Question

In a famous experiment, Matthew Meselson and Franklin Stahl grew E. coli cells in a medium containing only the "heavy" isotope of nitrogen, 15 N . These cells were then transferred to a medium with the "light" isotope of nitrogen, 14 N . The results of the experiment supported the hypothesis of semiconservative replication, which was proposed by Watson and Crick. Heavy DNA ( 15 N DNA), hybrid DNA, and light DNA ( 14 N DNA) can be separated by centrifugation. If cells containing 15 N DNA are transferred to a medium with only 14 N NH 4 Cl as a nitrogen source, what percent of daughter molecules are composed of hybrid DNA after 3 generations?

Answer 1

Answer:

3rd generation = 2 Hybrid DNA molecules + 6 Light DNA

In Percentage 2÷8 × 100 = 25%

25% of the DNA are hybrid

Explanation:

Semi conservative model proposed by Watson and Crick states that one strand of the DNA comes from parent DNA which act as a template for the other newly formed strand.

First generation:

If cells containing 15 N DNA are transferred to a medium with only 14 N then in initial first generation two parent strands becomes separated and act as template for newly synthesized strands. this means that in first generation both DNA molecules are hybrid, one strand of 15 N DNA and other strand of 14 N DNA

Ist generation = 2 Hybrid DNA molecules

Second generation:

In Second generation these two hybrid molecules become unzip. Two strands are 15N and two strands are 14N. All these 4 strands act as template and 4 new strands are synthesized. The molecules of Hybrid DNA is still 2 with 2 molecules of Light DNA.

2nd generation = 2 Hybrid DNA molecules + 2 Light DNA

Third generation:

In Third generation these two hybrid molecules and two light DNA molecules become unzip. Two strands are of 15N and six strands are of 14N. All these 8 strands act as template and 8 new strands are synthesized. The molecules of Hybrid DNA is still 2 with 6 molecules of Light DNA.

3rd generation = 2 Hybrid DNA molecules + 6 Light DNA

In Percentage 2÷8 × 100 = 25%

25% of the DNA are hybrid