Question

the measleson and stahls experment is continued for four generations in bacteria the ratio of N15 N15 nitrogen 15 nitrogen 14 nitrogen 14 nitrogen 14 containing DNA in 4th generation would be

Answer 1

Dear student

Messelson and Stahl performed the experiment to prove that DNA replicates semi-conservatively. The salient features of their experiment include

• E.coli was grown in a medium containing heavy isotope 15N as the nitrogen source.
• 15N was incorporated into newly synthesised DNA as well and the DNA became heavy DNA.
• Heavy DNA molecule can be differentiated from normal DNA by density gradient centrifugation using cesium chloride as the gradient.
• Then, cells were again transferred into a medium with 14N as nitrogen source. Samples were taken from this media and their DNA was extracted.
• E .coli divides every 20 minutes. Therefore, the DNA extracted after 20 minutes had a hybrid density.
• DNA extracted after 40 minutes had equal amount of hybrid and light intensities.
• DNA extracted after 60 minutes contained 25% hybrid (N15 N14) and 75% light (N14 N14) in the 1:3 ratio.
• The DNA extracted after fourth generation i.e after 80 minutes contained 12.5% N15 N14 and 87.5% N14 N14 in the 1:7 ratio.

 Hence, ratio of N15N15, N15N14 and N14N14 after fourth generation will be 0 : 1 : 7 Regards