Microbial Genetics

Structure of DNA and RNA

The structure of nucleic acids is fundamental to understanding microbial genetics. DNA and RNA are polymers composed of nucleotide monomers, each consisting of a sugar, a phosphate group, and a nitrogenous base.

• Bases: DNA contains adenine (A), thymine (T), cytosine (C), and guanine (G). RNA contains adenine (A), uracil (U), cytosine (C), and guanine (G).

• Backbone: Both DNA and RNA have a sugar-phosphate backbone. DNA uses deoxyribose; RNA uses ribose.

• Orientation: Nucleic acids are oriented 5' to 3'. DNA is typically double-stranded and antiparallel; RNA is usually single-stranded.

• Location in Cell: In prokaryotes, DNA is found in the nucleoid region; RNA is synthesized throughout the cytoplasm.

• Example: The double helix structure of DNA allows for complementary base pairing: A-T and C-G.

Prokaryotic Chromosome and Plasmids

Prokaryotes possess a single, circular chromosome and may contain plasmids, which are small, circular DNA molecules.

• Chromosome: Contains essential genes for cell function; located in the nucleoid.

• Plasmids: Carry non-essential genes, often for antibiotic resistance or virulence; replicate independently.

• Example: Escherichia coli often carries plasmids encoding resistance to antibiotics.

Operon Model of Transcriptional Control in Prokaryotes

The operon model explains how groups of genes are regulated together in prokaryotes.

• Operon: A cluster of genes under control of a single promoter and operator.

• Transcriptional Control: Regulatory proteins bind to the operator to control gene expression.

• Example: The lac operon is regulated by the presence or absence of lactose.

Inducible vs. Repressible Operons

Operons can be classified based on their regulatory mechanisms.

• Inducible Operon: Usually off; can be turned on by an inducer (e.g., lac operon).

• Repressible Operon: Usually on; can be turned off by a repressor (e.g., trp operon).

• Example: The lac operon is inducible; the trp operon is repressible.

Lactose Operon and Tryptophan Operon

The lac and trp operons are classic examples of gene regulation in prokaryotes.

• Lac Operon: Controls genes for lactose metabolism; induced by lactose.

• Trp Operon: Controls genes for tryptophan synthesis; repressed by tryptophan.

• Example: When lactose is present, the lac operon is activated; when tryptophan is abundant, the trp operon is repressed.

Mutation: Types and Effects

Mutations are changes in the DNA sequence that can affect gene function.

• Definition: A mutation is a heritable change in the nucleotide sequence of DNA.

• Types:

  • Point Mutation: Change in a single nucleotide (e.g., substitution).

  • Frameshift Mutation: Insertion or deletion of nucleotides, altering the reading frame.

  • Silent Mutation: No effect on protein sequence.

  • Missense Mutation: Changes one amino acid.

  • Nonsense Mutation: Creates a stop codon, truncating the protein.

• Effects: Can be beneficial, neutral, or harmful.

• Example: Sickle cell anemia is caused by a missense mutation.

Mutagens and Examples

Mutagens are agents that cause mutations.

• Definition: Mutagens are physical or chemical agents that increase mutation rates.

• Examples: UV radiation, X-rays, chemicals like ethidium bromide.

• Example: UV light causes thymine dimers in DNA.

DNA Repair Mechanisms

Cells have multiple mechanisms to repair DNA damage.

• Thymine Dimer Repair: Photoreactivation or excision repair removes UV-induced dimers.

• Base-Excision Repair: Removes damaged bases and replaces them.

• Mismatch Repair: Corrects errors made during DNA replication.

• SOS Response: Emergency repair mechanism; can be error-prone.

• Example: Photolyase enzyme repairs thymine dimers using visible light.

Positive and Negative Selection for Mutants

Selection techniques are used to isolate mutants in microbial genetics.

• Positive Selection: Only mutants with a desired trait grow (e.g., antibiotic resistance).

• Negative Selection: Mutants are identified by their inability to grow under certain conditions (e.g., auxotrophs).

• Example: Replica plating is used for negative selection.

Ames Test for Identifying Carcinogens

The Ames test is a biological assay to assess the mutagenic potential of chemical compounds.

• Principle: Uses mutant strains of Salmonella that cannot synthesize histidine.

• Procedure: Chemical is tested for its ability to induce mutations that restore histidine synthesis.

• Interpretation: Increased revertant colonies indicate mutagenicity.

• Example: Used to screen chemicals for carcinogenic potential.

Horizontal vs. Vertical Gene Transfer

Gene transfer can occur between generations (vertical) or between cells (horizontal).

• Vertical Gene Transfer: Transmission of genetic material from parent to offspring.

• Horizontal Gene Transfer: Transfer of genes between cells of the same generation.

• Example: Conjugation, transformation, and transduction are forms of horizontal gene transfer.

Transformation

Transformation is the uptake of naked DNA from the environment by a bacterial cell.

• Process: DNA fragments are incorporated into the recipient's genome.

• Example: Streptococcus pneumoniae can acquire capsule genes via transformation.

Transduction

Transduction involves the transfer of DNA from one bacterium to another via bacteriophages.

• Generalized Transduction: Any gene can be transferred.

• Specialized Transduction: Only specific genes are transferred.

• Example: Lambda phage transfers genes between E. coli cells.

Bacterial Conjugation

Conjugation is the transfer of DNA via direct cell-to-cell contact, often mediated by plasmids.

• F Factor: Fertility plasmid required for conjugation.

• F+ Cells: Possess F plasmid; can donate DNA.

• F- Cells: Lack F plasmid; receive DNA.

• HFR Cells: High-frequency recombination cells; F factor integrated into chromosome.

• Example: Conjugation between F+ and F- cells results in transfer of plasmid DNA.

Transposons and Transposition

Transposons are DNA sequences that can move from one location to another within the genome.

• Transposon: "Jumping gene"; can disrupt gene function.

• Transposition: Movement of transposon within or between DNA molecules.

• Example: Insertion sequences and composite transposons can carry antibiotic resistance genes.