Question

Suppose that in another world, DNA does not undergo semiconservative replication, but instead a conservative mechanism is used. In a repeat of the Meselson and Stahl experiment, bacterial cells that are grown in 15N medium for many generations then transferred to 14N medium for two generations will have the following distribution of DNA molecules in a CsCl gradient:
a. ½ heavy, ½ light
b. ¼ heavy, ½ intermediate, ¼ light
c. ¼ heavy, ¾ light
d. ¾ heavy, ¼ light
e. Impossible to predict

Answer 1

Answer:

c. ¼ heavy, ¾ light

Explanation:

DNA replication follows semi conservative model which means that both the strands separate during replication and the newly formed DNA molecule has one old and one new strand. If instead of semi conservative, conservative mechanism is used, old strands will not separate from each other and the two newly synthesised DNA strands will come together to form a DNA molecule.

Here the cells were first grown in heavy 15N medium so all the DNA molecules had 15N and thus were heavy. Then the cells were transferred to 14N medium where only lighter 14N were available for replication process.

In first generation, 14N DNA molecules were synthesised from 15N DNA molecules so 1/2 DNA molecules were 15N and other half were 14N. In the second generation, 14N molecules will give rise to new 14N molecules and 15N molecules will also give rise to 14N molecules hence 1/4 molecules will be 15N (heavy) and 3/4 will be 14N (light).