I. DNA Replication

Overview of DNA Replication

DNA replication is a semiconservative process in which each daughter molecule contains one parental strand and one newly synthesized strand. Replication begins at an origin of replication where helicase unwinds the double helix, initiating a replication fork. Single-strand binding proteins stabilize the separated strands, while topoisomerase (gyrase) relieves supercoiling ahead of the fork.

• RNA primer: Synthesized by primase to provide a 3' hydroxyl group for DNA polymerase III, which adds deoxyribonucleotides in the 5'→3' direction.

• Leading strand: Synthesized continuously toward the fork.

• Lagging strand: Synthesized discontinuously as Okazaki fragments away from the fork. These fragments are extended by DNA polymerase III and joined by DNA ligase.

• DNA polymerase I: Removes RNA primers and replaces them with DNA.

• Prokaryotes have a single origin of replication; eukaryotes possess multiple origins.

Example: In Escherichia coli, DNA replication proceeds bidirectionally from a single origin, with enzymes coordinating synthesis and repair.

II. Transcription

Transcription Process

Transcription converts the information in DNA into complementary RNA. It occurs in three stages: initiation, elongation, and termination.

• Initiation: RNA polymerase binds to the promoter region, aided by a sigma (σ) factor in prokaryotes.

• Elongation: RNA polymerase synthesizes the template strand 3'→5', adding complementary ribonucleotides.

• Termination: Occurs by Rho-dependent (Rho protein binds RNA and displaces polymerase) or Rho-independent (GC-rich hairpin followed by poly-U sequence destabilizes polymerase) mechanisms.

• Eukaryotic RNA processing: Includes 5' capping, poly(A) tail addition, and splicing to remove introns.

Example: In eukaryotes, mature mRNA is produced after splicing and exported from the nucleus for translation.

III. Translation

Translation Mechanism

Translation interprets mRNA codons to assemble amino acids into a polypeptide. Ribosomes act as the catalytic platform, consisting of rRNA and protein subunits: 70S (30S + 50S) in prokaryotes and 80S (40S + 60S) in eukaryotes.

• Initiation: The small ribosomal subunit binds the mRNA at the start codon (AUG, encoding methionine). In prokaryotes, the Shine-Dalgarno sequence aligns the ribosome; in eukaryotes, the Kozak consensus sequence is used.

• Elongation: tRNAs deliver amino acids to the ribosome, matching codons via anticodons. Peptide bonds form at the P site, and the ribosome translocates along the mRNA.

• Termination: Occurs when a stop codon (UAA, UAG, UGA) is reached, releasing the completed polypeptide.

• Polysome: Multiple ribosomes translate a single mRNA simultaneously, increasing protein yield.

Example: In bacteria, translation can begin before transcription is complete due to the absence of a nuclear membrane.

IV. Gene Regulation and Operons

Operon Structure and Function

Bacteria coordinate gene expression through operons, polycistronic units comprising a promoter, operator, and one or more structural genes. Regulation occurs at the level of transcription initiation.

• Lac operon: An inducible, catabolic operon. When lactose is absent, the repressor binds the operator, preventing transcription. When lactose is present, it binds the repressor, allowing transcription of genes for lactose catabolism.

• Trp operon: A repressible, anabolic operon. In the absence of tryptophan, the repressor is inactive, allowing transcription of genes for tryptophan biosynthesis. When tryptophan accumulates, it activates the repressor, blocking transcription.

Example: The lac operon enables bacteria to metabolize lactose only when it is available, conserving energy.

V. Antimicrobial Agents and Drug Mechanisms

Antibiotic Classes and Targets

Antibiotics exploit differences between prokaryotic and eukaryotic cells.

• Cell-wall inhibitors: e.g., β-lactams (penicillins, cephalosporins) block synthesis of peptidoglycan, leading to lysis of growing Gram-positive bacteria.

• Protein synthesis inhibitors: Target ribosomal subunits unique to bacteria:

  • 30S targeting: aminoglycosides (inhibit initiation and misreading), tetracyclines (block tRNA docking).

  • 50S targeting: macrolides (inhibit translocation), chloramphenicol (blocks peptide bond formation).

• Antifungal agents: e.g., amphotericin B and nystatin bind ergosterol in fungal membranes, disrupting integrity.

• MIC (Minimum Inhibitory Concentration): Lowest concentration preventing visible growth.

• Therapeutic Index (TI): ; higher TI = safer drug.

Example: Penicillin is effective against Gram-positive bacteria due to its action on peptidoglycan synthesis.

VI. Prokaryotic Cell Biology and Reproduction

Prokaryotic Cell Division and Traits

Binary fission is the primary mode of prokaryotic reproduction. The circular chromosome replicates, attaches to opposite cell poles, and cytokinesis partitions the cytoplasm, producing two identical daughter cells.

• Snapping division: Observed in Corynebacterium, where the inner cell wall ruptures but the outer wall remains intact, generating angular fragments.

• Endospores: Produced by Bacillus and Clostridium under adverse conditions; highly resistant to heat, desiccation, and chemicals.

• Archaea: Lack peptidoglycan, possess ether-linked membrane lipids, and often thrive in extreme environments.

• Actinobacteria: Notable for producing antibiotics (e.g., Streptomyces) and geosmin, which imparts an earthy odor.

Example: Endospore formation allows Bacillus species to survive harsh conditions for extended periods.

VII. Exam Focus Summary

Key Concepts to Memorize

• DNA Replication: Directionality (template 3'→5', synthesis 5'→3'), enzyme functions (helicase, gyrase, primase, DNA polymerases, ligase), Okazaki fragments, origin differences (prokaryotes vs. eukaryotes).

• Transcription: Initiation by RNA polymerase, Rho-dependent/independent termination, RNA synthesis direction, eukaryotic mRNA processing.

• Translation: Ribosome subunits (prokaryotes: 70S = 30S + 50S; eukaryotes: 80S = 40S + 60S), start codon (AUG), tRNA sites (A, P, E), stop codons (UAA, UAG, UGA), polysome formation.

• Operons: Inducible (lac) vs. repressible (trp), components (regulator, promoter, operator, structural genes).

• Antibiotics & Drugs: Cell wall and protein synthesis inhibitors, antifungal agents, MIC, TI.

• Prokaryotic Traits: Binary fission, snapping division, endospores, archaeal features, actinobacteria and antibiotic production.

Additional info: These notes synthesize and expand upon the provided study guide, ensuring coverage of all major microbiology topics relevant to cell biology, genetics, and antimicrobial mechanisms.