Microbial Genetics: Structure, Function, and Transfer of Genetic Material

Microbial Genetics

Introduction to Microbial Genetics

Microbial genetics is the study of how microorganisms inherit traits, how genetic information is expressed, and how genes are replicated and transferred. Understanding microbial genetics is essential for grasping how bacteria adapt, evolve, and interact with their environments.

• Genetics: The study of genes, their functions, and inheritance.

• Chromosomes: Structures containing DNA that carry hereditary information.

• Genes: Segments of DNA encoding functional products, usually proteins.

• Genome: The complete genetic information in a cell.

• Genomics: The sequencing and molecular characterization of genomes.

Structure and Function of the Genetic Material

The Genetic Code and Central Dogma

The genetic code is a set of rules that determines how nucleotide sequences are converted into amino acid sequences in proteins. The central dogma of molecular biology describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

• Central Dogma: DNA → RNA → Protein

• Gene Expression: When the product encoded by a gene is produced, the gene is said to be expressed.

Genotype and Phenotype

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable characteristics resulting from gene expression.

DNA and Chromosomes in Bacteria

• Bacteria typically have a single, circular chromosome composed of DNA and associated proteins.

• Example: Escherichia coli has a chromosome with 4.6 million base pairs, which is highly supercoiled.

• The genome includes protein-coding genes and noncoding regions such as short tandem repeats (STRs).

The Flow of Genetic Information

Vertical Gene Transfer

• Genetic information is passed from one generation to the next.

Transcription and Translation

• Genetic information in DNA is transcribed into mRNA, which is then translated into proteins.

DNA Replication

Structure of DNA

• DNA forms a double helix with a backbone of deoxyribose-phosphate.

• Strands are held together by hydrogen bonds: A-T and C-G.

• Strands are antiparallel (run in opposite directions).

Mechanism of DNA Replication

• Each strand serves as a template for a new strand (semiconservative replication).

• Key enzymes:

  • Topoisomerase and Gyrase: Relax DNA strands.

  • Helicase: Separates DNA strands.

  • DNA Polymerase: Adds nucleotides in the 5' → 3' direction, initiated by an RNA primer.

  • DNA Ligase: Joins Okazaki fragments on the lagging strand.

• Replication is bidirectional and highly accurate due to proofreading by DNA polymerase.

Equation (Semiconservative Replication):

$\text{Parental DNA} \xrightarrow{\text{Replication}} \text{One Parental Strand} + \text{One New Strand}$

RNA and Protein Synthesis

Types of RNA

• mRNA (Messenger RNA): Carries genetic code from DNA to ribosomes.

• tRNA (Transfer RNA): Transports amino acids during protein synthesis.

• rRNA (Ribosomal RNA): Integral part of ribosomes.

Transcription in Prokaryotes

• Transcription is the synthesis of a complementary mRNA strand from a DNA template.

• Steps:

  1. Initiation: RNA polymerase binds to the promoter region.

  2. Elongation: RNA polymerase synthesizes RNA in the 5' → 3' direction.

  3. Termination: Transcription stops at the terminator sequence.

Translation

• Translation converts mRNA into a polypeptide chain (protein).

• Codons: Triplets of mRNA nucleotides; 64 possible codons (61 sense codons for amino acids, 3 stop codons).

• Start Codon: AUG (codes for methionine).

• Stop Codons: UAA, UAG, UGA.

• Degeneracy: Most amino acids are encoded by more than one codon.

Steps of Translation

1. Initiation: Ribosomal subunits, mRNA, and initiator tRNA assemble at the start codon.

2. Elongation: tRNAs bring amino acids to the ribosome, matching codons with anticodons; peptide bonds form between amino acids.

3. Termination: When a stop codon is reached, the ribosome releases the completed polypeptide.

Additional info: In bacteria, translation can begin before transcription is complete because both processes occur in the cytoplasm.

Transcription in Eukaryotes

• Transcription occurs in the nucleus; translation occurs in the cytoplasm.

