Principles of Microbial Genetics: Structure, Function, and Regulation

Principles of Genetics

Genomes, Genes, and Genetic Material in Microbes

The genome is the complete set of genetic material within an organism. In microbes, genetic material is primarily found in the chromosome, but may also exist as plasmids (small, circular, extra-chromosomal DNA) or within organelles such as mitochondria and chloroplasts in eukaryotic microbes.

• Gene: A discrete DNA sequence encoding instructions for a specific protein; the fundamental unit of heredity.

• Plasmids: Extra-chromosomal DNA elements, often carrying genes for antibiotic resistance or other adaptive traits.

Genotype, Phenotype, Genome, and Proteome

The genotype is the full collection of genes inherited by an organism, serving as its genetic blueprint. The phenotype is the observable expression of these genes. The proteome is the complete set of proteins a cell can produce, representing the functional output of the genome.

• Genotype: Internal genetic makeup.

• Phenotype: Observable traits resulting from gene expression.

• Proteome: All proteins produced by a cell.

• Link: DNA (genotype) directs protein synthesis (proteome) via transcription and translation, determining phenotype.

Nucleotides and DNA Structure

Nucleotides are the building blocks of nucleic acids (DNA and RNA), each consisting of a phosphate group, a pentose sugar (deoxyribose in DNA), and a nitrogenous base.

• DNA Nucleotides: Adenine (A), Guanine (G), Thymine (T), Cytosine (C)

• Similarities: All share the same phosphate and deoxyribose sugar.

• Differences: Differ in their nitrogenous base.

• Pyrimidines: Single-ringed (Thymine, Cytosine)

• Purines: Double-ringed (Adenine, Guanine)

• Complementary Base Pairing: Adenine pairs with Thymine (2 H-bonds), Guanine pairs with Cytosine (3 H-bonds)

DNA Replication

Antiparallel Structure and Okazaki Fragments

DNA strands are antiparallel: one runs 5' to 3', the other 3' to 5'. This orientation affects replication:

• Leading Strand: Synthesized continuously in the direction of replication fork opening.

• Lagging Strand: Synthesized discontinuously in short segments called Okazaki fragments.

Key Enzymes in DNA Replication

• Helicase: Unwinds the DNA double helix by breaking hydrogen bonds.

• Primase: Synthesizes short RNA primers to provide a starting point for DNA polymerase.

• DNA Polymerase III: Main enzyme for adding nucleotides to the new DNA strand.

• DNA Polymerase I: Removes RNA primers and replaces them with DNA nucleotides.

• Ligase: Joins Okazaki fragments and seals nicks in the sugar-phosphate backbone.

• Topoisomerase: Relieves supercoiling tension ahead of the replication fork; in bacteria, topoisomerase IV separates circular chromosomes post-replication.

Semiconservative Replication

DNA replication is semiconservative: each new DNA molecule contains one original (parental) strand and one newly synthesized strand.

Protein Synthesis

Transcription and Translation

• Transcription: Synthesis of RNA from a DNA template. Products include mRNA, tRNA, rRNA, and regulatory RNAs.

• Translation: Synthesis of proteins using mRNA as a template. Requires mRNA, tRNA, rRNA, and ribosomes.

Roles of RNA Types and Genetic Coding

• mRNA: Carries genetic code from DNA to ribosome.

• tRNA: Brings specific amino acids to the ribosome; contains anticodon complementary to mRNA codon.

• rRNA: Structural and catalytic component of ribosomes.

• Codon: Three-nucleotide sequence on mRNA specifying an amino acid.

• Anticodon: Three-nucleotide sequence on tRNA complementary to mRNA codon.

Differences Between DNA and RNA Nucleotides

• Sugar: RNA contains ribose; DNA contains deoxyribose.

• Bases: RNA uses uracil (U) instead of thymine (T).

Stages of Transcription

• Initiation: RNA polymerase binds to the promoter region (e.g., Pribnow box in bacteria, TATA box in eukaryotes).

• Elongation: RNA polymerase synthesizes the RNA strand by adding complementary nucleotides.

• Termination: RNA polymerase stops at a termination signal, releasing the RNA transcript.

• Transcription Factors: Proteins that help recruit RNA polymerase to the promoter (especially in eukaryotes).

