Question

You grow a bacterial culture in a media containing N^15 and transfer it to a media with N^14. After two rounds of replication you perform a CsCl density gradient centrifugation of the DNA. How many bands will you observe what will be their intensity?

(a) One, very intense

(b) Two, equally intense

(c) Three with middle one more intense than upper and lower

(d) Three equally intense

This question was addressed to me during an online interview.

I'm obligated to ask this question of DNA Replication Prokaryotes topic in division DNA Replication of Cytogenetics

Answer 1

Right option is (b) Two, equally intense

Easy explanation: In semi conservative replication initially the DNA was N^15-N^15.  After 1st round of replication both strands were N^14-N^15, making only a single band as they have same density. In the next round we will get N^14N^14, and  N^14-N^15 making two bands which are equally intense for having 1:1 concentration.