Phosphorus is required to synthesize the deoxyribonucleoside triphosphates used in DNA replication. A geneticist grows some E. coli in a medium containing nonradioactive phosphorus for many generations. A sample of the bacteria is then transferred to a medium that contains a radioactive isotope of phosphorus ( 3232P). Samples of the bacteria are removed immediately after the transfer and after one and two rounds of replication. Assume that newly synthesized DNA contains 3232P and the original DNA contains nonradioactive phosphorus. What will be the distribution of radioactivity in the DNA of the bacteria in each sample? Will radioactivity be detected in neither, one, or both strands of the DNA?Think about what this looks like after 1 round of replication. Two rounds? Also, what would this look like if the conserved model or dispersive model of replication were correct instead?

Explanation:

1. In the initial sample which is immediately removed after addition of radioactive isotope of phosphorus ( 3232P), hence there is  no incorporation of 3232P into the DNA because replication in the medium containing 3232P has not yet occurred.  

2. After one round of replication in  radioactive isotope of phosphorus ( 3232P) containing  medium, here  only one newly synthesized strand of DNA molecule will contain 3232P, while the other strand  will not contain any  radioactive isotope of phosphorus ( 3232P),  because DNA replication occurs in semi-conservative way.

3. After two rounds of replication in medium which contains radioactive isotope of phosphorus ( 3232P), here 50% of the DNA molecules will have  radioactive isotope 3232P in both strands, while the rest 50% will contain 3232P in only one strand and nonradioactive phosphorous in the other strand.

digestion

it excretes wastes