Microbial Genetics - Lecture Notes

Core Principles and Terminology of Microbial Genetics

• Genetics: The science of heredity and the study of genes, including how they carry biological information, how that information is expressed within an organism, and how genetic material is replicated.

• Genome: The complete genetic complement or total hereditary information contained within a cell. In bacteria, this includes the chromosome as well as any existing plasmids.

• Chromosomes: Cellular structures composed of DNA and associated proteins that physically carry hereditary information; chromosomes harbor individual functional units known as genes.

  • Bacterial chromosomes typically exist as a single, circular double-stranded DNA molecule.
  • Eukaryotic chromosomes are linear structures compacted around basic histone proteins inside a membrane-delimited nucleus.

• Genes: Specific segments of chromosomal or plasmid DNA that encode functional products, which are typically proteins (or functional non-coding RNAs such as ribosomal or transfer RNA).

• Genotype: The specific genetic constitution or full collection of genes within an organism.

• Phenotype: The observable, physical, or physiological properties of an organism resulting from the expression of its genotype.

• Genomics: The molecular characterization, sequencing, and comparative analysis of whole genomes.

• The Central Dogma of Molecular Biology: The foundational paradigm governing the directional flow of genetic information: $\text{DNA} \rightarrow \text{mRNA} \rightarrow \text{Protein} \rightarrow \text{Function}$.

  • DNA directs the synthesis of messenger RNA via transcription.
  • Messenger RNA directs the synthesis of specific polypeptides via translation.
  • Folded, functional proteins (such as structural components and enzymes) execute cellular biochemical functions.
  • Mutations alter nucleotide sequences within DNA, producing altered mRNA transcripts, which yield modified proteins with altered or lost biological functions.

• Operon Regulation: Bacterial gene expression is rigorously controlled by operons—coordinated clusters of genes under the control of a single promoter and operator region.

• Medical and Biotechnological Implications of Microbial Genetics:
  • Pathogenesis and Bacterial Disease: Many bacterial infections stem directly from the expression of genes encoding toxic proteins that destroy host tissues. For example, Vibrio cholerae harbors genes encoding an enterotoxin that triggers secretory diarrhea and massive dehydration, which can be fatal if left untreated.
  • Antibiotic Resistance Mechanisms: Spontaneous mutations in the bacterial genome and subsequent horizontal acquisition of resistance genes confer resistance to antimicrobial agents. Staphylococcus aureus acquired resistance to beta-lactam antibiotics like penicillin. Subsequent development of methicillin to treat penicillin-resistant strains drove the emergence of Methicillin-resistant Staphylococcus aureus (MRSA), which represents a major cause of severe healthcare-associated infections.
  • Recombinant Biotechnology: Microorganisms can be genetically modified to introduce human genes that code for therapeutic proteins. Recombinant human insulin (e.g., Humulin R) used for the clinical management of diabetes mellitus is synthesized commercially in engineered bacterial systems.

DNA Architecture and Organization

• Double Helix Geometry: Deoxyribonucleic acid (DNA) is structured as a right-handed double-stranded helix consisting of repeating nucleotide monomer subunits.

• Sugar-Phosphate Backbone: Each polynucleotide chain features an alternating backbone consisting of five-carbon deoxyribose sugar molecules covalently linked by phosphate groups via phosphodiester bonds.

• Complementary Base Pairing: The two strands are held together across the helical axis by hydrogen bonds connecting specific nitrogenous bases:

  • Adenine ($\text{A}$), a purine, pairs strictly with Thymine ($\text{T}$), a pyrimidine, forming $2$ hydrogen bonds.
  • Guanine ($\text{G}$), a purine, pairs strictly with Cytosine ($\text{C}$), a pyrimidine, forming $3$ hydrogen bonds.
  • The linear sequence of these four nitrogen-containing bases provides the precise genetic code and instructions for the cell.

