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.

• 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 Between Genotype and Phenotype

The genotype of a microbe determines its phenotype through the expression of genes as proteins, which carry out cellular functions. Changes in genotype, such as mutations, can lead to changes in phenotype.

• Example: Escherichia coli (E. coli) can be classified by its ability to ferment lactose (Lac+ or Lac−), which is a phenotypic trait determined by the presence or absence of genes encoding lactose-metabolizing enzymes.

• Blood Agar Test: Bacterial colonies can be differentiated by their ability to cause hemolysis, a phenotype resulting from the expression of hemolysin genes.

• Urease Test: Klebsiella species can be identified by their ability to produce urease, an enzyme that hydrolyzes urea to ammonia and carbon dioxide, resulting in a color change in the medium (purplish-pink).

Flow of Genetic Information

Central Dogma of Molecular Biology

Genetic information flows from DNA to RNA to protein, a process known as gene expression. This flow is essential for the maintenance and function of all living cells.

• Transcription: The process by which a gene's DNA sequence is copied into messenger RNA (mRNA).

• Translation: The process by which the mRNA sequence is used to synthesize a protein.

• Key Enzymes: RNA polymerase (for transcription), ribosomes (for translation).

Equation:

Gene Expression

Mechanisms and Components

Gene expression involves the coordinated action of various molecular components to produce functional proteins from genetic instructions.

• 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.

• Genetic Code: The genetic code is redundant, with 64 codons encoding 20 amino acids and start/stop signals.

Example: The mRNA sequence codes for methionine, the universal start codon for translation.

Mutation

Types and Effects of Mutations

Mutations are permanent changes in the DNA sequence. They can be spontaneous or induced by mutagens (chemicals, radiation). Mutations may be silent, lethal, or beneficial, and are classified by their molecular nature.

• Base Substitution (Point Mutation): A single nucleotide is replaced by another, which may result in:

  • Silent Mutation: No change in the amino acid sequence.

  • Missense Mutation: Changes one amino acid in the protein.

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

• Frameshift Mutation: Insertion or deletion of a nucleotide shifts the reading frame, altering downstream amino acids.

Equation:

Summary Table: Genotype vs. Phenotype

Additional info:

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

• Vertical gene transfer is the transmission of genetic material from parent to offspring, while horizontal gene transfer involves the movement of genetic material between organisms other than by descent.