If E.coli (N¹⁵- N¹⁵ in DNA)is allowed to grow in ${\mathrm{N}}^{14}$ medium for 80 minutes, then what would be the proportions of light and hybrid densities of DNA molecules?

Generation 0 (Initial): E. coli with N15 - N15 DNA in N15 medium. All DNA is of a heavy density (N¹⁵-N¹⁵).

Generation time of E.coli is 20 mins. Therefore, no. generations in 80 mins= 80/20=4

Generation 1 (20 minutes): After one generation of growth (20 minutes), the DNA would be replicated, resulting in hybrid DNA. The DNA is newly synthesized and contains one heavy strand (N¹⁵) and one light strand (N¹⁴). This is called a hybrid DNA molecule (intermediate density).

Generation 2 (40 minutes): After two generations of growth (40 minutes), the DNA distribution would be:
50% newly synthesized DNA containing two light strands (N¹⁴-N¹⁴, light density).
50% hybrid DNA (intermediate density).

Generation 3 (60 minutes): After three generations of growth (60 minutes), the DNA distribution would be:
75% newly synthesized DNA containing two light strands (N¹⁴-N¹⁴, light density).
25% hybrid DNA (intermediate density).

Generation 4 (80 minutes): After four generations of growth (80 minutes), the DNA distribution would be:

87.5% newly synthesized DNA contains light strand DNA(14).
12.5% hybrid DNA (intermediate density). (2 hybrid DNA).