• Antiparallel Strand Orientation:
  • The two complementary strands of the double helix run in opposite chemical directions.
  • One strand is oriented in the $5' \rightarrow 3'$ direction (terminating with a free phosphate group at the $5'$ carbon of deoxyribose), while the complementary partner strand is oriented in the $3' \rightarrow 5'$ direction (terminating with a free hydroxyl group on the $3'$ carbon of deoxyribose).
  • The sugar-phosphate backbone of one strand is upside down relative to the backbone of the opposing strand.

The Flow of Genetic Information

• Three Major Pathways of Information Flow:
  • Gene Expression: Genetic information within DNA is transcribed into mRNA and translated into functional polypeptides and metabolic enzymes necessary for the cell to metabolize, grow, and maintain homeostasis.
  • Recombination (Horizontal Gene Transfer):
  • Involves the transfer and integration of genetic elements between donor and recipient cells within the same generation.
  • Produces recombinant cells containing novel genetic combinations, which can accelerate the dissemination of virulence factors and antibiotic resistance genes.
  • Replication (Vertical Gene Transfer):
  • Flow of genetic information from a parent cell to daughter offspring cells during cell division.
  • Duplicates the genome to ensure accurate hereditary preservation across generations.

Mechanisms and Enzymatic Machinery of DNA Replication

• Semiconservative Nature of Replication: Each parental double-stranded DNA molecule is unzipped so that each single strand serves as a template for synthesizing an exact complementary daughter strand, yielding two identical double helices that each contain one conserved parental strand and one newly synthesized daughter strand.

• Replication Energetics:

  • DNA polymerization is an endergonic anabolic process powered directly by its incoming substrates: nucleoside triphosphates (dATP, dCTP, dGTP, dTTP).
  • When a free nucleoside triphosphate aligns with its complementary template base, it hydrolyzes its terminal two phosphate groups as pyrophosphate ($\text{PP}_i$).
  • The hydrolysis of these high-energy phosphoanhydride bonds provides the exergonic driving energy required to form the covalent phosphodiester bond linking the incoming nucleotide's $5'$ phosphate to the preceding nucleotide's $3'$ hydroxyl ($\text{OH}$) group.

• Mechanics at the Replication Fork:
  • Unwinding and Relaxation: Helicase breaks weak hydrogen bonds between complementary base pairs to unwind the double helix, creating a Y-shaped replication fork. Topoisomerase and DNA gyrase relieve positive supercoiling and torsional strain accumulating ahead of the advancing fork. Single-stranded DNA-binding proteins stabilize the separated template strands to prevent reannealing.
  • Directionality Constraints: DNA polymerases can add new deoxyribonucleotides exclusively to a free $3'\text{-OH}$ group, meaning nascent DNA synthesis proceeds strictly in the $5' \rightarrow 3'$ direction.
  • Leading Strand Synthesis: The strand running toward the replication fork is synthesized continuously by DNA polymerase requiring only a single initial RNA primer.
  • Lagging Strand Synthesis: The strand running away from the fork must be synthesized discontinuously:
  • Primase (an RNA polymerase) synthesizes short RNA primers at intervals along the exposed lagging template.
  • DNA polymerase extends these primers in the $5' \rightarrow 3'$ direction away from the fork, forming discontinuous segments known as Okazaki fragments.
  • DNA polymerase enzymatically degrades and removes the ribonucleotide primers, filling the gaps with deoxyribonucleotides.
  • DNA ligase forms covalent phosphodiester bonds to seal the remaining nicks between Okazaki fragments.

