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Microbial Genetics Overview

Chapter Overview

• Chapter 8: Microbial Genetics

• Focus on genetic material structure, function, and mechanisms guiding genetic processes.

Structure and Function of Genetic Material

Learning Objectives

• Define key terms:

  • Genetics: Study of genes and how they carry and express information.

  • Genome: Complete set of genetic information in a cell.

  • Chromosome: DNA structures that carry hereditary information.

  • Gene: DNA segment encoding functional proteins.

  • Genetic Code: Rules for translating nucleotide sequences into amino acids.

  • Genotype vs Phenotype:

    • Genotype: Genetic makeup of an organism.

    • Phenotype: Observable expression of genes.

DNA Replication

Mechanism Overview

• Topoisomerase and Gyrase: Relax DNA strands.

• Helicase: Unwinds double-stranded DNA into two strands.

• Semi-Conservative Replication: Each strand serves as a template for a new complementary strand.

• Leading vs Lagging Strand:

  • Leading: Continuous synthesis.

  • Lagging: Discontinuous synthesis with Okazaki fragments.

Key Enzymes Involved

• DNA Polymerase: Adds nucleotides to forming strands; proofreads DNA.

• Helicase: Unwinds DNA.

• Ligase: Joins Okazaki fragments.

• Primase: Synthesizes RNA primers for replication initiation.

RNA and Protein Synthesis

RNA Types

• rRNA: Component of ribosomes.

• tRNA: Brings amino acids to ribosomes.

• mRNA: Carries genetic code from DNA to ribosomes.

Transcription in Prokaryotes

• Initiated by RNA polymerase binding to the promoter.

• Synthesized in the 5' to 3' direction.

• Terminates at a specific sequence called the terminator.

Translation Process

• Initiation: Begins at the start codon (AUG).

• Reading Codons: mRNA codons are read by tRNA, matching anticodons to codons.

• Peptide Bonding: Amino acids are linked by peptide bonds to form polypeptides.

• Continues until a stop codon is reached, releasing the newly formed protein.

Gene Regulation in Bacteria

Operon Concept

• Operon: A cluster of genes controlled by a promoter and an operator.

• Types of Genes:

  • Constitutive: Always active.

  • Inducible: Turned on by presence of specific substrates (e.g., lactose).

  • Repressible: Remains active until repressed by metabolites (e.g., tryptophan).

Mutations

Types of Mutations

• Base Substitution: Single nitrogen base is replaced.

  • Silent Mutation: No change in protein.

  • Missense Mutation: Results in a different amino acid.

  • Nonsense Mutation: Produces a premature stop codon.

  • Frameshift Mutation: Addition or deletion of bases that shifts reading frame.

Mutagens

• Agents inducing mutations:

  • Chemical mutagens (e.g., nitrous acid).

  • Radiation (e.g., UV light causing thymine dimers).

Genetic Transfer Mechanisms

Types of Transfer

• Vertical Gene Transfer: From parent to offspring.

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

  • Transformation: Uptake of "naked" DNA from the environment.

  • Conjugation: Transfer of DNA between bacteria via direct contact (sex pili).

  • Transduction: Transfer via bacteriophages, involving generalized or specialized forms.

Plasmids and Transposons

Plasmids

• Plasmids: Circular, self-replicating DNA, smaller than chromosomes.

  • Types:

    • Conjugative: Impart genes for pili and plasmid transfer.

    • Dissimilation: Code for enzymes for unusual compound catabolism.

    • Resistance Factors (R factors): Carry genes for antibiotic resistance.

Transposons

• Jumping Genes: Sequences that can move within and between DNA molecules.

• Contain genes necessary for their movement (transposase).

Conclusion

Review Topics

• Processes of DNA replication, transcription, and translation.

• Distinctions between prokaryotes and eukaryotes in genetic processes.

• Mutation types and DNA repair mechanisms.

• Gene transfer techniques and their implications in genetic diversity.