When cells are grown in the presence of radioactive thymidine it is incorporated

Question

When cells are grown in the presence of radioactive thymidine it is incorporated into DNA during S phase. The results shown in the graph below come from an experiment in which cells are grown for 30 minutes in the presence of radioactive thymidine then the medium is replaced with medium containing non-radioactive thymidine (in other words, a pulse-chase procedure (for a brief description of pulse-chase experiments, see page 747 of your text)). The pulse chase approach allows investigators to track biomolecules labeled at the time of the pulse. At different times after replacement of the medium, the cells are examined by microscope to determine the number of cells undergoing mitosis (mitotic cells are easy to recognize as their chromosomes are obviously condensed). The fraction of mitotic cells that have incorporated the radioactive thymidine can also be determined and plotted as a function of time after thymidine labeling (see below). a. At 10 hours –essentially 100% of the mitotic cells contain radioactive DNA-what phase of the cell cycle were these cells in when the radioactive thymidine was present? (4 points) b. Why are there no mitotic cells that contain radioactive DNA at the initial time points? ( 4 points)

Explanation / Answer

a). All the cells with the radioactive thymidine in the DNA indicate the "S" phase.

b). The radioactive DNA will be taken up during the S phase. Initially (at t = 0), the cells do not contain radioactive DNA because the first cycle should be completed to see the radioactive labeled DNA.

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