BIOL211 – Fundamentals of Microbiology Notes

BIOL211 – Fundamentals of Microbiology

  • Instructor: Dr/Professor Mercado

  • Semester: Fall 2025

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Chapter 7: Microbial Genetics

Date: Oct 6th, 2025

RNA Polymerase

  • Coding Strand:

    • The strand of DNA that has a sequence complementary to mRNA.

  • Template Strand:

    • The strand of DNA that RNA polymerase binds to synthesize mRNA.

Genetic Mutations

  • Definition:

    • Errors in the mRNA sequence.

  • Types of Mutations:

    • Single Base Mutations:

      • Changes in a single nucleotide of DNA.

    • Neutral Mutation:

      • A single base mutation with no effect on the organism's fitness.

    • Missense Mutation:

      • A single base mutation causing a change in the amino acid sequence.

    • Nonsense Mutation:

      • A single base mutation resulting in a stop codon, truncating the protein.

    • Frameshift Mutation:

      • Occurs due to deletion or insertion of nucleotides resulting in a complete alteration of the amino acid sequence.

    • Spontaneous Mutation:

      • Random changes in the DNA sequence, generally rare.

Mutagens

  • Definition:

    • Agents that increase the likelihood of mutations in the DNA sequence.

  • Types of Mutagens:

    • Chemical Mutagens:

      • Examples include intercalating agents that insert between DNA bases.

    • Radiation:

      • UV radiation can cause thymine dimers, leading to mutations.

Summary of Mutations

  • Mutations:

    • Changes in DNA sequence from errors during replication.

  • Types of Mutations Recap:

    • Neutral, Missense, Nonsense, Frameshift, Spontaneous

  • Mutagens:

    • Increase the likelihood of mutations.

Learning Objectives

  • Define operons and their role in gene expression in bacteria.

  • Distinguish between inducible and repressible operons.

  • Define conjugation and understand its significance in bacterial genetics.

Gene Expression

  • Definition:

    • The biological conversion of the nucleotide sequence of DNA into a protein.

  • Genes:

    • Composed of DNA, serve as information molecules and are expressed to produce specific proteins.

Regulation of Gene Expression

  • Regulated Genes:

    • Genes whose expression can be controlled (turned ON or OFF).

  • Non-Regulated Genes:

    • Always expressed, also referred to as constitutive genes.

Operons in Bacteria

  • Definition:

    • Clusters of genes that are transcribed together and typically involved in a related process.

  • Features of Operons:

    • Each gene in an operon codes for a specific protein.

    • Contains regions that regulate activity.

  • Prokaryotic vs Eukaryotic:

    • Prokaryotic (bacterial) genes are organized into operons; eukaryotic genes are regulated individually.

Inducible Operons

  • Definition:

    • A system where genes are typically OFF but can be turned ON by an inducer molecule.

  • Components:

    • Inducer:

      • A substrate that can sequester a repressor protein, facilitating gene expression.

    • Promoter:

      • A DNA sequence where RNA polymerase binds to initiate transcription.

    • Operator:

      • A regulatory sequence acting as a binding site for repressor or inducer proteins.

    • Repressor:

      • A protein that inhibits transcription when bound to the operator.

Lac Operon: A Classic Example of Inducible Operon

  • Conditions:

    • No Lactose Present:

      • Repressor is active; transcription does not occur.

    • Lactose Present:

      • Lactose binds to repressor (via allolactose), enabling transcription of lacZ, lacY, and lacA genes by RNA polymerase.

Repressible Operons

  • Definition:

    • A system that is normally ON but can be shut OFF by a corepressor molecule.

  • Components:

    • Corepressor:

      • A substrate that can sequester the enhancer protein, stopping transcription.

    • Promoter, Operator, Enhancer:

      • Similar roles to the components described for inducible operons.

Trp Operon: A Classic Example of Repressible Operon

  • Conditions:

    • Low Tryptophan Levels:

      • RNA polymerase transcribes trpE, trpD, trpC, trpB, and trpA genes.

    • High Tryptophan Levels:

      • Tryptophan binds to the repressor protein, inhibiting transcription by binding to the operator.

Quorum Sensing

  • Definition:

    • A regulatory mechanism where bacterial cells sense population density to modulate gene expression.

  • Process:

    • Bacteria produce autoinducers that bind to receptors influencing operon activity.

    • At low population density, autoinducer diffuses away; at high density, it accumulates and activates gene expression.

Eukaryotic Gene Regulation

  • Mechanisms:

    • Utilize various methods to activate and inhibit gene expression.

    • Inactive Genes:

      • DNA is tightly coiled around proteins; thus, these genes are not expressed.

    • Active Genes:

      • DNA is loosely coiled allowing for transcription.

    • Role of RNA:

      • microRNA can regulate gene expression.

Summary of Gene Regulation

  • Bacteria express genes as operons (inducible or repressible).

  • Quorum sensing influences gene expression in bacteria.

  • Eukaryotic gene expression involves DNA coiling and RNA regulation mechanisms, which are still under investigation.

Genetic Variation

  • Definition:

    • Variability in genetic material among living organisms.

  • Importance:

    • Contributes to both genotype (the genetic makeup) and phenotype (the expressed traits) of organisms.

    • Genotype:

      • The specific genetic configuration of an organism.

    • Phenotype:

      • The observable expression of the genotype.

Reproduction in Unicellular Organisms

  • Process:

    • Bacteria reproduce asexually through binary fission.

    • A parental cell divides to form two identical daughter cells, retaining the same DNA as the parent.

Horizontal Gene Transfer

  • Definition:

    • A method for achieving genetic recombination in asexually reproducing organisms.

  • Mechanisms:

    1. Transformation:

      • Uptake of free DNA from the environment by a bacterial cell (recipient) from a dead bacterial cell (donor).

    2. Conjugation:

      • Direct transfer of plasmids between prokaryotic cells.

    3. Transduction:

      • Gene transfer facilitated by bacteriophages.

    4. Transposons:

      • Segments of DNA that can move (or