Chapter 7: Microbial Genetics

Introduction

Microbial genetics is the study of how microorganisms inherit traits and how their genetic material is structured, replicated, and altered. Understanding microbial genetics is essential for comprehending microbial evolution, antibiotic resistance, and biotechnology applications.

Structure and Function of Prokaryotic and Eukaryotic Genomes

Prokaryotic Genomes

• Chromosomes: Prokaryotes typically possess a single, circular chromosome located in the nucleoid region of the cytoplasm. They are haploid, meaning they have one copy of each gene.

• Plasmids: Small, circular DNA molecules that replicate independently of the chromosome. Plasmids often carry genes that confer advantages, such as antibiotic resistance, but are not essential for normal growth.

Eukaryotic Genomes

• Nuclear Chromosomes: Eukaryotes have multiple, linear chromosomes contained within a membrane-bound nucleus. They are typically diploid, with two copies of each chromosome.

• Extranuclear DNA: Mitochondria and chloroplasts contain their own circular DNA, resembling prokaryotic chromosomes. These organelles code for a small percentage of the cell's genes.

• Plasmids: Some fungi, algae, and protozoa may also carry plasmids.

Comparison Table: Prokaryotic vs. Eukaryotic Genomes

DNA Structure and Replication

DNA Structure

• Nucleotides: The building blocks of DNA, each consisting of a phosphate group, a deoxyribose sugar, and a nitrogenous base (adenine, guanine, cytosine, or thymine).

• Sugar-Phosphate Backbone: Nucleotides are linked by covalent bonds between the sugar of one nucleotide and the phosphate of the next, forming a backbone.

• Base Pairing: Adenine pairs with thymine (A-T), and guanine pairs with cytosine (G-C) via hydrogen bonds.

DNA Replication

• Semiconservative Process: Each new DNA molecule consists of one original (parental) strand and one newly synthesized (daughter) strand.

• Key Enzyme: DNA polymerase synthesizes the new strand by adding complementary nucleotides to the template strand.

• Energy Source: Triphosphate deoxyribonucleotides provide both the monomers and the energy for polymerization.

Equation:

Genotype and Phenotype

• Genotype: The complete set of genes in an organism's genome; the genetic blueprint.

• Phenotype: The observable physical and functional traits of an organism, determined by gene expression.

• Not all genes are expressed at all times; gene regulation determines which genes are active.

The Central Dogma of Genetics

The central dogma describes the flow of genetic information within a cell:

• Replication: DNA is copied to produce identical DNA molecules.

• Transcription: DNA is transcribed into RNA.

• Translation: RNA is translated into polypeptides (proteins).

Equation:

Additional info: In some viruses, reverse transcription (RNA to DNA) can occur.

Transcription and Translation

Transcription

• Prokaryotes: Occurs in the nucleoid; a single RNA polymerase and transcription factor are involved.

• Eukaryotes: Occurs in the nucleus, mitochondria, and chloroplasts; involves three nuclear RNA polymerases and multiple transcription factors. mRNA is processed before translation.

Translation

• Codons: Sequences of three nucleotides in mRNA that specify amino acids.

• Start Codon: AUG (codes for methionine) is the universal start codon.

• Genetic Code: The set of rules by which nucleotide sequences are translated into amino acid sequences.

Equation:

Regulation of Genetic Expression

• Some genes are constitutively expressed (always active).

• Other genes are regulated and expressed only in response to environmental changes, conserving energy.

• Regulation can occur at the transcriptional or translational level.

Mutations

Definition and Types

• Mutation: A change in the nucleotide base sequence of a genome.

• Point Mutation: Affects a single base pair. Types include:

  • Silent Mutation: No change in the amino acid sequence.

  • Missense Mutation: Results in a different amino acid.

  • Nonsense Mutation: Creates a premature stop codon.

• Frameshift Mutation: Insertion or deletion of nucleotides that alters the reading frame, often resulting in nonfunctional proteins.

Causes and Frequency

• Spontaneous Mutations: Occur naturally due to errors in DNA replication or repair.

• Induced Mutations: Caused by mutagens such as radiation or chemicals.

• Mutations are rare due to the high fidelity of DNA polymerases (about 1 in 10 million genes).

• Some RNA viruses have higher mutation rates because their polymerases lack proofreading ability.

Example: Influenza and SARS-CoV-2 have high mutation rates, affecting vaccine development.

Genetic Recombination and Gene Transfer

Genetic Recombination

• The exchange of genetic material between different DNA molecules, leading to genetic diversity.

Vertical vs. Horizontal Gene Transfer

• Vertical Gene Transfer: Transmission of genetic material from parent to offspring during reproduction.

• Horizontal Gene Transfer: Movement of genetic material between organisms other than by descent. Mechanisms include:

  • Transformation: Uptake of free DNA from the environment.

  • Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

  • Conjugation: Direct transfer of DNA between bacteria via cell-to-cell contact, often involving plasmids and F factors.

Conjugation Details

• F Factor: A plasmid that enables a bacterium to form a pilus for DNA transfer.

• F+ Cells: Bacteria possessing the F plasmid; can donate DNA.

• Hfr Cells: Bacteria with the F plasmid integrated into their chromosome; can transfer chromosomal genes during conjugation.

Summary Table: Types of Horizontal Gene Transfer

Applications and Importance

• Understanding microbial genetics is crucial for addressing antibiotic resistance, genetic engineering, and evolutionary biology.

• Mutations and gene transfer contribute to microbial diversity and adaptation.