Microbial Genetics: Core Concepts and Mechanisms

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Microbial Genetics

Central Dogma of Biology

The central dogma of biology describes the flow of genetic information within a biological system. It explains how DNA is transcribed into RNA, which is then translated into proteins.

DNA → RNA → Protein
This process is fundamental to all living organisms and underpins gene expression.
Example: The gene for insulin in humans is transcribed into mRNA and then translated into the insulin protein.

Genetic Terms

Key genetic terms are essential for understanding microbial genetics.

Term	Definition
Gene	A segment of DNA that encodes a functional product, usually a protein.
Genome	The complete set of genetic material in an organism.
Genotype	The genetic makeup of an organism.
Phenotype	The observable characteristics of an organism resulting from the genotype.

Genotype vs. Phenotype

Genotype refers to the genetic constitution, while phenotype is the physical expression of those genes.

Genotype: Determines potential traits.
Phenotype: Actual traits observed, influenced by genotype and environment.
Example: A bacterium may have a gene for antibiotic resistance (genotype), but only express resistance (phenotype) when exposed to the antibiotic.

Bacterial DNA Characteristics & Short Tandem Repeats

Bacterial DNA is typically circular and found in the nucleoid region. Short tandem repeats (STRs) are repeating sequences that contribute to genetic diversity.

Circular DNA: Most bacteria have a single, circular chromosome.
STRs: Involved in genome organization and can affect gene regulation.
Example: STRs are used in microbial genotyping for epidemiological studies.

Vertical Gene Transfer

Vertical gene transfer is the transmission of genetic material from parent to offspring during reproduction.

Occurs during cell division (binary fission in bacteria).
Ensures genetic continuity across generations.

DNA Replication: Purpose and Mechanism

DNA replication ensures that each daughter cell receives an identical copy of the genome.

Purpose: Genetic continuity and cell division.
Mechanism: Semi-conservative process involving DNA polymerase.
Equation:

Basic Process of DNA Replication

DNA replication involves several steps:

Initiation at the origin of replication.
Unwinding of DNA by helicase.
Synthesis of new strands by DNA polymerase.
Leading and lagging strand synthesis.
Termination and ligation of fragments.

Types of RNA and Their Functions

RNA molecules play distinct roles in gene expression.

RNA Type	Function
mRNA (messenger RNA)	Carries genetic information from DNA to ribosomes for protein synthesis.
tRNA (transfer RNA)	Brings amino acids to the ribosome during translation.
rRNA (ribosomal RNA)	Forms the core of ribosome structure and catalyzes protein synthesis.

Transcription

Transcription is the process by which RNA is synthesized from a DNA template.

Produces mRNA, tRNA, and rRNA.
Equation:

Translation

Translation is the synthesis of proteins from mRNA by ribosomes.

Produces polypeptides (proteins).
Equation:

Process of Translation

Translation involves initiation, elongation, and termination phases.

Initiation: Ribosome assembles on mRNA.
Elongation: tRNAs bring amino acids, forming a polypeptide chain.
Termination: Release of the completed protein.

Mutation and Mutagen

A mutation is a change in the DNA sequence. A mutagen is an agent that causes mutations.

Mutations: Can be spontaneous or induced.
Mutagens: Chemicals, radiation, or biological agents.
Example: UV light is a mutagen that causes thymine dimers in DNA.

Types of DNA Mutations

Mutation	Description
Point Mutation	Change in a single nucleotide base.
Insertion	Addition of one or more nucleotide bases.
Deletion	Loss of one or more nucleotide bases.
Frameshift	Insertion or deletion that alters the reading frame.

Crossing Over and Genetic Diversity

Crossing over is the exchange of genetic material between homologous chromosomes, increasing genetic diversity.

Occurs during meiosis in eukaryotes.
Results in new combinations of alleles.

Vertical vs. Horizontal Gene Transfer

Gene transfer can occur vertically (parent to offspring) or horizontally (between organisms).

Vertical: Inheritance during reproduction.
Horizontal: Transfer between unrelated organisms (e.g., transformation, conjugation, transduction).

Plasmids and Transposons

Plasmids are extrachromosomal DNA molecules; transposons are DNA sequences that can move within the genome.

Plasmids: Often carry antibiotic resistance genes.
Transposons: Can disrupt genes and contribute to genetic variation.

Types of Plasmids

Conjugative plasmids: Carry genes for transfer between cells.
Resistance plasmids: Provide resistance to antibiotics.
Virulence plasmids: Carry genes that enhance pathogenicity.

Genetic Variability in Bacteria

Bacteria achieve genetic variability through mutations, gene transfer, and recombination.

Allows adaptation to changing environments.

Mechanisms of Gene Transfer: Transformation, Conjugation, Transduction

Transformation: Uptake of free DNA from the environment.
Conjugation: Direct transfer of DNA between cells via pilus.
Transduction: Transfer of DNA by bacteriophages (viruses).

Example: Antibiotic resistance genes can spread via conjugation.