Lecture notes
LAC OPERON
- Module
- Biomedical Sciences (BB2704)
- Institution
- Brunel University (BU)
GENE EXPRESSION IN PROKARYOTES - LAC OPERON
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Control of gene expression in prokaryotes (Lac operon)Gene regulation: refers to the ability of cells to control their level of gene expression
In an organism, some proteins are required continuously, the genes encoding them
are expressed constitutively. Many other proteins are made only when they’re
needed as they’re only required for specific purposes, the genes encoding them are
inducible/expressed under particular conditions. Expression of inducible genes can
be either positively or negatively regulated. The regulation of the expression of
inducible genes involves the actions of regulatory proteins. They activate/induce or
repress the expression of inducible genes…
- Positive regulation (induction) is often used in the catabolism of substrates for
energy production; the presence of the substrate stimulates expression of
genes in the metabolic pathway
- Negative regulation (repression) is used in anabolic pathways, whereby the
absence of the substrate represses the expression of the genes in the
metabolic pathway
There are many potential control points in the gene expression pathway as
organisms regulate gene expression at a variety of levels…
- Transcriptional regulation
- Post transcriptional regulation
• includes co-transcriptional control – RNA splicing
• mRNA stability and transport
- During translation
- Post-translational regulation
• protein stability
• activation or inactivation (e.g phosporylation, ADP-rybosylations etc)
In bacteria, the most common way to regulate gene expression is by affecting the
rate at which transcription is initiated, meaning the rate of transcription can be
increased or decreased. Regulation of transcription requires action of inducers or
repressors.
In bacteria genes for enzymes of metabolic pathways are arranged in groups of 2 or
more genes, these groups are called ‘operons’. The genes forming the operon are
located adjacent to each other in the DNA and are under the control of a single
promoter; a single promoter is responsible for the transcription of 2 or more genes.
- All genes located together in an operon can be transcribed as a single unit
- An operon encodes a single polycistronic mRNA
- The ‘operator’ is the binding site of a repressor/regulatory protein
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