Chapter 20 describes a blotting method known as Northern blotting, which can be used to detect RNA transcribed from a particular gene or a particular operon. In this method, a specific RNA is detected by using a short segment of cloned DNA as a probe. The DNA probe is complementary to the RNA that the researcher wishes to detect. After the probe DNA binds to the RNA within a blot of a gel, the RNA is visualized as a dark band. For example, a DNA probe complementary to the mRNA of the lac operon could be used to specifically detect the lac operon mRNA on a gel blot. As shown here, the method of Northern blotting can be used to determine the amount of a particular RNA transcribed under different types of growth conditions. In this Northern blot, bacteria containing a normal lac operon were grown under different types of conditions, and then the mRNA was isolated from the cells and subjected to Northern blotting, using a probe that is complementary to the mRNA of the lac operon.

Based on your understanding of the regulation of the lac operon, explain these results. Which is more effective at shutting down the lac operon, the binding of the lac repressor or the removal of CAP? Explain your answer based on the results shown in the Northern blot.

2 Lane 1. Growth in media containing glucose Lane 2. Growth in media containing lactose Lane 3 Growth in media containing glucose and lactose Lane 4. Growth in media that doesnt contain glucose or lactose