Chapter_6_PPT

Chapter 06: Microbial Genetics

Section 6.1: Introduction to Genetics and Genes

• Learning Outcomes:

  • Define terms: genome and gene.

  • Differentiate between genotype and phenotype.

  • Summarize the steps of bacterial DNA replication and the enzymes involved.

Levels of Genetic Study

• Organism Level: Focus on entire organisms.

• Cell Level: Examination of cell structures and genes.

• Chromosome Level: Genes organized within chromosomes.

• Molecular Level: Study of DNA and RNA sequences.

Introduction to Genetics

• Genetics: Study of heredity and variation.

  • Transmission of traits from parents to offspring.

  • Expressions of these traits.

  • Structure and function of genetic material, and its change over time.

The Nature of the Genetic Material

• Genome: Total genetic material in an organism.

  • Mostly in chromosomes; some in plasmids and organelles (mitochondria, chloroplasts).

  • Cells mainly contain DNA; viruses may have DNA or RNA.

• Genomics: Study of entire genome.

Genome Location and Forms in Cells and Viruses

• General locations of genetic material vary between cellular structures (not to scale).

Levels of Structure and Function of the Genome

• Chromosome: Structure containing DNA.

  • Eukaryotic: Found in nucleus, varies in number and appearance (linear).

  • Bacterial: Usually a single, circular chromosome.

Genes and Genetics

• Genes: Fundamental units coding for proteins or RNA.

  • Categories:

    • Structural Genes: Code for proteins.

    • RNA Genes: Code for RNA machinery.

    • Regulatory Genes: Control gene expression.

  • Only 2% of genes in organisms code for proteins.

Genotype Versus Phenotype

• Genotype: Total genetic makeup of an organism.

• Phenotype: Physical expression of genotype traits.

  • More genes exist in genotype than expressed in phenotype.

The Size and Packaging of Genomes

• Genome sizes vary:

  • E. coli: Single chromosome with 4000-5000 genes, 1 mm long when unwound.

  • Human Cell: 23,000 genes on 46 chromosomes.

The Structure of DNA

• Basic unit: Nucleotide (phosphate, deoxyribose sugar, nitrogenous base).

  • Bases: Purines (Adenine, Guanine) pair with Pyrimidines (Thymine, Cytosine).

Important Characteristics of DNA

• Antiparallel Arrangement: One strand runs 5' to 3'; the opposite runs 3' to 5'.

DNA Replication

• Semiconservative Replication: Each daughter strand contains one original strand.

  • Key Steps in DNA Replication:

    • Enzymes Involved:

      • Helicase: Unzips DNA.

      • Primase: Synthesizes RNA primer.

      • DNA Polymerase III: Adds nucleotides to new chain.

      • DNA Polymerase I: Removes primer and repairs mismatches.

      • DNA Ligase: Joins Okazaki fragments.

Replication Fork and Primer

• Replication Fork: Area where DNA strands are unwound.

• Primer: RNA segment that initiates replication.

Applications of the DNA Code: Transcription and Translation

• Learning Outcomes:

  • Discuss changes in the “central dogma” of genetics.

  • Identify differences between RNA and DNA.

  • Illustrate transcription steps.

• Central Dogma:

  • DNA -> RNA (transcription) -> Protein (translation).

  • Exceptions: RNA viruses and retroviruses.

The Gene–Protein Connection

• Proteins determine phenotype; their structure/function dictated by DNA.

• Proteomics: Study of expressed proteins.

Transcription and Translation Components

• Participants: mRNA, tRNA, rRNA, enzymes, raw materials.

• mRNA: A transcript of a structural gene, synthesized during transcription.

Genetic Code and Translation

• Codons: Groups of three nucleotides that specify amino acids.

  • Start Codon: AUG

  • Stop Codons: UAA, UAG, UGA.

  • The universal nature of the genetic code across organisms.

• Translation Process: Initiation, elongation, termination.

Differences Between Prokaryotic and Eukaryotic Processes

• Prokaryotes: Cotranscriptional translation, simultaneous processes.

• Eukaryotes: Transcription in the nucleus, translation in the cytoplasm, mRNA typically codes for one protein.

Mutations in Microbial Genetics

• Mutations: Changes in nucleotide sequence, important in evolution.

  • Types:

    • Spontaneous: Natural errors during replication.

    • Induced: Caused by mutagens.

  • Categories:

    • Point mutations (silent, missense, nonsense).

    • Frameshift mutations: Insertions or deletions changing reading frame.

Importance of Mutations

• Impact genetic diversity and evolution; some beneficial, others detrimental.

Gene Regulation

• Operons: Sets of genes regulated together.

  • Inducible Operons: Turned on by substrates for enzymes.

  • Repressible Operons: Turned off by final product of a pathway.

Horizontal Gene Transfer

• Methods: Conjugation, transformation, transduction.

• Conjugation: Direct transfer of DNA through pilus.

• Transformation: Uptake of free DNA from the environment.

• Transduction: DNA transfer via bacteriophages.

Conclusion

• Understanding microbial genetics is critical in studying heredity, gene expression, and evolution in microorganisms.