You expose growing cells to 3H-thymidine (a radioactive nucleoside) just as they

Question

You expose growing cells to 3H-thymidine (a radioactive nucleoside) just as they enter S-phase, then wash the labeled base off the cells and let them go through a second S-phase before looking at the chromosomes. How would you expect the 3H-thymidine to be distributed over a pair of homologous chromosomes if recombination has NOT occurred? Choose the correct answer from the choices below, and clearly explain your reasoning (you may find it helpful to track the radioactivity by drawing the results from each round of replication).

A. One chromatid of one homolog

B. One chromatid each of both homologs

C. Both chromatids of one homolog

D. Both chromatids of both homologs

E. Some other pattern

Explanation / Answer

Let us denote the radioactive strand by red color and normal strand as blue.

The original cells have both normal strands (blue)

First division after growing in radioactive 3-H thymidine, we get the following: One red and one blue strand.

During the next DNA replication, one chromatid has the radioactive strand as template and other chromatid gets the non-radioactive strand as template. Since there is no more radioactivity in the culture, the new strands synthesized will not be radioactive. Hence, the correct answer is B, One chromatid each from both homologs.

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