8 Microbial Genetics: Structure, Function, and Regulation of Genetic Material

Microbial Genetics

Introduction to Genetics

Genetics is the science of heredity, focusing on how genetic information is stored, expressed, and transmitted in microorganisms. Understanding microbial genetics is essential for grasping how bacteria adapt, evolve, and interact with their environment, including their roles in disease and biotechnology.

• Genetics: The study of genes, how they carry information, how information is expressed, and how genes are replicated.

• Genome: All the genetic information in a cell.

• Chromosome: Structures containing DNA that physically carry hereditary information; chromosomes contain genes.

• Gene: Segments of DNA that encode functional products, usually proteins.

• Genotype: The genetic makeup of an organism; the set of genes it carries.

• Phenotype: The expression of the genes; the observable traits.

• Genomics: The sequencing and characterization of genomes.

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system: DNA is transcribed into RNA, which is then translated into protein. This process is fundamental to gene expression and cellular function.

• DNA → RNA → Protein: The sequence of events for gene expression.

• Genetic code: The set of rules by which information encoded in DNA or RNA sequences is translated into proteins by living cells.

Genotype and Phenotype

The genotype refers to the genetic constitution of an organism, while the phenotype is the observable expression of those genes. Mutations or changes in the genotype can lead to changes in the phenotype.

• Genotype: The alleles present in an organism.

• Phenotype: The physical or biochemical characteristics that result from gene expression.

Structure and Function of Genetic Material

DNA Structure

DNA is a double helix composed of two antiparallel strands of nucleotides. The backbone consists of deoxyribose-phosphate, and the strands are held together by hydrogen bonds between complementary bases (A-T and C-G).

• Antiparallel strands: The two DNA strands run in opposite directions (5' to 3' and 3' to 5').

• Base pairing: Adenine pairs with thymine, and cytosine pairs with guanine.

Prokaryotic Chromosomes and Plasmids

Bacteria typically have a single, circular chromosome that is highly supercoiled to fit within the cell. Plasmids are small, circular DNA molecules that exist independently of the chromosome and often carry genes beneficial for survival, such as antibiotic resistance.

• Chromosome: Contains essential genes for survival.

• Plasmid: May carry genes for antibiotic resistance, virulence factors, or metabolic pathways.

DNA Replication

Mechanism of DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division. It is semiconservative, meaning each new DNA molecule consists of one parental and one newly synthesized strand. Replication is highly accurate due to proofreading by DNA polymerase.

• Initiation: Begins at the origin of replication.

• Enzymes involved: Helicase unwinds DNA, primase synthesizes RNA primers, DNA polymerase adds nucleotides, ligase joins Okazaki fragments.

• Leading strand: Synthesized continuously.

• Lagging strand: Synthesized discontinuously as Okazaki fragments.

• Energy source: Hydrolysis of nucleoside triphosphates provides energy for DNA synthesis.

Bacterial DNA Replication

Most bacterial DNA replication is bidirectional, starting from a single origin and proceeding in both directions around the circular chromosome. Each daughter cell receives one complete copy of the DNA molecule.

• Bidirectional replication: Increases efficiency and speed of DNA synthesis.

• Proofreading: DNA polymerase corrects errors, ensuring high fidelity.

Gene Expression: Transcription and Translation

Transcription

Transcription is the synthesis of a complementary mRNA strand from a DNA template. In prokaryotes, transcription occurs in the cytoplasm and is often coupled with translation.

• Initiation: RNA polymerase binds to the promoter region.

• Elongation: RNA polymerase synthesizes RNA in the 5' to 3' direction.

• Termination: Transcription stops at the terminator sequence.

Translation

Translation is the process by which the sequence of codons in mRNA is used to assemble amino acids into a polypeptide chain, forming a protein. This process occurs at the ribosome and involves mRNA, tRNA, and rRNA.

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

• Start codon: AUG (methionine).

• Stop codons: UAA, UAG, UGA (signal termination of translation).

• tRNA: Brings amino acids to the ribosome and matches them to the mRNA codon via its anticodon.

• Peptide bonds: Link amino acids together to form proteins.

Regulation of Gene Expression

Operons and Gene Regulation

Gene expression in bacteria is tightly regulated, often at the level of transcription. Operons are clusters of genes under the control of a single promoter and operator, allowing coordinated expression.

• Inducible operon: Usually off; turned on by an inducer (e.g., lac operon).

• Repressible operon: Usually on; turned off by a corepressor (e.g., trp operon).

• Catabolite repression: Inhibits the use of alternative carbon sources when glucose is present.

Type	Default State	Regulation	Example
Inducible	Off	Turned on by inducer	Lac operon
Repressible	On	Turned off by corepressor	Trp operon

Mutations and Genetic Variation

Types of Mutations

Mutations are permanent changes in the DNA sequence. They can be spontaneous or induced by mutagens and may have neutral, beneficial, or harmful effects.

• Base substitution (point mutation): One base is replaced by another.

• Missense mutation: Results in a different amino acid.

• Nonsense mutation: Results in a stop codon.

• Frameshift mutation: Insertion or deletion of bases shifts the reading frame.

Mutagens and DNA Repair

Mutagens are agents that cause mutations, such as chemicals or radiation. Cells have repair mechanisms, including proofreading by DNA polymerase and excision repair, to correct errors and maintain genetic stability.

• Photolyase: Repairs UV-induced thymine dimers.

• Nucleotide excision repair: Removes and replaces damaged DNA segments.

Genetic Transfer and Recombination

Horizontal and Vertical Gene Transfer

Genetic information can be transferred vertically (from parent to offspring) or horizontally (between cells of the same generation). Horizontal gene transfer increases genetic diversity and can spread traits such as antibiotic resistance.

• Vertical gene transfer: Transmission of genes from parent to offspring during cell division.

• Horizontal gene transfer: Transfer of genes between cells via transformation, transduction, or conjugation.

Plasmids and Transposons

Plasmids are small, self-replicating DNA molecules that can carry genes for antibiotic resistance or other traits. Transposons are mobile genetic elements that can move within and between DNA molecules, facilitating genetic diversity and the spread of resistance genes.

• Conjugative plasmid: Carries genes for sex pili and plasmid transfer.

• Dissimilation plasmid: Encodes enzymes for the catabolism of unusual compounds.

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

• Transposons: "Jumping genes" that move within the genome.

Mechanisms of Genetic Recombination

• Transformation: Uptake of naked DNA from the environment.

• Conjugation: Direct transfer of DNA between bacteria via sex pili.

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

Summary Table: Key Terms and Concepts

Term	Definition
Genotype	Genetic makeup of an organism
Phenotype	Observable traits
Mutation	Permanent change in DNA sequence
Plasmid	Small, circular DNA molecule in bacteria
Operon	Cluster of genes under a single promoter
Transposon	Mobile genetic element
Transformation	Uptake of naked DNA
Conjugation	DNA transfer via cell-to-cell contact
Transduction	DNA transfer by bacteriophage