After bacteria are grown for many generations on a medium containing normal, non

Question

After bacteria are grown for many generations on a medium containing normal, nonradioactive phosphorus (P), some are transferred to a medium where all the phosphorus is in the form of its radioactive isotope (32P). The bacteria use this 32P to synthesize (among other things) the deoxyribonucleotide sthey need for DNA replication. The bacteria grow on this 32P medium for three generations (assume a 20 min generation time) and a sample of bacteria is removed at the end of each round of replication. DNA duplexes (double stranded DNA) from these samples are isolated and analyzed for 32P content.

- For each of the three samples of bacteria (time 20 mins, time 40 mins, time 60 mins) estimate the proportion of DNA duplexes that would have:

All of their nucleotides containing 32P

Some of their nucleotides containing 32P & some containing normal P.

All of their nucleotides containing normal P

...

part 2

Assume that the conditions of the experiment previously - are the same except that following the second round of replication the bacteria are transferred back to a medium where all of the phosphorus is nonradioactive P. Describe the distribution of radioactive and nonradioactive P in the DNA duplexes in the sample taken after the next (third) round of replication. In what proportion would the various kinds of DNA duplexes occur in the sample?

Explanation / Answer

Part 1 (Remember bacteria undergo semi-conservative DNA replication)

After 20 minutes the proportion of DNA duplexes that would have:

After 40 minutes the proportion of DNA duplexes that would have:

After 60 minutes the proportion of DNA duplexes that would have:

Part 2

25 % DNA double helix will be P

75 % DNA double helix will be 32P and P

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