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Summary Chapter 14 - Gene Regulation in Bacteria
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Chapter 14 of Genetics by Brooker.
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Chapter 14 - Gene regulation in bacteriaGene regulation: The phenomenon in which the level of gene expression can vary under
different conditions.
Unregulated genes = constitutive genes
14.1 Overview of transcriptional regulation
Regulatory proteins:
- Repressor: regulatory protein that binds to the DNA and inhibits transcription
- Activator: regulatory protein that increases the rate of transcription
Small effector molecules:
- Inducer: causes transcription to increase
- Corepressor: binds to the repressor protein, thereby causing the protein to bind to the
DNA
- Inhibitor: Prevents it from binding.
Repressor + Inducer → Transcription
Activator + Inducer → Transcription
Repressor + Corepressor → No transcription
Activator + Inhibitor → No transcription
14.2 Regulation of the lac operon
The phenomenon of enzyme adaptation is due to the synthesis of cellular proteins.
Enzyme adaptation refers to the observation that a particular enzyme appears within a living
cell only after the cell has been exposed to the substrate for that enzyme.
The lac operon encodes proteins involved in lactose metabolism. An operon is a group
of two or more genes that are transcribed from a single promoter. An operon encodes a
polycistronic mRNA, a RNA that contains the sequences of two or more genes. The lac
operon contains:
- CAP site: DNA sequence recognized by an activator protein called catabolite
activator protein (CAP).
- Lac promoter
- Operator site: A sequence of bases that provides a binding site for a repressor
protein called lac repressor
- Lac Z: enzyme cleaves lactose into galactose and glucose
- Lac Y: lactose permease
- Lac A : encodes galactoside transacetylase
- Lac I: encodes lac repressor and situated in front of the lac operon
The lac operon is regulated by a repressor protein. The ability of lac repressor to bind to
the operator site depends on whether or not allolactose is bound to it. When four molecules
of allolactose bind to the repressor, a conformational change occurs that prevents lac
repressor from binding to the operator site. Under these circumstances, RNA polymerase is
free to transcribe the operon. The operon has been induced.
Allosteric regulation: the action of a small effector molecule. Their functioning is controlled by
effector molecules that bind to the proteins' allosteric sites.
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