1. Gene expression in prokaryotes
Functional nit of DNA containing a cluster of genes
under a single promoter
Group of genes that are transcribed at the same time
Multiple genes are on and off simultaneously due to
the regulation of one promoter
2. Multiple genes are on and off simultaneously
due to the regulation of one promoter
3. Lac operon
Consists of three genes each involved in the process of
sugar lactose
One of them the gene for the enzyme β-galactosidase
Hydrolysis Lactose in to Glucose and Glactose
E.coli can use either glucose (mono) or Lactose (Di), How
ever Lactose must be hydrolyzed first in to glucose
4. Gene Regulation -- The Lac Operon
transacetylase
Structural genes (genes that code
for polypeptides)
Promoter: A site where RNA polymerase attaches in order to transcribe
mRNA.
I gene is called a regulator gene: is transcribed to make a mRNA which is
translated to a repressor protein. There is a termination signal at the end of the I
gene.
Operator: It is a short sequence of bases that acts like a switch that can be
recognized by repressor protein
5. Situations
When glucose is absent and lactose is present
An Activator protein is needed which stabilises RNA polymerase
The activator protein only works when glucose is absent
In this way E.coli only makes enzymes to metabolise other sugar
in the absence of glucose
Activator protein: Bound to DNA
Repressor protein: Lifted off
operator site
RNA polymerase: Sits on the
promoter site
Lac operon: Transcription
6. Situations
When glucose is present and lactose is absent
Activator protein: Not bound to
DNA
Repressor protein: Bound to
operator site
RNA polymerase: Blocked by the
repressor
Lac operon: No transcription
A repressor protein is continuously synthesised , It sits on a
sequence of DNA just in front of the Lac operon, the Operator site.
The repressor protein block the promoter site where the RNA
polymerase settles before it starts transcribing
7. Situations
When glucose is present and lactose is present
Activator protein: Not bound to
DNA
Repressor protein: Lifted off
operator site
RNA polymerase: keeps falling off
promoter site
Lac operon: No transcription
This explains how the lac operon is transcribed only lactose is present
But it dose not explain why the operon is not transcribed when both
present
When glucose and lactose are present RNA polymerase can sit on the
promoter site but it is unstable and keeps it falling off,
