Question

You start with a cell that is grown in normal media. Subsequently DNA is allowed to replicate in an environment with all necessary enzymes, dATP, dCTP, dGTP, and radioactively labeled dTTP (3H thymidine) for 1 cell cycle, (resulting in 2 cells) and then switched back to normal, nonradioactive medium. Cell are allowed to replicate once more, so you end up with 4 cells. How many of these 4 cells with have radioactive dTTP incorporated in their DNA?

Select one:

a. this can not be determined

b. 1

c. 2

d. 0

e. 4

Answer 1

Option c: 2

The DNA in the first cell does not have any radioactive dTTP.

When it is allowed to replicate in an environment that contains radioactive dTTP, due to the semi-conservative nature of DNA replication, each new strand will contain radioactive dTTP. Thus, following one division cycle, both cells with contain radioactive dTTP in the newly synthesised DNA strands.

In the next cycle, since no radioactive substrate is provided, there will be no incorporation of radioactive dTTP. Thus, all newly synthesised strands will lack radioactivity.

From the resulting four cells, only two cells will have radioactive dTTP incorporated in their DNA.