Page 55 - Cell biology PDG 2024

such proteins to the DNA can regulate transcription by either preventing
or increasing transcription from a particular promoter.
Regulation of transcription in bacteria
In bacteria, genes are often clustered in groups i.e. genes that are
expressed at the same time are located next to each other and all of them
are controlled by the same promoter.
Example: The lac operon, it is a group of genes coding for proteins
needed for the uptake and breakdown of the sugar lactose. It consists of
three genes lac z, lac y and lac a. All are controlled by a single promoter.
Bacteria take the nutrient glucose to supply it with energy, but if glucose
is not available in the environment and lactose is present, the bacteria will
take up lactose and use it as a source of energy instead of glucose. So the
proteins for taking up and breaking down lactose are only needed when
glucose is not present and lactose is available.
The bacterial cells will express the genes of the lac operon only when
glucose is absent and lactose is present. At times when lactose is absent,
the cells do not need to express these genes

Figure 22: the Lac operon

How do bacteria achieve this? The transcription of the genes of the Lac
operon is controlled by a repressor protein that binds to a region on the
DNA that is downstream the lac promoter. Remember that the promoter
is where the RNA polymerase must bind to begin transcription. The place
where the repressor is bound is called the operator (O). When the
repressor is bound at the operator, it prevents the RNA polymerase from
transcribing the genes of the lac operon

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