Microbial Genetics: Structure, Function, and Mechanisms

Microbial Genetics

Big Picture: Genetics

Genetics is the science of heredity, focusing on how genetic information is carried, expressed, and altered in microorganisms. The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein, and mutations can disrupt this process, leading to changes in gene expression and function. Operons regulate gene expression in bacteria, allowing coordinated control of groups of genes.

• Central Dogma: DNA → RNA → Protein → Function

• Mutations: Changes in DNA sequence can alter mRNA, protein, and cellular function.

• Operons: Groups of genes regulated together, can be inducible or repressible.

Structure and Function of Genetic Material

The genetic material of bacteria is organized in chromosomes, which contain genes encoding functional products. The genome encompasses all genetic information in a cell, and genomics is the study of genome sequences and their molecular characteristics. Genotype refers to the genetic makeup, while phenotype is the observable expression of genes.

• Chromosomes: Structures containing DNA and associated proteins.

• Genes: Segments of DNA encoding proteins or functional products.

• Genome: All genetic information in a cell.

• Genotype vs. Phenotype: Genotype is the DNA sequence; phenotype is the expressed traits.

The Genetic Code and Central Dogma

The genetic code is a set of rules for converting nucleotide sequences into amino acid sequences. The central dogma describes the process by which genetic information is transcribed from DNA to RNA and then translated into protein.

• Transcription: DNA → mRNA

• Translation: mRNA → Protein

• Gene Expression: When a gene's product is produced, the gene is considered expressed.

DNA and Chromosomes in Bacteria

Bacterial chromosomes are typically single, circular DNA molecules, highly supercoiled for efficient packaging. For example, E. coli has a chromosome with 4.6 million base pairs.

• Supercoiling: DNA is twisted to fit inside the cell.

• Associated Proteins: Help organize and stabilize DNA.

DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division. The double helix structure allows each strand to serve as a template for a new strand. Replication is highly accurate due to proofreading by DNA polymerase.

• Antiparallel Strands: DNA strands run in opposite directions.

• Base Pairing: Adenine pairs with Thymine, Cytosine with Guanine.

• Replication Fork: The site where DNA is unwound and replicated.

• Leading Strand: Synthesized continuously.

• Lagging Strand: Synthesized discontinuously in Okazaki fragments.

• Enzymes: Helicase, DNA polymerase, primase, ligase, topoisomerase, and gyrase.

RNA and Protein Synthesis

RNA is a single-stranded nucleic acid with ribose sugar and uracil instead of thymine. Three main types of RNA participate in protein synthesis: mRNA, tRNA, and rRNA.

• mRNA: Carries genetic code from DNA to ribosomes.

• tRNA: Transports amino acids to ribosomes.

• rRNA: Integral part of ribosome structure.

Transcription in Prokaryotes

Transcription is the synthesis of a complementary mRNA strand from a DNA template. It involves initiation (RNA polymerase binds to promoter), elongation (RNA strand grows), and termination (RNA polymerase reaches terminator).

Translation

Translation converts mRNA into protein. Codons (three-nucleotide sequences) specify amino acids. The process begins at the start codon (AUG) and ends at stop codons (UAA, UAG, UGA). tRNA molecules bring amino acids to the ribosome, matching codons with anticodons.

• Degeneracy: Multiple codons can code for the same amino acid.

• Peptide Bonds: Join amino acids during protein synthesis.

• Coupled Transcription-Translation: In bacteria, translation can begin before transcription is complete.

Regulation of Bacterial Gene Expression

Gene expression in bacteria is regulated by operons, which can be constitutive (always on), inducible (turned on by inducers), or repressible (turned off by repressors).

• Constitutive Genes: Expressed at a fixed rate.

• Inducible Genes: Expression turned on by environmental signals.

• Repressible Genes: Expression turned off by corepressors.

Operon Model of Gene Expression

The operon model describes how groups of genes are regulated together. The promoter is where transcription begins, and the operator is the regulatory region. In inducible operons (e.g., lac operon), genes are transcribed only when an inducer is present. In repressible operons (e.g., trp operon), genes are transcribed until a corepressor turns them off.

• lac Operon: Inducible; metabolizes lactose.

• trp Operon: Repressible; synthesizes tryptophan.

Mutation and Genetic Variation

Mutations are permanent changes in DNA sequence, which can be neutral, beneficial, or harmful. Mutagens are agents that cause mutations, while spontaneous mutations occur without mutagens.

• Silent Mutation: No effect on protein function.

• Missense Mutation: Changes one amino acid.

• Nonsense Mutation: Creates a stop codon.

• Frameshift Mutation: Insertion or deletion shifts reading frame.

Mutagens and Radiation

Chemical mutagens and radiation can cause mutations. Ionizing radiation breaks DNA, while UV radiation causes thymine dimers. Cells can repair UV-induced damage using photolyases or nucleotide excision repair.

Mutation Rate and Identification

The mutation rate is the probability of a gene mutating during cell division. Mutagens increase mutation rates. Mutants can be identified by direct (positive) or indirect (negative) selection. Auxotrophs are mutants with nutritional requirements absent in the parent strain.

Ames Test

The Ames test uses bacteria to detect chemical carcinogens by measuring mutation reversion rates.

Genetic Transfer and Recombination

Genetic information can be transferred vertically (parent to offspring) or horizontally (between cells of the same generation). Horizontal gene transfer mechanisms include transformation, conjugation, and transduction, all contributing to genetic diversity.

• Transformation: Uptake of naked DNA from the environment.

• Conjugation: Transfer of plasmids via cell-to-cell contact.

• Transduction: Transfer of DNA via bacteriophages.

Plasmids

Plasmids are self-replicating, circular DNA molecules found in bacteria. They may carry genes for antibiotic resistance, toxin production, or unusual metabolic pathways. Resistance (R) factors encode antibiotic resistance and can be transferred horizontally.

• Conjugative Plasmid: Carries genes for sex pili and plasmid transfer.

• Dissimilation Plasmids: Encode enzymes for catabolism of unusual compounds.

• R Factors: Encode resistance to antibiotics.

Genes and Evolution

Mutations and recombination generate genetic diversity, which is the foundation for evolution. Natural selection acts on populations, favoring organisms best suited to their environment.