Ras mRNA (mRNA transcribed from the Ras gene) is found at a concentration of 100 molecules per nerve cell at time zero. At time zero, you irradiate the cells with UV light such that transcription is turned off. After 40 minutes, you determine that the concentration of Ras mRNA is now 25 molecules per cell. What is the half-life of Ras mRNA in these cells?

The half-life of Ras mRNA in nerve cells is determined to be 20 minutes, as the concentration drops to one-fourth in 40 minutes, indicating two half-lives have passed.

Explanation:

To determine the half-life of Ras mRNA in nerve cells, we can use the given data where the concentration of Ras mRNA drops from 100 molecules per cell to 25 molecules per cell after being irradiated with UV light, which inhibits transcription.

Since the concentration of mRNA falls to one-fourth of its original value after 40 minutes, we need to calculate the time it takes for the amount to reduce by half, known as the half-life.

By definition, after one half-life, the concentration would drop to 50 molecules per cell (half of 100), and after two half-lives, to 25 molecules (half of 50). Since this occurs over a period of 40 minutes, it means that the mRNA has undergone two half-lives, so one half-life would be 20 minutes.