Question

A. In a Meselson-Stahl type experiment, bacterial cells grown in 15N medium for many generations were washed and then allowed to grow in 14N medium. DNA was extracted just after transfer (0 generation) and then after 1 and 2 generations. All these samples were analyzed by CsCl gradient centrifugation. From DNA isolated from generations 0, 1 and 2 show.

B.Position of bands in the centrifuge tubes

C. 14N and 15N labelled DNA strands

Answer both questions for each mode of replication, conservative and dispersive mode of replication.

Answer 1

bacteria cells were grown in 15N medium for many generations.

for 0 generation:

In case of conservative replication the DNA will have both the strands with 15 N as it is the parent DNA grown in 15N media.

In semiconservative the Two strands will have only 15N

Dispersive: this will also have only 15N in both the strands as no replication occur in 147N media.

So, the bands will appear at similar position towards bottom in the tube.

After 1 generation:

Conservative : There will be two DNA molecule one having both strands labelled with 15N while the other DNA with both strands labelled with 14N.

So on CsCl2 gradient centrifugation, there will be two distincts bands one below(15N) and one above (14N).

Semiconservative: IN this case there will be two DNA molecules each having one strand labelled with 14N and the other strand labelled with 15 N.

So the band will appear in between the two bands appear in case of conservative.

Dispersive: In first generation, the two strands of DNA will be labelled with both 15N and 14N dispersed all over strands.

So the band will appear similar to the semiconservative that is in between heavy 15 N bands and light 14N bands.

Second generation:

Conservative: There will be formation of four DNA molecules: one DNA molecule will have both the strands labelled with 15N while the rest three new will have 14N in their both the strands.

So, Rwo distinctt bands of heavy and light chain of 15N and 14N respectively will be sen on tube below and above respectively.

Semiconservative: Four DNA molecule among which 2 will have both the strands labelled with 14N.

While, the rest two will have one strand labelled 14N and other 15N.

So two bands will be appeared in the centrifugation tube, light and heavy band.

Dispersive : All four DNA molecules will have dispersed 14N and 15N labelled strands

So two bands will be observed on the CsCl2 gradient tube .