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Gene Regulation in Prokaryotes

• Prokaryotes control gene expression primarily by regulating transcription.
• Many genes are co-regulated through operon systems.
• There are three types of genes:
  • Constitutive: Always transcribed, with gene products always needed.
  • Repressible: Usually on but can be turned off, often by the end product of the metabolic pathway.
  • Inducible: Usually off but can be turned on, typically by the presence of a specific substrate.

Lac Operon

• Composed of genes responsible for lactose metabolism.
• Functions as an inducible system.
• Normally "off" because lactose is not commonly present.
• Induced when lactose is present.

Mutation

• A change in DNA sequence.
• Can be spontaneous or induced.
• May or may not alter the amino acid sequence of a protein.
• Types of mutations:
  • Point Mutations: Single base change.
    • Silent: No change in amino acid sequence.
    • Missense: Single amino acid change.
    • Nonsense: Introduction of a premature stop codon.
  • Frameshift: Insertion or deletion of a base, changing the reading frame downstream.

Causes of Mutations

• Chemical mutagens:
  • Base-pair mutagens: Alter base-pairing rules.
  • Base analogs: Incorporated as nucleotides but have altered base-pairing rules.
  • Frameshift mutagens: Intercalate into DNA and cause DNA polymerase to make insertions or deletions.
• Radiation:
  • Ionizing: Damages DNA, creates free radicals.
  • Nonionizing: Creates thymine dimers.

Detecting Mutagens: Ames Test

• Utilizes an auxotrophic strain of Salmonella typhimurium.
• Requires histidine (his-) for growth.
• Measures the frequency of revertants to his+ phenotype, which is increased in the presence of mutagens.

Horizontal Genetic Exchange

• Transfer of genetic information between members of the same generation.
• Occurs exclusively in prokaryotes.

Three Methods of Horizontal Recombination

• Transformation: Uptake of "naked" DNA by competent cells.
• Transduction: Transfer of DNA by bacteriophages.
  • Generalized Transduction: Bacteriophage packages host DNA into new phage particles.
• Conjugation: Transfer of genetic material via a sex pilus.
  • F+ Conjugation: F plasmid containing the genes for sex pilus is transferred from F+ cell to F- cell.

Transformation

• Competent cells are able to take up "naked" DNA.
• Recombination can occur when DNA sequences are homologous, replacing the old DNA sequence with the new one
• Results in the recipient cell acquiring a new trait.

Transduction

• Bacteriophage packages host DNA into new phage particles.
• Recipient cell acquires new traits.

Conjugation

• Requires a sex pilus encoded by the F plasmid.
• In standard F+ conjugation, a copy of the F plasmid is transferred from the donor (F+) cell to the recipient (F-) cell, making the recipient cell F+.