Microbial Genetics

Overview of Genetics

Microbial genetics is the study of how microorganisms inherit traits, how their genetic information is expressed, and how it can be altered. The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein, ultimately determining cellular function.

• Genetics: The study of genes, their expression, and replication.

• Genome: All genetic material in a cell.

• Chromosome: Structure containing DNA and genes.

• Gene: Segment of DNA encoding a functional product, usually a protein.

• Genomics: Sequencing and molecular characterization of genomes.

• Genetic Code: Rules for converting nucleotide sequences into amino acid sequences.

• Genotype: Genetic makeup of an organism.

• Phenotype: Expression of the genotype as observable traits.

Structure and Function of Genetic Material

DNA is the primary genetic material in bacteria, typically organized as a single circular chromosome. The DNA is highly supercoiled and contains both protein-coding genes and noncoding regions such as short tandem repeats (STRs).

• DNA Structure: Double helix with antiparallel strands held together by hydrogen bonds between complementary bases (A-T, C-G).

• Chromosome: Contains all hereditary information.

• STRs: Noncoding, repeating sequences (2-5 base pairs/repeat).

The Flow of Genetic Information

Genetic information flows from DNA to RNA (transcription) and then to protein (translation). This process is essential for cell function and growth.

• Vertical Gene Transfer: From parent to offspring.

• Horizontal Gene Transfer: Between cells of the same generation.

DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division. It is highly accurate due to the proofreading activity of DNA polymerase.

• Double Helix: Two antiparallel strands.

• Replication Fork: Site where DNA is unwound and new strands are synthesized.

• Leading Strand: Synthesized continuously.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments.

• Key Enzymes: Helicase (unwinds DNA), DNA polymerase (synthesizes DNA), DNA ligase (joins fragments), gyrase/topoisomerase (relaxes supercoiling).

• Energy: Provided by hydrolysis of nucleoside triphosphates.

RNA and Protein Synthesis

RNA is synthesized from DNA and is essential for protein synthesis. There are three main types of RNA: mRNA, tRNA, and rRNA.

• mRNA: Carries genetic code from DNA to ribosomes.

• tRNA: Transports amino acids during translation.

• rRNA: Integral part of ribosomes.

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 synthesis), and termination (RNA polymerase reaches terminator).

Translation

Translation is the process by which mRNA is decoded to synthesize proteins. Codons (three-base sequences) specify amino acids. The process begins at the start codon (AUG) and ends at stop codons (UAA, UAG, UGA).

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

• tRNA: Matches codons with amino acids via anticodon.

• Peptide Bonds: Join amino acids into polypeptides.

Transcription and Translation in Eukaryotes

In eukaryotes, transcription occurs in the nucleus and translation in the cytoplasm. Genes contain exons (coding regions) and introns (noncoding regions). Introns are removed from the RNA transcript by snRNPs, and exons are spliced together.

Regulation of Bacterial Gene Expression

Gene expression in bacteria is regulated at both pre-transcriptional and post-transcriptional levels. Genes can be constitutive (always expressed), inducible, or repressible.

• Operon: A set of operator and promoter sites and structural genes they control.

• Inducible Operon: Genes are off unless an inducer is present.

• Repressible Operon: Genes are on unless a corepressor activates the repressor.

Mutations and Genetic Variation

Mutations are permanent changes in the DNA sequence. They can be neutral, beneficial, or harmful, and are a source of genetic diversity.

• Mutagens: Agents that cause mutations.

• Spontaneous Mutations: Occur without mutagens.

• Types of Mutations: Silent, missense, nonsense, and frameshift.

Genetic Transfer and Recombination

Genetic recombination is the exchange of genes between two DNA molecules, contributing to genetic diversity. Genes can be transferred vertically (parent to offspring) or horizontally (between cells of the same generation).

• Plasmids: Self-replicating circular DNA, often carrying antibiotic resistance genes.

• Transposons: DNA segments that move within and between genomes.

Applications and Implications

Alteration of bacterial genes and gene expression can cause disease, prevent disease treatment, or be manipulated for human benefit (e.g., biotechnology).

Additional info: These notes expand on brief points from the original slides, providing definitions, examples, and context for each concept. Images included are directly relevant to the explanations and reinforce key concepts in microbial genetics.