Control of Genes

Control of Genes


Even simple organisms such as bacteria have as many as 3000 genes, producing protein products such as enzymes, cell wall components, building proteins, ribosomal proteins and many others. During a typical cell cycle a bacterial cell may have to make energy releasing proteins all the time, DNA synthesis proteins only during DNA replication, sugar digesting enzymes when it encounters sugar in its environment, and cell wall proteins only when it is about to grow and enlarge. Different proteins are needed at different times, some constantly but many only at specialized moments in the life of the bacterial cell. It would be both wastefull and inefficient to make all proteins continuously. The cell, therefore, needs some method of controling when and where these many different proteins are made

Bacterial Genes

Francois Jacob and Jacques Monod were the first scientists to successfuly investigate how cells control the production of proteins. They used bacteria (because of their simplicity) and developed a model of how these types of cells regulate their genes in response to changes in their environment. Bacteria are opportunists and respond quickly to new sources of food or the presence of dangerous chemicals. Their method of genetic regulation allows them to adapt very rapidly to to what is going on around them.

Jacob and Monod studied how bacteria respond to the sudden appearance of a sugar (called Lactose) in their growth medium. To digest this sugar the cells must make an enzyme called Beta-galactosidease. Since Lacatose sugar is rare, and not always available, it would be wasteful to make this specialized enzyme when it was not needed, so most bacterial cells do not make it until needed.


These scientists discovered that the sudden appearance of Lactose sugar cause the bacterial cells to turn on transcription (making mRNA) of the gene for the enzyme. This type of control of a gene is called transcriptional control, and turned out to be one of the major ways all cells control their genes and the production of proteins.