• Enzymes in DNA Replication, Expression, and Repair (Table 8.1):

  • DNA Gyrase: Relaxes supercoiling ahead of the replication fork.
  • DNA Ligase: Makes covalent bonds to join DNA strands, Okazaki fragments, and new segments during excision repair.
  • DNA Polymerases: Synthesizes DNA; proofreads and repairs damaged or mismatched DNA.
  • Endonucleases: Cut the phosphodiester backbone within a strand of DNA; facilitate excision repair and insertion events.
  • Exonucleases: Hydrolyze and remove nucleotides sequentially from exposed ends of DNA strands; facilitate repair.
  • Helicase: Unwinds double-stranded DNA into single strands at the replication fork.
  • Methylase: Adds methyl groups to selected nitrogenous bases in newly synthesized DNA strands to distinguish parental strands and protect against restriction cleavage.
  • Photolyase: Uses visible light energy to split and repair ultraviolet-induced pyrimidine (thymine) dimers.
  • Primase: An specialized RNA polymerase that synthesizes short complementary RNA primers on a single-stranded DNA template to initiate DNA synthesis.
  • Ribozyme: Catalytic RNA molecule that excises non-coding introns and splices protein-coding exons together.
  • RNA Polymerase: Transcribes complementary RNA strands from a DNA template strand.
  • snRNP: Small nuclear ribonucleoprotein particle (RNA-protein complex) that splices pre-mRNA by removing introns and joining exons.
  • Topoisomerase: Relaxes supercoiling ahead of the replication fork; resolves and decatenates linked circular daughter DNA chromosomes at the completion of replication.
  • Transposase: Cleaves the DNA backbone at insertion sequences to facilitate genetic transposition, generating single-stranded sticky ends.

• Bidirectional Replication in Bacteria:

  • Bacterial DNA replication initiates at a defined chromosomal locus termed the origin of replication ($\text{oriC}$).
  • Replication proceeds bidirectionally, establishing two replication forks moving in opposite directions around the circular chromosome.
  • The two forks advance until they reach the termination region ($\text{ter}$), where replication terminates, producing two interlinked circular chromosome copies that are separated by topoisomerase prior to partitioning into daughter cells.
  • High fidelity is preserved throughout this process due to the intrinsic $3' \rightarrow 5'$ exonucleolytic proofreading capability of DNA polymerase.

RNA Classes and the Mechanism of Transcription

• Chemical Properties of RNA:

  • Generally single-stranded.
  • Contains ribose pentose sugars instead of deoxyribose.
  • Uses the pyrimidine Uracil ($\text{U}$) instead of Thymine ($\text{T}$); Uracil forms complementary hydrogen bonds with Adenine ($\text{A}$).

• Major Classes of RNA:

  • Messenger RNA (mRNA): Carries the genetic message transcribed from a gene to the ribosome, serving as the template for polypeptide assembly.
  • Ribosomal RNA (rRNA): Forms structural and catalytic components of the ribosome, aligning mRNA and catalyzing peptide bond synthesis.
  • Transfer RNA (tRNA): Cloverleaf-shaped adaptor molecules that carry specific activated amino acids to the ribosome, matching their anticodons to corresponding mRNA codons.

• The Transcription Process in Prokaryotes:
  • Initiation: RNA polymerase binds to a specific non-coding promoter sequence upstream of the gene on the DNA double helix. RNA polymerase unwinds the local DNA, creating an open transcription bubble.
  • Elongation: RNA polymerase moves along the template strand, synthesizing a complementary single-stranded RNA molecule in the $5' \rightarrow 3'$ direction using ribonucleoside triphosphates (ATP, CTP, GTP, UTP). Only one DNA strand acts as the template strand. Behind the advancing enzyme, the nascent RNA strand peels away, and the DNA double helix rewinds.
  • Termination: Transcription continues until the polymerase encounters a specific terminator sequence, triggering the release of RNA polymerase and the newly synthesized complete RNA transcript, allowing the DNA duplex to close completely.

