Regulation of gene expression

All the cells of our body contain identical genes. However, the cells of our muscles look different and fulfil very different functions to, for example, the cells of our eye. So how is this possible?The answer is that not all the ≅20 000 genes are expressed or “switched on” in every cell: specific genes are expressed at specific times in specific cells.The control of gene expression is highly complex and a detailed description is beyond the scope of this course.However, in order to provide you with a flavour of this carefully orchestrated system, we will briefly consider three mechanisms which contribute to the regulation of gene expression:

Epigenetic regulation

The epigenome plays a critical role in the regulation of gene expression both through direct modification of DNA (such as DNA methylation) or through chromatin remodeling (how tightly the DNA is wrapped around the histone proteins).

The production of transcription factors

Transcription factors form complexes and bind to the promoters of genes to initiate transcription.Some components of the transcription complex are always present in cells whilst others are only formed in response to specific stimuli enabling cells to respond to changes in their environment and take on specific/targeted roles.

The regulation of protein production

The central dogma describes how DNA encodes RNA which in turn determines the type and order of the constituent amino acids of a protein. Given that it is the different proteins which determine a cell’s characteristics, regulation of protein production, and in particular transcription, is key to the regulation of gene expression. Diagram of gene expression Click to expand
_{© St George’s, University of London}

Talking point

The control of gene expression is really complex and difficult to understand. I sometimes find it helps to use an analogy to help visualise what is going on. For me, the analogy I use is starting and driving a car.The genome is the fully formed car. The epigenetic signal is the switch which causes a whole load of mechanical things to spring into action (as you can tell, I’m no car mechanic!) including the delivery of petrol to the engine (analogous to the transcription factors).These then cause the engine to turn (transcription) and the car can be driven (gene expression).Can you think of another or a better analogy?