Chapter 8 Lecture: Microbial Genetics (Part One)

Overview

• This lecture covers DNA replication, transcription, translation, and mutations.
• Central Dogma of Molecular Biology: DNA → RNA → Protein.
• Mutations can be detrimental, beneficial, or neutral.
• Gene expression controlled by operons.

Genetics Basics

• Genetics: Study of genes, gene function, and gene transmission.
• Central Dogma: DNA is transcribed into RNA, which is translated into protein.
• Mutations: Changes in DNA sequence; affect RNA and protein produced.
• Gene expression: Turning genes into functional products (proteins).

DNA Structure & Replication

• DNA: Double helix made of nucleotides (sugar, phosphate, base).
  • Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).
  • Strands are anti-parallel (5’ → 3’ direction).
• DNA Replication: Occurs during cell division.
  • Enzymes involved:
    • Topoisomerase & Gyrase: Relax DNA strands.
    • Helicase: Unwinds DNA, creating replication fork.
    • DNA Polymerase: Synthesizes new DNA strands.
  • Strands:
    • Leading Strand: Continuous synthesis toward replication fork.
    • Lagging Strand: Discontinuous synthesis, forms Okazaki fragments.
    • DNA Ligase: Joins Okazaki fragments.
  • Replication is bidirectional and involves proofreading.

RNA and Protein Synthesis

• Transcription: DNA → RNA (specifically mRNA).

  • RNA Polymerase synthesizes mRNA using DNA template.
  • Promoter: DNA sequence where transcription begins.
  • Terminator: DNA sequence where transcription ends.

• Translation: mRNA → Protein.

  • Ribosome: Site of protein synthesis.
  • tRNA: Transfers amino acids; has anticodon complementary to mRNA codons.
  • mRNA Codons: Triplets that specify amino acids.
  • Start Codon: AUG (Methionine).
  • Stop Codons: UAA, UAG, UGA.
  • Process involves ribosomal subunits assembling on mRNA, with tRNA bringing amino acids to form a polypeptide chain.
  • In bacteria, transcription and translation can occur simultaneously.

Gene Regulation

• Constitutive Genes: Always expressed.
• Inducible Genes: Normally off, can be turned on.
• Repressible Genes: Normally on, can be turned off.
• Operons:
  • Promoter: Initiates transcription.
  • Operator: Controls transcription.
  • Example: Lac operon (inducible, regulated by lactose presence).

Additional Regulation Methods

• Methylation: Adding methyl groups to nucleotides can turn genes off.
• Post-transcriptional Control:
  • Riboswitches: Bind molecules, changing mRNA structure to block translation.
  • MicroRNA: Base pair with mRNA, causing degradation.

Mutations

• Types:
  • Base Substitution (Point Mutation): Single nucleotide change.
    • Missense Mutation: Changes amino acid.
    • Nonsense Mutation: Creates stop codon, truncating protein.
  • Frameshift Mutation: Insertion/deletion shifts reading frame.
• Mutagens: Agents causing mutations (chemical, spontaneous).
  • Example: Nitrous acid causes adenine to pair with cytosine.

Conclusion

• Mutations are integral to microbial genetics, affecting DNA and protein functions.
• Understanding these processes aids in disease prevention, treatment, and biotechnology applications.