Gene Expression

Gene Expression - Seeing expression in a single cell...

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Seeing expression in a single cell Bacteria are small; bacteriologists are large. This obvious fact guides how microbiologists study our subjects. Our ultimate goal is to understand what goes on in a cell, but it is very challenging to tell what is going on at such a small scale. Typically, we study a huge population of cells that are (in principle) genetically identical, that have (in principle) grown under identical conditions, and that respond (in principle) to stimuli in an identical manner. We then sample this large population, and deduce the behavior of the “average” cell. In principle, this system works well, and we have learned a huge amount about how a cell works by exploiting this technique. However, it is useful to bear in mind the difference between the averaged behavior of a large population and the idiosyncratic behavior of the single cell. Much of the research I’ve done has focused on changes in gene expression in response to changes in the environment. Most often, the changes in gene expression result from changes in transcription, the process of making a messenger RNA (mRNA) from a DNA template. Transcription begins when the enzyme RNA polymerase binds to a DNA sequence called a promoter, and then proceeds down the DNA molecule, making mRNA. The rate at which this happens varies enormously. A strong promoter can lead to the production of 1,000 times as much RNA as a weak promoter in the same amount of time. But instead of opening up a single cell and seeing 1 or 1,000 mRNAs, I’m used to measuring this by examining a culture of trillions of cells, and seeing that it has made 10,000 times as much of some product as another culture. Looking at these large populations, it is easy to lose track of the molecular events going on at the promoter. One
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Gene Expression - Seeing expression in a single cell...

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