Control of Gene Activity in Prokaryotic Cells
I. The activity of genes is controlled by the cell and the environment. A. Inducible genes are inactive unless circumstances cause them to be activated (“turned on”). B. Repressible genes are active unless circumstances cause them to be inactivated (“turned off”). C. Constitutive gene functions are active continually, with no control exerted. This is generally an abnormal situation.
II. In prokaryotic cells (and viruses) the control of gene activity is often in the form of operons. A. Operons are a form of transcriptional control.
B. An operon consists of the structural gene (or genes) which actually code for specific proteins and the controlling elements associated with the control of those genes. An operon typically contains several genes, all under the same control mechanism. C. Though rather similar controlling systems have been found for some eukaryotic genes, control mechanisms in eukaryotes are generally more diverse and more complex, and except for a few examples in simple eukaryotic organisms like yeasts, multiple genes are not found to function under a single control mechanism. In other words, eukaryotic cells do not have operons.
III. The first operon investigated was the lac operon in E. coli. This work came from Jacob and Monod (1959 Journal of Molecular Biology V. 1). A. The primary carbohydrate source for the cell is the sugar glucose, but there are a large number of sugars which can be used if there isn’t enough glucose available to support the energy needs of the cell. Sugars like lactose are “backup” carbohydrates. This means that the cell only metabolizes lactose if glucose is low and lactose is plentiful. B. The cell uses a negative control system (the lac operon) to respond to the availability of lactose in the environment. Negative control means that it’s a system in which the active substance acts to turn off function. It is very wasteful to manufacture the enzymes needed to metabolize lactose if there is no lactose to be used, so under normal conditions the genes for producing the enzymes for metabolizing lactose need to be inactive. They should be activated only when lactose is plentiful. So the purpose of the operon is to keep these genes turned off if there’s no lactose around, and to turn them on if lactose is plentiful. 1. The lac operon contains three genes which code for enzymes necessary for the metabolism of the sugar lactose. These three genes are side by side on the DNA molecule, and they are transcribed as a single, polycistronic message. The operon consists of a promoter, an operator, the structural genes, a termination sequence, and a repressor gene. a. The lac z gene codes for beta-galactosidase. b. The lac y gene codes for permease
c. The lac a gene codes for transacetylase
2. There is a single promoter region which precedes the lac z gene (the first gene in the sequence). The promoter region is where the RNA polymerase binds for transcription. 3. Between the Promoter and the lac z gene is a region called the Operator. This is the primary control site for the operon. 4. The repressor gene (lac I) is not contiguous with the other portions of the operon. The repressor codes for the production of a diffusible repressor protein. So our entire operon could be diagrammed like this: [pic]
lac I is the name for the repressor gene; P is the promoter, O the operator, and T the termination sequence. Note that the prokaryotic genome is a circular DNA molecule, and we are viewing just one segment of that circle. Also note that the normal condition for a prokaryotic cell is to have a single copy of its genome in the cell. 5. When glucose is plentiful and/or lactose is low, the desired situation is for these genes to be repressed (inactive)....
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