¹⁴N¹⁴N → ¹⁵N¹⁴N    ¹⁴N¹⁴N   ¹⁴N¹⁴N   ¹⁴N¹⁴N ¹⁵N¹⁴N    ¹⁴N¹⁴N   ¹⁴N¹⁴N   ¹⁴N¹⁴N (6 light DNA and 2 hybrid DNA molecules)

After 4^th generation

2(¹⁵N¹⁴N) → 2 (¹⁵N¹⁴N    ¹⁴N¹⁴N)

6(¹⁴N¹⁴N) → 6 (¹⁴N¹⁴N   ¹⁴N¹⁴N)

Total no. of light DNA= 14

Total no. of Hybrid DNA= 2

% of light DNA= 87.5%

% of light DNA= 12.5%