Control of Gene Expression
Prokaryote

Gene

  • a hereditary unit occupying a specific position (locus) within the genome.
  • Set of segments of nucleic acid that contains the information necessary to produce a functional RNA product in a controlled manner

Structure of a gene

  • Promoter regions are base pair sequences that specify where transcription beings.
  • Coding sequence is a base pair sequence that include coding information for the RNA specified by the gene
  • The terminator sequence that specifies the end of the RNA transcript

Phenotype = the observed characteristic of the
organism

  • Determined by an individuals genotype and expressed genes

Genotype = the genetic makeup of an individual
organism

  • Through interactions through the environment a genotype is expressed in a phenotype

Gene expression - The process by which DNA direct protein synthesis.

  • Includes 2 stages
    • Transcription
    • Translation

Conditions inside the cell and its surroundings can influence the cells DNA Screen Shot 2019-04-17 at 12.01.52

Genetic Orchestra

  • Prokaryotes and eukaryotes alter gene expression in response to their changing environment
  • In multicellular eukaryotes gene expression regulate development and is responsible for differences in cell types - cell differentiation
  • RNA molecules play many roles in regulating gene expression in eukaryotes
  • Bacteria respond to environmental change by regulating transcription
  • Natural selection has favoured bacteria that produce only products needed by that cell.
  • Cell can regulate the production of enzymes by feedback inhibition or by gene regulation
  • Gene expression in bacteria is controlled by the operon model
  • Operon theory gene that function together are regulated together and are known as an operon

Gene terminology

  • Constitutive genes
    • these are genes that are constantly switched on. Transcribed continuously
    • genes for glucose metabolism
  • Facultative genes
    • genes that are transcribed only when needed

Operon theory

  • Several distinct genes situated in tandem
  • All controlled by a common regulatory region
  • The message produced from an operon is polycistronic in that the information for all of the structural genes will reside on one mRNA molecule.
  • Regulation of these genes is coordinated
  • Transcription depends on a common regulatory region

  • Structural genes - codes for proteins and RNA molecules required for normal enzymatic function


  • Regulator gene - code for proteins and RNA molecules which regulate the expression of structural genes

  • Cistron - region of DNA that encodes for a single polypeptide (or functional RNA molecule)
  • Polcistronic RNA - a single mRNA encoding several different polypeptide chains

Operons

  • Entire stretch of DNA that includes the operator, the promoter and the genes that they control.
  • A cluster of functionally related genes. Can be under coordinated control by a single on-off switch
  • Regulatory switch is a segment of DNA called an operator usually positioned within the promoter.
  • RNA polymerase must bind to the promoter site and continue past the operator site to transcribe mRNA
  • Can be switched off by a protein repressor
  • Repressor prevents gene transcription by bind to the operator and blocking RNA polymerase
    -Repressor is the product of a separate regulatory gene
  • Repressor can be in active or inactive form, depending on the presence of other molecules
  • Glucose is the main fuel used in E.coil
  • Used as a carbon and energy source
  • If glucose not available lactose is used instead

3 proteins important for lactose metabolism
These 3 proteins are all encoded in one single expressible unit called the lac operon


  1. Galactoside permease


    • Transport protein that pumps lactose into the cell

  2. B-galactosidase


    • Intracellular enzyme that breaks down lactose into galactose and glucose

  3. B-galactoside transacetylase


    • Enzyme that transfers an acetyl group from acetyl-CoA to B-galactosides

2 Types of Negative Gene Regulation

  • Repressible operon is one that is usually on
  • Binding of repressor to the operator shuts off transcription
  • The trp operon is a repressible operon
  • Inducible operon is one that is usually off
  • Molecule called an inducer inactivates the repressor and turns on transcription

What happens in the presence of lactose

  • Lactose enters E.coli small amounts of allolactose an isomer of lactose are formed
  • Allolactose bind to an allosteric site on the repressor protein causing a conformational change
  • As a result of this change the repressor can no longer bind to the operator region and falls off
  • RNA polymerase can then bind to the promoter and transcribe the lac genes
  • Allolactose is called an inducer because it turns on or induces the expression of the lac genes

If the lac I gene becomes mutated such that the repressor encoded no longer binds to allolactose the repressor would bins to the operator regardless of the presence or absence allolactose and the operon would never be transcribed at high levels.

Catabolite repression - Cell will not metabolise lactose if there is sufficient glucose

CAP = Catabolite Activator Protein site
Catabolite Activator Protein (CAP) is also known as Cyclic AMP Receptor Protein (CRP) and is coded for by the crp gene in E. coli.

  • CAP - ability of glucose to control the expression of a number of different inducible operons.
    • CAP bind to cAMP
    • Inverse relationship between glucose levels and cAMP levels in bacterial cells
    • Glucose levels high = cAMP levels low (and visa versa)
  • cAMP-CAP complex binds to a site on the DNA in the promoter region
  • Binding of the complex enhances the activity of the promoter, more transcripts are initiated, this is called positive regulation

Regulation

  • Constitutive genes are expressed at a fixed rate
  • Other genes are expressed only as needed
    • Repressible genes
    • Inducible genes
    • Catabolite repression