Microbial Genetics

Introduction to Microbial Genetics

Microbial genetics is the study of how microorganisms inherit and express their genetic information. This field explores the structure, function, and transmission of genetic material in microbes, focusing on how genes determine phenotypic traits and how genetic variation arises and spreads within microbial populations.

• Genetics: The scientific study of heredity and variation in organisms.

• Genome: The complete set of genetic material (DNA) in an organism.

• 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 is translated into proteins by living cells.

• Genotype: The genetic makeup of an organism; the information encoded in its DNA.

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

Gene Expression and the Flow of Genetic Information

Central Dogma: DNA to RNA to Protein

The flow of genetic information in cells follows the central dogma: DNA is transcribed into RNA, which is then translated into protein. This process determines how genetic information is expressed as phenotypic traits.

• Transcription: The process by which a gene's DNA sequence is copied to make an RNA molecule.

• Translation: The process by which the sequence of an mRNA molecule is used to assemble amino acids into a polypeptide (protein).

• Genetic Code: Consists of 64 codons (triplets of nucleotides) that specify 20 amino acids and start/stop signals for translation.

Equation:

Connecting Genotype to Phenotype

The genotype of a microbe determines which proteins it can produce, which in turn affects its phenotype. For example, the presence or absence of genes encoding enzymes for lactose fermentation or urea hydrolysis leads to observable differences in metabolic capabilities.

• Lactose Fermentation: E. coli with the Lac+ genotype can ferment lactose, producing acid and changing the color of lactose broth. Lac- strains cannot ferment lactose.

• Urease Activity: Klebsiella species with the urease gene produce a purplish-pink color in urea broth due to the breakdown of urea into ammonia and carbon dioxide.

Example: On blood agar, some bacteria exhibit hemolysis (breakdown of red blood cells), a phenotypic trait determined by the presence of hemolysin genes.

DNA Structure and Replication

DNA as Genetic Information

DNA stores genetic information in the sequence of its nucleotide bases. The complementary structure of DNA facilitates accurate duplication during cell division.

• Semiconservative Replication: Each new DNA molecule consists of one original strand and one newly synthesized strand.

• Prokaryotic Genome: Typically consists of a single, circular chromosome, but may also include plasmids.

Equation:

Gene Expression: Transcription and Translation

Transcription

Transcription is the synthesis of messenger RNA (mRNA) from a DNA template. In prokaryotes, transcription and translation are often coupled, occurring simultaneously in the cytoplasm.

• Sense (Coding) Strand: The DNA strand with the same sequence as the mRNA (except T is replaced by U in RNA).

• Antisense (Template) Strand: The DNA strand that serves as the template for RNA synthesis.

Translation

Translation is the process by which ribosomes synthesize proteins using the sequence encoded in mRNA. Transfer RNAs (tRNAs) bring amino acids to the ribosome, matching codons in the mRNA with the correct amino acids.

• Start Codon: AUG (codes for methionine)

• Stop Codons: UAA, UAG, UGA

• Polysome: A complex formed by multiple ribosomes translating a single mRNA simultaneously.

Mutations

Definition and Types of Mutations

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

• Point Mutation: A change in a single nucleotide base pair.

• Silent Mutation: Alters a codon but does not change the amino acid sequence.

• Missense Mutation: Results in a different amino acid in the protein.

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

• Frameshift Mutation: Insertion or deletion of a base shifts the reading frame, altering downstream amino acid sequence.

Equation (Frameshift Example):

Auxotroph: A mutant organism that requires a specific additional nutrient that the wild-type does not.

Genetic Transfer and Recombination

Vertical and Horizontal Gene Transfer

Genetic information can be transmitted vertically (from parent to offspring) or horizontally (between cells of the same generation). Horizontal gene transfer (HGT) is a major source of genetic diversity in prokaryotes.

• Vertical Transmission: Inheritance of genetic material from parent to progeny.

• Horizontal Transmission: Acquisition of genetic material from another organism without being its offspring.

Mechanisms of Horizontal Gene Transfer

• Transformation: Uptake of naked DNA from the environment by competent cells.

• Conjugation: Direct transfer of DNA between cells via cell-to-cell contact, often mediated by a sex pilus and conjugative plasmids (e.g., F factor in E. coli).

• Transduction: Transfer of DNA from one bacterium to another via bacteriophages (viruses that infect bacteria).

• Transposition: Movement of transposons (mobile genetic elements) within and between DNA molecules.

Plasmids and Transposons

• Plasmids: Small, circular, extrachromosomal DNA molecules that replicate independently and often carry genes for antibiotic resistance or other traits.

• Conjugative Plasmids: Plasmids that carry genes for pilus formation and DNA transfer (e.g., F factor).

• Transposons: Segments of DNA that can move from one location to another within a genome, sometimes carrying antibiotic resistance genes.

Recombination

Recombination is the process by which genetic material is exchanged between different DNA molecules, leading to new genetic combinations. In bacteria, homologous recombination is often mediated by RecA protein.

• Genetic Diversity: Recombination increases genetic diversity and can repair damaged DNA.

• Crossing Over: Exchange of genetic material between homologous DNA sequences.

Summary Table: Mechanisms of Horizontal Gene Transfer

Historical Example: Griffith's Experiment

Frederick Griffith (1928) demonstrated transformation in Streptococcus pneumoniae, showing that non-virulent bacteria could acquire virulence by uptake of DNA from heat-killed virulent cells.

Key Takeaways

• Genotype determines phenotype through gene expression.

• Mutations and horizontal gene transfer are major sources of genetic variation in microbes.

• Plasmids and transposons play important roles in the spread of antibiotic resistance and other traits.

• Understanding microbial genetics is essential for biotechnology, medicine, and understanding microbial evolution.