Microbial Genetics

Introduction to Genetics

Genetics is the scientific study of genes, heredity, and how traits are transmitted from parents to offspring. In microbiology, understanding genetics is crucial for exploring microbial diversity, adaptation, and evolution.

• Genome: The entire genetic complement of an organism, including all its genes and nucleotide sequences.

• Gene: A segment of DNA that encodes a functional product, usually a protein.

The Structure and Replication of Genomes

The Structure of Nucleic Acids

Nucleic acids are polymers of nucleotides, which are the building blocks of DNA and RNA. Each nucleotide consists of a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.

• Nucleoside: Consists of a sugar and a nitrogenous base.

• Nucleotide: Consists of a sugar, a nitrogenous base, and a phosphate group.

• Nitrogenous bases: Purines (Adenine, Guanine) and Pyrimidines (Thymine, Cytosine, Uracil).

Base Pairing and DNA Structure

Base pairing occurs between purines and pyrimidines, ensuring the stability and uniform width of the DNA double helix.

• Adenine (A) pairs with Thymine (T) in DNA or Uracil (U) in RNA.

• Guanine (G) pairs with Cytosine (C).

Prokaryotic and Eukaryotic Genomes

Prokaryotic Genomes

Prokaryotic cells (bacteria and archaea) typically have a single, circular chromosome located in the nucleoid region. They may also contain plasmids—small, circular DNA molecules that replicate independently and can confer advantages such as antibiotic resistance.

• Haploid: Single chromosome copy per cell.

• Plasmids: Fertility, resistance, bacteriocin, and virulence plasmids.

Eukaryotic Genomes

Eukaryotic cells have multiple, linear chromosomes contained within a nucleus. DNA is packaged with histone proteins into nucleosomes, forming chromatin. Eukaryotes are typically diploid (two chromosome copies per cell).

• Extranuclear DNA: Found in mitochondria and chloroplasts, resembling prokaryotic chromosomes.

• Some fungi, algae, and protozoa also carry plasmids.

Comparison Table: Microbial Genomes

DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division. It is semiconservative, meaning each new DNA molecule consists of one original and one new strand.

• Replication requires triphosphate deoxyribonucleotides for both building blocks and energy.

• Replication begins at the origin and proceeds in a 5′ to 3′ direction.

• Leading strand: Synthesized continuously.

• Lagging strand: Synthesized discontinuously in Okazaki fragments.

Additional Features of Bacterial DNA Replication

• Bidirectional replication from a single origin.

• Gyrases and topoisomerases relieve supercoiling.

• DNA methylation regulates gene expression, replication initiation, and DNA repair.

Eukaryotic DNA Replication

• Multiple origins of replication per chromosome.

• Four main DNA polymerases.

• Shorter Okazaki fragments.

• Only cytosine bases are methylated in plants and animals.

Gene Function: From Genotype to Phenotype

Genotype and Phenotype

The genotype is the set of genes in the genome, while the phenotype is the observable physical and functional traits of the organism.

Central Dogma: Transcription and Translation

Genetic information flows from DNA to RNA (transcription) and from RNA to protein (translation).

• Transcription: DNA is copied into RNA by RNA polymerase.

• Translation: Ribosomes use mRNA to synthesize polypeptides.

Types of RNA

• RNA primers

• Messenger RNA (mRNA)

• Ribosomal RNA (rRNA)

• Transfer RNA (tRNA)

• Regulatory RNA

• Ribozymes

Transcription in Prokaryotes

• Occurs in the nucleoid.

• Three steps: Initiation (sigma factor recognizes promoter), elongation (RNA polymerase synthesizes RNA 5′→3′), termination.

Transcription in Eukaryotes

• Occurs in the nucleus, mitochondria, and chloroplasts.

• Three types of nuclear RNA polymerases.

• mRNA is processed before translation: 5′ capping, 3′ polyadenylation, and splicing to remove introns.

Translation: Protein Synthesis

Translation is the process by which ribosomes synthesize polypeptides using the genetic information encoded in mRNA. It is universal and redundant, meaning multiple codons can code for the same amino acid.

• Participants: mRNA, tRNA, ribosomes (rRNA + proteins).

• Three stages: Initiation, elongation, termination.

• Initiation and elongation require GTP as an energy source.

Translation in Eukaryotes

• Initiation occurs when the ribosomal subunit binds to the 5′ guanine cap.

• The first amino acid is methionine (not formyl-methionine as in prokaryotes).

• Polypeptides can be synthesized into the rough endoplasmic reticulum.

Comparison Table: Replication, Transcription, Translation

Regulation of Genetic Expression

Gene Expression Control in Prokaryotes

Gene expression can be regulated at the level of transcription or translation. Most genes are expressed continuously, but others are regulated to conserve energy.

• Operon: A cluster of genes under the control of a single promoter and operator, allowing coordinated regulation.

• Inducible operons (e.g., lac operon): Activated by inducers, regulate catabolic pathways.

• Repressible operons (e.g., trp operon): Deactivated by repressors, regulate anabolic pathways.

Basic Roles of Operons

Regulation by RNA Molecules

• microRNAs (miRNAs): Bind complementary mRNA and inhibit translation.

• Small interfering RNAs (siRNAs): Bind and inactivate target RNA or DNA.

• Riboswitches: RNA molecules that change shape to regulate translation.

Mutations of Genes

Types and Effects of Mutations

A mutation is a change in the nucleotide sequence of a genome. Most mutations are deleterious, but some may confer advantages.

• Point mutations: Affect a single base pair (substitutions, insertions, deletions).

• Gross mutations: Involve larger changes (inversions, duplications, transpositions).

Table: Types of Point Mutations and Their Effects

Mutagens and DNA Repair

• Mutagens increase mutation rates (10–1000x).

• Types: Radiation (ionizing, nonionizing), chemical mutagens (nucleotide analogs, nucleotide-altering chemicals, frameshift mutagens).

• Cells have multiple DNA repair mechanisms to correct damage.

Identifying Mutants, Mutagens, and Carcinogens

• Mutants: Descendants of cells with uncorrected mutations.

• Wild-type: Cells normally found in nature.

• Methods: Positive selection, negative (indirect) selection, Ames test.

Genetic Recombination and Transfer

Genetic Recombination

Genetic recombination involves the exchange of nucleotide sequences between DNA molecules, resulting in new genetic combinations.

• Vertical gene transfer: DNA transmission from parent to offspring (e.g., binary fission).

• Horizontal gene transfer: DNA transfer between cells not in a parent-offspring relationship.

Horizontal Gene Transfer Mechanisms in Prokaryotes

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

• Transduction: Transfer of DNA via bacteriophages (generalized or specialized).

• Conjugation: Direct transfer of DNA between cells via physical contact (e.g., F plasmid).

Table: Natural Mechanisms of Horizontal Genetic Transfer

Transposons and Transposition

Transposons are DNA segments that can move from one location to another within a genome, causing frameshift mutations. They may carry additional genes (complex transposons) or just the transposase gene (insertion sequences).

Additional info: Understanding microbial genetics is essential for fields such as antibiotic resistance, biotechnology, and evolutionary biology.