Genetics and Molecular Biology in Microbiology: Key Concepts and Processes

Genetics in Microbiology

Genome, Genotype, and Phenotype

The genome is the complete set of genetic material in an organism, including all DNA (or RNA in viruses). Genotype refers to the genetic makeup (DNA sequence), while phenotype describes observable traits influenced by genotype and environment.

• Genome: All genetic material in a cell or virus.

• Genotype: DNA sequence of an organism.

• Phenotype: Observable characteristics.

Example: The genotype for antibiotic resistance may result in the phenotype of a bacterium surviving antibiotic treatment.

Gene Definition

A gene is a DNA segment encoding a functional product, such as a protein or RNA.

• Gene: Unit of heredity; sequence of nucleotides.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA → RNA → Protein → Phenotype.

• DNA is transcribed to RNA.

• RNA is translated to protein.

• Protein determines phenotype.

Equation: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Organization of Prokaryotic and Eukaryotic Genomes

Prokaryotic Genome Structure

Prokaryotes typically have circular double-stranded DNA, often organized in a single chromosome and plasmids.

• Size: 0.5–10 Mb

• Organization: Nucleoid, operons, polycistronic mRNA, no introns

• Location: Cytoplasm

Eukaryotic Genome Structure

Eukaryotes have linear double-stranded DNA, organized into multiple chromosomes within a nucleus.

• Size: Millions to billions of bp

• Organization: Nucleus, multiple chromosomes, introns/exons, histones

• Location: Nucleus, mitochondria, chloroplasts

Chromosomes in Prokaryotes and Eukaryotes

• Prokaryotes: Usually one circular chromosome; may have plasmids.

• Eukaryotes: Multiple linear chromosomes; organelles may have their own DNA.

Nucleic Acids: DNA and RNA

Nucleic Acid Structure

Nucleic acids are polymers of nucleotides. DNA and RNA differ in sugar and bases.

• DNA bases: A, T, C, G

• RNA bases: A, U, C, G

DNA Structure

DNA is a double helix with antiparallel strands. Phosphodiester bonds form the backbone; hydrogen bonds connect bases.

• Phosphodiester bonds: Link sugar and phosphate groups.

• Hydrogen bonds: Connect complementary bases (A-T, C-G).

DNA Replication and Enzymes

Leading and Lagging Strands

DNA replication is semi-conservative, with leading and lagging strands synthesized differently.

• Leading strand: Continuous synthesis.

• Lagging strand: Discontinuous synthesis (Okazaki fragments).

Key Enzymes in DNA Replication

• Helicase: Unwinds DNA.

• Primase: Synthesizes RNA primers.

• DNA Polymerase III: Adds nucleotides.

• DNA Polymerase I: Replaces RNA primers with DNA.

• Ligase: Seals nicks in DNA.

Gene Expression and Regulation

Gene Expression: Transcription and Translation

Gene expression uses DNA to make proteins via transcription and translation.

• Transcription: DNA → RNA (RNA polymerase)

• Translation: RNA → Protein (ribosome)

Types of RNA

• mRNA: Carries coding information.

• tRNA: Brings amino acids to ribosome.

• rRNA: Forms ribosomes, catalyzes peptide bond formation.

RNA Processing in Eukaryotes

Eukaryotic mRNA is processed before translation.

• 5' cap

• Poly-A tail

• Intron splicing

Genetic Code: Start and Stop Codons

• Start codon: AUG (methionine)

• Stop codons: UAA, UAG, UGA

Gene Regulation: Operons

• Constitutive genes: Always on.

• Facultative genes: Regulated, on/off.

• Operon: Cluster of genes under one promoter.

• Inducible operons: Turned on by substrate (e.g., lac operon).

• Repressible operons: Turned off by product (e.g., trp operon).

Pre- and Post-Transcriptional Regulation

• Pre-transcriptional: DNA methylation, transcription factor binding.

• Post-transcriptional: mRNA stability, degradation control.

Mutations and DNA Repair

Types of Mutations

• Substitution: One base replaced by another.

• Insertion: Addition of bases.

• Deletion: Removal of bases.

Silent, Missense, and Nonsense Mutations

• Silent mutation: No amino acid change; no effect.

• Missense mutation: Amino acid change; effect varies.

• Nonsense mutation: Creates stop codon; truncates protein.

Frameshift Mutations

• Insertion/deletion shifts reading frame; usually severe effect.

DNA Damage and Repair

• Spontaneous mutations: Natural errors.

• Induced mutations: Caused by mutagens.

• Repair mechanisms: Proofreading, mismatch repair, excision repair, recombination.

Ames Test

The Ames test detects mutagens by measuring reversion rates in bacteria.

Genetic Exchange in Bacteria

Horizontal vs. Vertical Gene Transfer

• Vertical transfer: Parent to offspring.

• Horizontal transfer: Between unrelated cells (e.g., via plasmids).

Plasmids

Plasmids are small, circular DNA molecules independent of chromosomes, important for resistance and cloning.

• Nature: Carry genes for antibiotic resistance, metabolism.

• Laboratory: Used in genetic engineering.

Conjugation

Conjugation is the transfer of DNA via pilus, increasing genetic diversity.

Recombination

Recombination is the exchange of DNA between molecules, creating new combinations.

Transformation

Transformation is the uptake of free DNA. In Griffith's experiments, harmless bacteria became pathogenic by DNA uptake.

Summary Table: Types of Mutations

Type	Definition	Effect
Silent	Base change, no amino acid change	No effect
Missense	Base change, amino acid change	Effect varies
Nonsense	Base change creates stop codon	Truncates protein
Frameshift	Insertion/deletion shifts reading frame	Usually severe

Additional info:

• Some context and definitions were expanded for clarity and completeness.

• Examples and applications were added to illustrate concepts.