Microbial Genetics Foundations

Introduction to Microbial Genomes

  • Genetics and Genomes: Genetics is the study of inheritance; a genome is the entire genetic complement of an organism, including genes and nucleotide sequences.

  • Nucleic Acid Structure: Polymers of nucleotides containing a phosphate, a pentose sugar, and a nitrogenous base (Adenine, Thymine/Uracil, Guanine, Cytosine).

  • Prokaryotic Genomes: Typically consist of a single, circular, haploid chromosome located in the nucleoid.

  • Plasmids: Small, independent DNA molecules not essential for metabolism but providing survival advantages (Fertility, Resistance, Bacteriocin, and Virulence plasmids).

  • Eukaryotic Genomes: Feature multiple linear nuclear chromosomes (often diploid) sequestered in a nucleus. Histones are present for packaging. Extranuclear DNA exists in mitochondria and chloroplasts (5%5\% of code).

DNA Replication Processes

  • Semiconservative Replication: New DNA consists of one original and one daughter strand.

  • Synthesis Direction: DNA polymerase acts only in the 5′→3′5' \rightarrow 3' direction.

  • Leading and Lagging Strands: The leading strand is synthesized continuously; the lagging strand is synthesized discontinuously via Okazaki fragments.

  • Bacterial Specifics: Replication is bidirectional. Enzymes 7gyrases and topoisomerases manage DNA supercoils.

  • Methylation: Used for gene expression control, initiation of replication, protection against viruses, and DNA repair.

Mechanisms of Gene Function

  • Central Dogma: DNA is transcribed into RNA, and RNA is translated into polypeptides.

  • Transcription: Process of copying DNA into RNA (mRNA, tRNA, rRNA, etc.) featuring Initiation, Elongation, and Termination steps.

  • Eukaryotic mRNA Processing: Includes capping, polyadenylation (Poly-A tail), and splicing (removing introns, joining exons).

  • Translation: Ribosomes use genetic information of nucleotide sequences (codons) to synthesize polypeptides.

  • The Genetic Code: Triplet codons represent specific amino acids. The start codon is usually AUG; stop codons include UAA, UAG, and UGA.

  • Ribosome Structure: Prokaryotes use 70S70S ribosomes (30S+50S30S + 50S); Eukaryotes use 80S80S ribosomes (40S+60S40S + 60S).

Gene Regulation and Operons

  • Operon Structure: Consists of a promoter, operator, and series of structural genes.

  • Inducible Operons: Must be activated by inducers (e.g., the Lac operon for lactose catabolism).

  • Repressible Operons: Transcribed continually until deactivated (e.g., the Trp operon for tryptophan synthesis).

  • Regulatory RNAs: microRNAs (miRNAs) and small interfering RNAs (siRNAs) can inhibit translation by binding to mRNA.

Mutations and DNA Repair Mechanisms

  • Mutation Types: Point mutations (substitutions or frameshift insertions/deletions) and Gross mutations (inversions, duplications).

  • Mutagens: Physical agents like ionizing radiation and nonionizing radiation (causing Thymine dimers), and chemical agents like nucleotide analogs.

  • DNA Repair: Includes direct repair (Light repair via photolyases), single-strand repair (Mismatch repair), and the SOS response for extensive damage.

  • Ames Test: A procedure using Salmonella to identify potential mutagens and carcinogens.

Horizontal Gene Transfer and Transposons

  • Transformation: The uptake of free DNA from the environment by competent cells.

  • Transduction: Transfer of DNA fragments between bacteria via a bacteriophage.

  • Bacterial Conjugation: Transfer of genetic material requiring physical contact through a pilus (e.g., F+F^+ cells or Hfr cells).

  • Transposons: DNA segments that move between locations (transposition). They range from simple insertion sequences to complex transposons containing additional genes.

Questions & Discussion

  • Question: DNA replication requires a large amount of energy, yet none of a cell’s ATP energy supply is used. Why isn’t it?

  • Response: (Context provided as a prompt for student reflection in the transcript).

  • Question: In bacteria, polypeptide translation can begin even before mRNA transcription is complete. Why can’t this happen in eukaryotes?

  • Response: (Context provided as a prompt for student reflection in the transcript).

  • Question: Changes in RNA sequences resulting from poor transcription are not as deleterious to an organism as changes to DNA resulting from mutations. Why is this the case?

  • Response: (Context provided as a prompt for student reflection in the transcript).

  • Question: Why is the genetic ancestry of microbes much more difficult to ascertain than the ancestry of animals?

  • Response: (Context provided as a prompt for student reflection in the transcript).

Genetics studies inheritance, while a genome contains the complete genetic material of an organism. Nucleic acids are made of nucleotides with a phosphate, sugar, and nitrogenous base.

Prokaryotic genomes have a single, circular chromosome in the nucleoid, while eukaryotic genomes have multiple linear chromosomes in a nucleus, with extranuclear DNA in mitochondria and chloroplasts.

DNA replication is semiconservative, with strands synthesized in a 5′→3′5' \rightarrow 3' direction. Leading strands are continuously synthesized; lagging strands are made in fragments. Gene function follows the central dogma: DNA is transcribed into RNA and then translated into proteins. Operons regulate gene expression with inducible and repressible types. Mutations can occur, and various mechanisms exist for DNA repair. Horizontal gene transfer allows genetic material exchange between bacteria.