Microbial Genetics: Foundations, Gene Expression, and Mutation

Microbial Genetics

Introduction to Microbial Genetics

Microbial genetics is the study of how microorganisms inherit traits and how their genetic material influences their structure, function, and behavior. This field is fundamental for understanding microbial physiology, evolution, and adaptation.

• Genetics: The science of heredity and variation in organisms.

• Genome: The complete set of genetic material in an organism, including all of its genes and non-coding sequences.

• Chromosome: A DNA molecule containing part or all of the genetic material of an organism. In bacteria, typically a single circular chromosome.

• Gene: A segment of DNA that encodes a functional product, usually a protein.

• Genetic Code: The set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins by living cells.

• Genotype: The genetic makeup of an organism; the specific set of genes it possesses.

• Phenotype: The observable characteristics or traits of an organism, resulting from the interaction of its genotype with the environment.

Genotype and Phenotype

Relationship and Examples

The genotype of a microbe determines its phenotype, which includes traits such as metabolic capabilities, morphology, and resistance to antibiotics. Phenotypic traits are often assessed using laboratory tests.

• Lactose Fermentation: E. coli can be identified by its ability to ferment lactose, producing acid and gas in lactose broths (Lac+ phenotype). Non-fermenters (Lac-) do not produce these changes.

• Blood Agar Hemolysis: Some bacteria produce hemolysins that lyse red blood cells, visible as clear zones around colonies on blood agar.

• Urease Activity: Klebsiella species can be identified by their ability to hydrolyze urea, producing a purplish-pink color in urea broth due to the production of ammonia.

These phenotypes are direct results of gene expression, where specific genes encode enzymes responsible for these reactions.

Gene Expression

Flow of Genetic Information

Genetic information in cells flows from DNA to RNA to protein, a process known as the central dogma of molecular biology.

• Transcription: The process by which a segment of DNA is copied into messenger RNA (mRNA) by the enzyme RNA polymerase.

• Translation: The process by which ribosomes synthesize proteins using the sequence of codons in mRNA.

In prokaryotes, transcription and translation are coupled, meaning they occur simultaneously in the cytoplasm.

Central Dogma Equation

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Genetic Code

The genetic code is redundant, with 64 codons encoding 20 amino acids and start/stop signals. Codons are triplets of nucleotides in mRNA that specify amino acids.

• Sense (coding) strand: The DNA strand whose sequence matches the mRNA (except T/U substitution).

• Antisense (template) strand: The DNA strand used as a template for mRNA synthesis.

Example:

• Sense DNA: 5'-AATGCCAGGGTAAA-3'

• Antisense DNA: 3'-TTACGGTCCCATTT-5'

• Sense RNA: 5'-AAUGCCAGGGUAAA-3'

Mutation

Definition and Types

A mutation is a permanent alteration in the DNA sequence of a gene. Mutations can be spontaneous or induced by mutagens (chemicals, radiation).

• Base Substitution (Point Mutation): Replacement of one nucleotide base with another. Types include:

  • Silent Mutation: No change in amino acid sequence.

  • Missense Mutation: Results in a different amino acid.

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

• Frameshift Mutation: Insertion or deletion of one or more bases, altering the reading frame and usually resulting in a nonfunctional protein.

Mutation frequency in bacteria is approximately one per $10^6$ replicated genes.

Summary Table: Key Concepts in Microbial Genetics

Term	Definition	Example/Application
Genotype	Genetic makeup of an organism	Lac+ gene in E. coli
Phenotype	Observable traits	Lactose fermentation, hemolysis
Mutation	Permanent change in DNA sequence	Missense mutation in antibiotic resistance gene
Transcription	DNA to RNA synthesis	mRNA production from urease gene
Translation	RNA to protein synthesis	Urease enzyme synthesis

Additional info:

• Polysome (polyribosome) formation refers to multiple ribosomes translating a single mRNA simultaneously, increasing efficiency of protein synthesis.

• Auxotrophs are mutants that require additional nutrients due to loss of biosynthetic capability.