The stages of the cell cycle for a cultured mammalian cell line require the

following periods of time: G1 = 8 hr, S = 5 hr, G2 = 1 hr, M = 1 hr. An

asynchronous culture of these cells is exposed to radioactive thymidine for

five minutes and then allowed to continue to grow in nonradioactive medium.

The figure below shows the results.

If the length of G2 were increased by one hour, which of the following

statements would be true regarding the slope of the line from the first

appearance of labelled mitotic cells until all cells are labelled?

It would increase.

It would decrease.

It would stay the same.

It would switch from a positive to a negative slope

Article

The cell cycle in cultured mammalian cells consists of several distinct phases with defined durations: G1 (8 hours), S (5 hours), G2 (1 hour), and M (1 hour). These phases collectively govern cell growth and division timing.

In a classic experiment, an asynchronous cell population is briefly exposed to radioactive thymidine, which incorporates into DNA during the S phase, marking cells synthesizing DNA at that precise interval. After the pulse, cells are returned to nonradioactive medium, and the appearance of labelled cells in mitosis is monitored.

The Experimental Setup

• Initial durations: G1 = 8 hr, S = 5 hr, G2 = 1 hr, M = 1 hr

• Pulse labeling: Radioactive thymidine for 5 minutes

• Tracking: Appearance of labelled mitotic cells over time

Understanding the Slope of Labelled Mitotic Cells

The slope referenced here corresponds to the rate at which labelled mitotic cells increase from the first appearance until all mitotic cells are labelled. This slope depends primarily on the length of the G2 phase because:

• Cells are labelled only during S phase.

• After S phase ends, cells pass through G2 before entering mitosis.

• The time between DNA labelling and the appearance of labelled mitotic cells reflects the G2 duration.

Impact of Increasing G2 from 1 to 2 Hours

If G2 is increased by 1 hour, it delays the time for cells to transition from DNA synthesis completion to mitosis. This lengthened delay spreads out the initial rise in labelled mitotic cells, slowing the rate at which they accumulate.

Explanation of Each Option

• It would increase: This would mean the rise of labelled mitotic cells is faster due to longer G2, which is incorrect. A longer G2 slows the progression to mitosis, reducing the slope.

• It would decrease: Correct. Increasing G2 increases the lag before labelled mitotic cells appear, spreading the increase over a longer time, thus decreasing the slope.

• It would stay the same: Incorrect. Changing the G2 length affects the timing from S phase to mitosis, altering the slope.

• It would switch from a positive to a negative slope: Incorrect. The slope reflects accumulation over time and will remain positive as more cells enter mitosis labelled; it will not become negative.

Summary

Increasing the G2 phase duration in a cultured mammalian cell line slows the progression from DNA synthesis to mitosis, decreasing the slope of labelled mitotic cells’ appearance after a pulse of radioactive thymidine.