Study Guide 13
Meiosis (and Genetic Variation)
Mendel & the Gene (linkage and crossing over)
How Do Genes Work
DNA = genes; the genetic code)
- Transcription and
Control of Gene Expression in Bacteria
Control of Gene Expression in Eukaryotes
Controlling Gene Expression in Bacteria
Gene expression is defined here as the synthesis of the final product of the information coded by a
It does not include controls on the activities of finished gene products
regulation of enzymes).
Thus, the control of rRNA gene expression would be assayed by measuring
and rate of synthesis
of rRNA in the cell.
Likewise, the control of expression of the
gene for glucose hexokinase (the first glycolytic enzyme) would be assayed by measuring the amount
and rate of synthesis of the enzyme.
If you detected that the levels of an enzyme rose during
development of an organism, or in response to some chemical treatment, you would want to know at
what point in the pathway to making the enzyme is regulation actually taking place.
An increase in
the amount of enzyme present (or the amount synthesized per unit time) occur by:
increasing the rate of transcription of the gene for the enzyme, making more mRNA available,
decreasing the rate of mRNA degradation, also making more mRNA available,
increasing the rate of translation by increasing the rate of loading of ribosomes on the mRNA,
increasing the rate of translation by increasing the rate of elongation of the polypeptide,
decreasing the rate of degradation of the protein,
Theoretically one or more of the steps above might be controlled.
In bacteria, gene expression is
), the most
common being transcriptional control, usually with a molecular on/off switch.
Since prokaryotes do
not undergo development in the familiar way of higher organisms, gene expression is usually
regulated in response to
(e.g., nutrients) or
the buildup of excess internal
2. The Control of Gene Expression by regulating transcription in Bacteria
Gene regulation was first seen in bacteria by investigators like Jacob & Monod who studied the
effects of mutations.
As with bread mold, many mutations were found to affect individual enzymes
in known biochemical pathways.
These were called
But some mutations
were discovered that caused a drop in the levels of multiple enzymes in the same biochemical
Because these mutations affected more than one enzyme, it was hypothesized that a
that had affected a gene that controlling the production of groups of related
In fact, genes performing related functions are often coordinately regulated. In bacteria,
this is facilitated by the organization of multiple genes into