Genetic Code: Start and Stop Codons

• Start Codon: AUG (codes for methionine) signals the start of translation.

• Stop Codons: UAA, UAG, UGA signal termination of translation; do not code for amino acids.

Wobble and Redundancy

Wobble refers to the flexibility in base pairing at the third position of a codon, allowing multiple codons to code for the same amino acid. This redundancy helps minimize the effects of mutations.

Stages of Translation and mRNA Processing

• Initiation: Small ribosomal subunit binds mRNA; first tRNA binds start codon.

• Elongation: Large subunit catalyzes peptide bond formation; polypeptide chain grows.

• Termination: Stop codon is reached; release factor disassembles the complex.

• Exons: Coding sequences in eukaryotic genes.

• Introns: Non-coding sequences removed during mRNA processing.

• Spliceosomes: Enzyme complexes that remove introns and join exons in eukaryotic mRNA.

• Alternative Splicing: Eukaryotes can process mRNA in multiple ways, producing diverse proteins from a single gene. Prokaryotes generally lack this capability.

Gene Regulation

Operons and Gene Regulation in Prokaryotes and Eukaryotes

• Operon: A cluster of genes under control of a single promoter, transcribed as one mRNA (common in bacteria and archaea).

• Eukaryotic Regulation: Each gene typically has its own promoter; regulation involves transcription factors and chromatin structure.

Inducible vs. Repressible Operons

Type	Default State	Regulation Mechanism	Example
Inducible	Off	Requires inducer to inactivate repressor	Lac operon (induced by lactose)
Repressible	On	Requires co-repressor to activate repressor	Tryptophan operon

• Lac Operon: Induced by lactose; repressed by a repressor protein in absence of lactose.

• Significance: Allows bacteria to conserve energy by producing enzymes only when substrate is available.

Gene Transfer and Recombination

Recombination

Recombination is the process by which new genetic material is incorporated into a microbe's genome, potentially creating new strains with novel traits.

Horizontal Gene Transfer Mechanisms

Mechanism	Description	Key Feature
Conjugation	Direct transfer of DNA between living cells via a pilus	Requires pilus; often transfers plasmids
Transduction	Transfer of DNA via bacteriophage (virus)	Virus acts as carrier of bacterial genes
Transformation	Uptake of free DNA from environment	Discovered by Griffith in Streptococcus pneumoniae

• Conjugation: Requires at least one cell to produce a pilus; can transfer plasmids and chromosomal DNA.

• Transduction: Bacteriophage transfers bacterial DNA between cells.

• Transformation: Uptake of naked DNA from environment; demonstrated by Griffith's experiment.

Mutations

Types of Mutations

• Mutation: Any change in the nucleotide sequence of DNA.

• Spontaneous Mutation: Occurs naturally due to replication errors.

• Induced Mutation: Caused by mutagens (chemicals, radiation).

Point Mutations and Their Effects

Type	Description	Effect
Missense	Single nucleotide change alters amino acid	May alter protein function
Nonsense	Single nucleotide change creates stop codon	Prematurely short, usually nonfunctional protein
Silent	Single nucleotide change does not alter amino acid	No effect due to genetic code redundancy (wobble)

Frameshift Mutations

Frameshift mutations result from insertion or deletion of nucleotides, altering the reading frame and usually producing nonfunctional proteins.

DNA Repair Mechanisms

• Photoreactivation: Photolyase enzymes use light energy to repair UV-induced damage.

• Excision Repair: Endonucleases remove mismatched or damaged nucleotides; DNA polymerase fills the gap using the complementary strand as a template.

Unrepaired mutations can be neutral, harmful, or (rarely) beneficial, contributing to evolution and adaptation.

Key Equations and Concepts

• Base Pairing: $A = T$, $G = C$ (in double-stranded DNA)

• Central Dogma: $DNA \rightarrow RNA \rightarrow Protein$

Example: Griffith's transformation experiment demonstrated that non-pathogenic Streptococcus pneumoniae could acquire virulence by uptake of DNA from heat-killed pathogenic strains, illustrating transformation and the concept of a "transforming principle."

Additional info: The notes above expand on the original content by providing definitions, examples, and tables for clarity and completeness, as well as highlighting the importance of each process in microbial genetics.