Mechanisms of Microbial Genetics

Central Dogma

  • Definition: The central dogma describes the flow of genetic information from DNA to RNA to protein.
  • Gene Expression: It involves replication, transcription, and translation.
  • Influences on Gene Expression: Environment, phenotype vs. genotype.

Bacterial DNA Replication

  • Location: Occurs in the cytoplasm at the nucleoid.
  • Process: Must happen before binary fission, resulting in two copies of DNA.
  • Key Components:
    • Template: Double-stranded DNA.
    • Players Involved: DNA polymerases, helicase, ligase, primase, topoisomerases.
    • Purpose: To replicate DNA for cell division.
  • Steps:
    1. Initiation: Begins at oriC, involving topoisomerase II and helicase.
    2. Elongation: DNA is synthesized by DNA pol III.
    3. Termination: Topoisomerase IV separates concatenated chromosomes.

Transcription in Bacteria

  • Process: DNA is transcribed to form RNA.
  • Template: Only one strand of DNA serves as the template.
  • Players Involved: RNA polymerase.
  • Steps:
    1. Initiation: Starts at the promoter.
    2. Elongation: RNA is synthesized complementary to the DNA template.
    3. Termination: RNA synthesis stops at the termination signal.
  • Polycistronic mRNA: Bacteria can encode multiple polypeptides on a single mRNA.

Translation in Prokaryotes

  • Location: Occurs at ribosomes in the cytoplasm.
  • Steps:
    1. Initiation: tRNA binds to the start codon with methionine.
    2. Elongation: tRNA brings amino acids corresponding to mRNA codons.
    3. Termination: Occurs when a stop codon is reached.

Comparison: Bacterial vs. Eukaryotic Replication

  • Genome Structure: Prokaryotes have a single circular chromosome; eukaryotes have multiple linear chromosomes.
  • Origins of Replication: Prokaryotes have one; eukaryotes have many.
  • Rate of Replication: Prokaryotes ~1000 nucleotides/sec; eukaryotes ~100 nucleotides/sec.
  • RNA Primer Removal: Prokaryotes utilize DNA pol I; eukaryotes utilize RNase H.
  • Presence of Telomerase: Not present in prokaryotes, present in eukaryotes.

Horizontal Gene Transfer

  • Transformation: Uptake of naked DNA from the environment.
  • Transduction: Gene transfer via bacteriophages.
  • Conjugation: Transfer of DNA between bacteria via a conjugation pilus.

Operons and Regulation

  • Definition: A cluster of genes transcribed as a single mRNA.
  • Transcription Factors: Proteins that regulate transcription.
  • Operators and Promoters: The operator is where repressors bind to regulate gene expression.
  • Types of Operons:
    • Repressible Operons: Turned off when the end product is present (e.g., trp operon).
    • Inducible Operons: Turned on in the presence of a substrate (e.g., lac operon).

Trp Operon Regulation

  • Function: Active when tryptophan is absent, blocked when tryptophan is present.
  • Mechanism: Repressor binds to operator in presence of tryptophan, blocking transcription.

Lac Operon Regulation

  • Function: Active when lactose is present; repressed in its absence.
  • Mechanism: Repressor binds to operator in absence of lactose, blocking transcription. Lactose presence weakens repressor’s binding.

Catabolite Repression

  • Definition: The presence of glucose inhibits the lac operon.
  • Role of cAMP: High cAMP levels indicate glucose absence, promoting transcription of the lac operon when lactose is available.