Microbial Genetics

Big Picture: Genetics

Microbial genetics explores how genes are structured, expressed, and altered in microorganisms. Understanding these processes is crucial for explaining microbial diversity, disease mechanisms, and biotechnological applications.

• Central Dogma of Molecular Biology: Describes the flow of genetic information: DNA → mRNA → Protein → Function.

• Mutations: Changes in DNA sequence can alter protein function and microbial traits.

• Gene Expression Controlled by Operons: Operons regulate groups of genes, allowing coordinated expression in response to environmental signals.

• Alteration of Genes: Can cause disease, prevent disease, or be manipulated for human benefit.

Structure and Function of Genetic Material

Genetic material in microorganisms is organized to ensure accurate inheritance and expression of traits.

• Genetics: Study of genes, their information, expression, and replication.

• Chromosomes: Structures containing DNA and genes; bacteria typically have a single circular chromosome.

• Genes: DNA segments encoding functional products, usually proteins.

• Genome: All genetic information in a cell.

• Genetic Code: Rules for converting nucleotide sequences into amino acid sequences.

Genotype and Phenotype

Genotype refers to the genetic makeup, while phenotype is the observable expression of those genes.

• Genotype: The DNA sequence of an organism.

• Phenotype: The physical and functional expression of genes.

DNA and Chromosomes in Bacteria

Bacterial chromosomes are typically single, circular DNA molecules associated with proteins.

• Vertical Gene Transfer: Genetic information passed from parent to offspring.

The Flow of Genetic Information

Genetic information flows through replication, expression, and recombination.

• Replication: DNA is copied for cell division.

• Expression: DNA is transcribed and translated to produce proteins.

• Recombination: Exchange of genetic material between cells.

DNA Replication

DNA replication is a highly accurate, energy-dependent process ensuring genetic continuity.

• Double Helix: DNA consists of two antiparallel strands held by hydrogen bonds (A-T, C-G).

• Replication Fork: Site where DNA strands are separated and copied.

• Enzymes: Topoisomerase, gyrase, helicase, DNA polymerase, primase, and ligase coordinate replication.

• Leading Strand: Synthesized continuously.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments.

• Energy: Provided by hydrolysis of nucleoside triphosphates.

• Bidirectional Replication: Most bacterial DNA replication proceeds in both directions from the origin.

• Proofreading: DNA polymerase corrects errors, ensuring high fidelity.

RNA and Protein Synthesis

RNA is essential for protein synthesis, acting as a messenger, translator, and structural component.

• Types of RNA:

  • mRNA: Carries genetic code from DNA to ribosomes.

  • tRNA: Transports amino acids during translation.

  • rRNA: Integral part of ribosomes.

• Transcription: Synthesis of mRNA from DNA template by RNA polymerase, starting at promoter and ending at terminator.

• Translation: mRNA is decoded into protein; codons (three-nucleotide sequences) specify amino acids.

• Start Codon: AUG; Stop Codons: UAA, UAG, UGA.

• Peptide Bonds: Join amino acids during translation.

• Simultaneous Transcription and Translation: In bacteria, translation can begin before transcription is complete.

Transcription and Translation in Eukaryotes

In eukaryotes, transcription occurs in the nucleus and translation in the cytoplasm. Genes contain exons (coding) and introns (non-coding).

• snRNPs: Remove introns and splice exons together.

Regulation of Bacterial Gene Expression

Gene expression is regulated at multiple levels to respond to environmental changes.

• Constitutive Genes: Expressed at a fixed rate.

• Inducible Genes: Expressed only when needed.

• Repressible Genes: Turned off when not needed.

• Operon Model: Includes promoter, operator, and structural genes.

• Inducible Operon: Genes are off unless an inducer is present (e.g., lac operon).

• Repressible Operon: Genes are on unless a corepressor activates the repressor (e.g., trp operon).

Epigenetic and Post-Transcriptional Control

Gene expression can be regulated by methylation and RNA-based mechanisms.

• Methylation: Turns genes off; not permanent.

• Riboswitches: mRNA regions that bind substrates and alter translation.

• miRNAs: Bind mRNA, making it double-stranded and preventing translation.

Mutations and Genetic Variation

Mutations are permanent changes in DNA sequence, contributing to genetic diversity.

• Types of Mutations:

  • Base Substitution (Point Mutation): Change in one base.

  • Missense Mutation: Changes amino acid.

  • Nonsense Mutation: Creates stop codon.

  • Frameshift Mutation: Insertion/deletion shifts reading frame.

• Mutagens: Agents causing mutations (e.g., chemicals, radiation).

• Spontaneous Mutations: Occur without mutagen.

Mutagenic Agents

Chemicals and radiation can induce mutations.

• Nitrous Acid: Causes adenine to bind with cytosine.

• Ionizing Radiation: Breaks DNA backbone.

• UV Radiation: Causes thymine dimers.

• Repair Mechanisms: Photolyases and nucleotide excision repair correct DNA damage.

Genetic Transfer and Recombination

Genetic recombination increases diversity and can occur vertically or horizontally.

• Vertical Gene Transfer: Parent to offspring.

• Horizontal Gene Transfer: Between cells of same generation.

• Plasmids: Self-replicating DNA, often carrying beneficial genes.

• Transposons: DNA segments that move within or between genomes.

Mechanisms of Genetic Transfer in Bacteria

Bacteria can exchange genetic material through transformation, conjugation, and transduction.

• Transformation: Uptake of naked DNA from environment.

• Conjugation: Transfer of plasmids via cell-to-cell contact (sex pili).

• Transduction: DNA transfer via bacteriophage; can be generalized or specialized.

Genes and Evolution

Mutations and recombination drive microbial evolution, providing raw material for natural selection.

• Diversity: Essential for adaptation and survival.

Key Terms and Concepts Table

Important Equations

• DNA Replication Energy: Hydrolysis of nucleoside triphosphates provides energy:

• Central Dogma:

Example: Lac Operon Regulation

The lac operon in E. coli is an inducible operon. In the absence of lactose, a repressor binds to the operator, preventing transcription. When lactose is present, allolactose (the inducer) binds to the repressor, inactivating it and allowing transcription of genes needed to metabolize lactose.

Example: Mutation Impact

A base substitution mutation can change a codon, resulting in a different amino acid in the protein. For example, a mutation changing GAA (glutamic acid) to GUA (valine) can alter protein function and lead to disease.

Example: Horizontal Gene Transfer

Conjugation allows bacteria to transfer plasmids carrying antibiotic resistance genes, increasing the spread of resistance in microbial populations.