• Genes contain exons (coding regions) and introns (noncoding regions).

• After transcription, introns are removed by small nuclear ribonucleoproteins (snRNPs), and exons are spliced together.

Regulation of Bacterial Gene Expression

Types of Genes

• Constitutive genes: Expressed at a fixed rate (always "on").

• Inducible genes: Expressed only when needed.

• Repressible genes: Can be turned off when not needed.

Pre-Transcriptional Control

• Repression: Inhibits gene expression, usually mediated by repressor proteins.

• Induction: Turns on gene expression, initiated by inducers.

The Operon Model

• Operon: A set of operator and promoter sites and the structural genes they control.

• Promoter: Site where RNA polymerase initiates transcription.

• Operator: Site that controls transcription of structural genes.

Operon Type	Regulation	Example
Inducible	Off unless inducer is present	lac operon
Repressible	On unless corepressor is present	trp operon

• lac operon: In the absence of lactose, the repressor binds the operator, blocking transcription. In the presence of lactose, allolactose (inducer) inactivates the repressor, allowing transcription.

• trp operon: Excess tryptophan acts as a corepressor, activating the repressor to block transcription.

Mutation

Definition and Types

• Mutation: A permanent change in the DNA sequence.

• Can be neutral, beneficial, or harmful.

• Mutagens: Agents that cause mutations.

• Spontaneous mutations: Occur without mutagens.

Type of Mutation	Description
Silent	Does not affect protein function (often due to genetic code degeneracy)
Missense	Base substitution changes one amino acid
Nonsense	Base substitution creates a stop codon
Frameshift	Insertion or deletion shifts reading frame, altering downstream amino acids

Chemical and Physical Mutagens

• Chemical mutagens: Nucleoside analogs (e.g., AZT for HIV) can be incorporated into DNA, causing errors.

• Ionizing radiation: (X-rays, gamma rays) causes DNA breaks and oxidation.

• UV radiation: Causes thymine dimers, which block replication and transcription.

• Nucleotide excision repair: Enzymes remove and replace damaged DNA.

Genetic Transfer and Recombination

Genetic Recombination

• Exchange of genes between DNA molecules, increasing genetic diversity.

• Crossing over: DNA segments break and rejoin, inserting foreign DNA into chromosomes.

Vertical vs. Horizontal Gene Transfer

• Vertical gene transfer: Genes passed from parent to offspring.

• Horizontal gene transfer: Genes transferred between cells of the same generation (unique to prokaryotes).

Mechanism	Description
Conjugation	Direct transfer of DNA via cell-to-cell contact
Transformation	Uptake of naked DNA from the environment
Transduction	Transfer of DNA by bacteriophage (virus)

Plasmids and Transposons

Mobile Genetic Elements

• Plasmids: Self-replicating, circular DNA molecules (1-5% the size of a chromosome), often carry genes for antibiotic resistance or toxin production.

• Conjugative plasmid (F factor): Carries genes for sex pili and plasmid transfer.

• Transposons: DNA segments that can move from one location to another within a genome.

Mechanisms of Genetic Transfer in Bacteria

Transformation

• Genes are transferred as naked DNA from one bacterium to another.

• First demonstrated by Griffith's experiment with Streptococcus pneumoniae.

• Occurs when bacteria take up DNA fragments from lysed cells and incorporate them by recombination.

Conjugation

• Plasmids are transferred via direct cell-to-cell contact.

• Gram-negative bacteria use sex pili; Gram-positive bacteria use surface molecules.

• Donor cells (F+) transfer plasmids to recipient cells (F-), converting them to F+.

Transduction

• DNA is transferred from donor to recipient via a bacteriophage.

• Generalized transduction: Random bacterial DNA is packaged into phage particles.

• Specialized transduction: Specific bacterial genes are transferred by the phage.

Example: The spread of antibiotic resistance genes among bacteria is often mediated by plasmids and transposons through horizontal gene transfer mechanisms.