Translation and the Genetic Code

• The Genetic Code and Codon Organization:
  • The genetic code consists of codons—triplets of three adjacent mRNA nucleotides that specify an individual amino acid.
  • A total of $64$ codons exist in the universal genetic code ($4^3 = 64$ variations):
  • $61$ sense codons code for the $20$ biological amino acids.
  • $3$ nonsense (stop) codons do not code for amino acids but signal the termination of translation: $UAA$, $UAG$, and $UGA$.
  • The universal start codon is $AUG$, which codes for the amino acid formylmethionine in bacteria (and methionine in eukaryotes).
  • Degeneracy of the Code: The genetic code is degenerate (redundant), meaning that most individual amino acids are encoded by multiple synonymous codons (e.g., Leucine is encoded by six distinct codons: $UUA$, $UUG$, $CUU$, $CUC$, $CUA$, and $CUG$).

• Molecular Components of Translation:

  • tRNA Structure: Contains a CCA-$3'$ stem that covalently binds a specific amino acid, and an anticodon loop bearing three nucleotides complementary to a specific mRNA codon.
  • Ribosomal Architecture: Prokaryotic $70S$ ribosomes consist of a small $30S$ subunit and a large $50S$ subunit. The assembled ribosome has three operational sites:
  • A (Aminoacyl) site: Binds incoming charged aminoacyl-tRNA.
  • P (Peptidyl) site: Holds the tRNA carrying the growing nascent polypeptide chain.
  • E (Exit) site: Houses the uncharged, deacylated tRNA prior to its release into the cytosol.

• Step-by-Step Translation Cycle:

  • Initiation:
  • The $30S$ subunit, mRNA transcript, and an initiator tRNA carrying formylmethionine ($\text{fMet}$) assemble at the start codon ($AUG$).
  • The initiator tRNA binds directly into the P site via its complementary anticodon ($UAC$).
  • The $50S$ large ribosomal subunit joins the complex to create an intact, functional $70S$ initiation ribosome.
  • Elongation:
  • The second codon of the mRNA sits positioned in the vacant A site.
  • A matching aminoacyl-tRNA binds to the A site through codon-anticodon complementary base pairing.
  • Peptidyl transferase activity within the large subunit forms a covalent peptide bond between the amino acid at the P site and the amino acid at the A site, releasing the amino acid from the P-site tRNA.
  • The ribosome moves downstream along the mRNA by three nucleotides ($1$ codon) in the $5' \rightarrow 3'$ direction (translocation).
  • Translocation shifts the uncharged tRNA to the E site, where it dissociates, while the tRNA bearing the growing peptide chain moves from the A site into the P site.
  • The empty A site now exposes the next consecutive codon to repeat the cycle.
  • Termination:
  • Elongation cycles continue until a nonsense/stop codon ($UAA$, $UAG$, or $UGA$) moves into the ribosomal A site.
  • No complementary tRNA exists for a stop codon; instead, protein release factors bind to the A site.
  • Release factors hydrolyze the ester bond linking the finished polypeptide chain to the P-site tRNA, releasing the complete protein.
  • The translation complex dissociates, freeing the mRNA transcript, the ribosomal subunits, and release factors for new rounds of translation.

Coupled Transcription and Translation in Bacteria

• Cytoplasmic Spatial Unity:

  • Bacteria lack a nuclear envelope; genomic DNA, RNA polymerase, ribosomes, and tRNAs all share the same cytoplasmic compartment.
  • This allows translation of an mRNA transcript to begin at its $5'$ end before RNA polymerase has finished synthesizing its $3'$ end.

• Polyribosome (Polysome) Dynamics:

  • A single mRNA molecule can be translated simultaneously by multiple consecutive ribosomes moving in tandem along the transcript in the $5' \rightarrow 3'$ direction.
  • As each ribosome traverses down the mRNA strand, its attached nascent polypeptide chain elongates proportionally.
  • Ribosomes closest to the $5'$ start codon display short polypeptide fragments, whereas ribosomes nearing the $3'$ termination codon carry nearly completed, long polypeptides.
  • Coupled transcription and translation enables rapid bacterial growth, fast phenotypic responses to environmental stimuli, and efficient protein production in prokaryotic cells.