Question

Suppose you label bacteriophage T2 with 15N for 10 rounds of bacterial infection 15N will label DNA (i.e., you use 15 N instead of P^32) and not protein. a) What percentage of the viral DNA strands will be labeled with 15 N? b) Now, after labeling bacteriophage T2 for 10 rounds of infection with l5N, your lab partner forgot to include 15N in the growth media for an unknown number of rounds of replication. You take out a sample and find that 12.5% of the viral strands are labeled with 15 N. How many rounds of bacterial infection did you lab partner allow the culture to go?

Answer 1

Solution: As we know that 15N will be incorporated in the DNA (unlike P32, which is incorporated in protein. and also, a) we know that after 1 round of bacterial infection, the % of labelled DNA will be 50% and after 10 rounds, almost all the viral DNA is labelled with 15N

b) we have the data that only 12.5 % of viral strands are labelled. after each round of labelled DNA incorporation. half of the DNA sample remain radioactive. let's say we start with the 100% fraction of labelled DNA, so after 1 round of replication, only 50 % of DNA remained as radioactive. Similarly, after 2 rounds, only 25% DNA remained as unlabeled and after 3 rounds of DNA replication, only 12.5% DNA will remain as labelled. so, after 3 rounds of replication with unlabeled DNA (or 3 rounds of forgetting additional 15N incorporation), only 12.5^% DNA remained as unlabeled.