Microbial Genetics

Introduction to Microbial Genetics

Microbial genetics is the study of how microorganisms inherit traits, how their genetic information is expressed, and how genetic changes drive microbial diversity and evolution. Understanding microbial genetics is essential for grasping how bacteria adapt, cause disease, and can be manipulated for biotechnology.

Structure and Function of Genetic Material

Key Definitions

• Genetics: The study of genes, how they carry information, how information is expressed, and how genes are replicated.

• Genome: All the genetic information in a cell.

• Chromosome: Structures containing DNA that physically carry hereditary information; chromosomes contain genes.

• Gene: Segments of DNA that encode functional products, usually proteins.

• Genotype: The genetic makeup of an organism.

• Phenotype: The expression of the genes; the observable characteristics.

• Genomics: Sequencing and molecular characterization of genomes.

DNA and Chromosomes

• Bacteria typically have a single, circular chromosome made of DNA and associated proteins.

• Example: Escherichia coli has a chromosome with 4.6 million base pairs, highly supercoiled for compactness.

• The genome includes protein-encoding genes and noncoding regions such as short tandem repeats (STRs).

The Genetic Code and Central Dogma

• The genetic code is a set of rules that determines how a nucleotide sequence is converted to an amino acid sequence of a protein.

• Central dogma: DNA is transcribed into mRNA, which is then translated into protein.

DNA Replication

Mechanism of DNA Replication

• DNA forms a double helix with antiparallel strands held together by hydrogen bonds (A-T, C-G).

• Replication is semiconservative: each new DNA molecule consists of one old and one new strand.

• Key enzymes include helicase (unwinds DNA), DNA polymerase (synthesizes new DNA), primase (synthesizes RNA primers), and ligase (joins fragments).

• Replication is bidirectional in bacteria and highly accurate due to proofreading by DNA polymerase.

RNA and Protein Synthesis

Types of RNA

• mRNA (messenger RNA): Carries genetic code from DNA to ribosomes.

• tRNA (transfer RNA): Brings amino acids to the ribosome during translation.

• rRNA (ribosomal RNA): Integral part of ribosomes.

Transcription (Prokaryotes)

• RNA polymerase binds to the promoter to initiate transcription.

• Only one DNA strand is transcribed into mRNA.

• Transcription stops at the terminator sequence.

Translation

• mRNA is read in codons (three nucleotides) to specify amino acids.

• Translation begins at the start codon (AUG) and ends at stop codons (UAA, UAG, UGA).

• tRNA anticodons pair with mRNA codons, and amino acids are joined by peptide bonds.

• In bacteria, translation can begin before transcription is complete.

Transcription and Translation in Eukaryotes

• Transcription occurs in the nucleus; translation in the cytoplasm.

• Genes contain exons (coding) and introns (noncoding); introns are removed by snRNPs.

Regulation of Bacterial Gene Expression

Operons and Gene Regulation

• Operon: A unit of DNA containing a promoter, operator, and structural genes.

• Inducible operon: Genes are off until induced (e.g., lac operon).

• Repressible operon: Genes are on until repressed (e.g., trp operon).

Positive and Epigenetic Regulation

• Catabolite repression: Inhibits use of alternative carbon sources when glucose is present.

• Epigenetic control: Methylation of DNA can turn genes off; this modification can be inherited but is reversible.

Post-Transcriptional Control

• Regulation after mRNA is made, e.g., riboswitches and microRNAs (miRNAs) that can block translation or degrade mRNA.

Mutations and Genetic Variation

Types of Mutations

• Mutation: Permanent change in DNA sequence.

• Base substitution (point mutation): One base is replaced by another.

• Missense mutation: Results in a different amino acid.

• Nonsense mutation: Results in a stop codon.

• Frameshift mutation: Insertion or deletion shifts the reading frame.

Mutagens and DNA Repair

• Mutagens: Agents that cause mutations (e.g., chemicals, radiation).

• DNA repair mechanisms include photolyase (light repair) and nucleotide excision repair.

Mutation Rate and Detection

• Mutation rate: Probability of mutation per gene per cell division.

• Mutagens increase mutation rates by 10–1000 times.

• Mutants can be detected by direct (positive) or indirect (negative) selection.

• The Ames test uses bacteria to identify potential carcinogens by measuring mutation reversion rates.

Genetic Transfer and Recombination

Mechanisms of Genetic Exchange

• Vertical gene transfer: Genes passed from parent to offspring.

• Horizontal gene transfer: Genes transferred between cells of the same generation.

• Genetic recombination: Exchange of genes between DNA molecules, increasing diversity.

Mobile Genetic Elements

• Plasmids: Small, self-replicating DNA molecules; may carry genes for antibiotic resistance or metabolism of unusual substances.

• Transposons: DNA segments that can move within and between DNA molecules, sometimes carrying antibiotic resistance genes.

Horizontal Gene Transfer Mechanisms

• Transformation: Uptake of naked DNA from the environment.

• Conjugation: Transfer of DNA via direct cell-to-cell contact, often involving plasmids.

• Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

Genes and Evolution

Genetic Variation and Natural Selection

• Mutations and recombination generate genetic diversity.

• Natural selection acts on this diversity, favoring traits that enhance survival and reproduction in specific environments.