Operons
erons
Operons
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
Operons
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