A multicellular organism, even the simplest one, contains different kinds of cells. It grew from a single zygote by cell division. This division conserves the number of chromosomes and the quantity of DNA. Although all the cells contain the same DNA, there are some differences in their roles and their contents. We have shown that, in a chapter about cell differentiation, genes are expressed only in the cells that require their expression, and when they are needed. A gene for insulin is present in all the cells of the organism, but insulin is produced only in the pancreas. Insulin is produced in the pancreas in response to certain levels of glucose in the blood. How is this gene regulation brought about?
By studying prokaryotes, Francois Jacob and Jacques Monod discovered some of the most valuable concepts in gene regulation. In bacteria, like Escherishia coli, many of the enzymes are synthesised all the time, while some of the enzymes are synthesised only when needed as is the case for the enzyme that controls the metabolism of lactose. Usually, E. coli uses glucose. In the absence of glucose, and in presence of lactose, the bacteria's growth will pause and then restart. The pause reflects the synthesis of the enzyme responsible for the metabolism of lactose. The expression of the gene for this enzyme is induced by lactose.
A gene (represented in Yellow in Figure 1) is preceded by a small sequence, which is called the operator (green). The RNA polymerase (purple) can start polymerising RNA from any part of any gene, and even an intron. In most cases, the synthesis stops because the polymerase is not locked properly on the DNA. It is only when the polymerase can stick to an operator that it can be locked, and can start polymerising RNA on the whole gene.
Monod and Jacob demonstrated that the gene for this enzyme is constantly repressed by a protein (a repressor). This repressor is fixed to the DNA and prevents the transcription of the gene into RNA. In the presence of allactose (from lactose), the repressor is removed, and transcription can take place.
The repressor is a protein whose shape varies depending on the presence of allactose (Blue). In the absence of allactose, the repressor (red in Figure 1 and 2) has a shape that fits the operator. The RNA polymerase cannot access the operator, and the synthesis of RNA cannot take place. In the presence of allactose, the repressor changes shape and does not fit the operator anymore. The repressor detaches from the DNA, and the polymerase can lock itself on the operator. The synthesis of RNA can start.
When the RNA is synthesised, it is used up and translated into protein. The protein synthesised enables the cell to metabolise glucose.