Chapter 6: Molecular Information Flow and Protein Processing

Translation: Process and Components

Translation is the process by which the genetic code carried by mRNA is decoded to produce a specific polypeptide or protein. This process occurs in the ribosome and involves several key molecules and steps.

• Initiation: The ribosome assembles around the target mRNA. The first tRNA is attached at the start codon.

• Elongation: tRNAs bring amino acids to the ribosome, matching the mRNA codons, and the polypeptide chain is elongated.

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

• Templates: mRNA serves as the template for translation.

• tRNA: Transfer RNAs carry specific amino acids to the ribosome and match them to the codons in mRNA via their anticodon.

• rRNA: Ribosomal RNAs are structural and catalytic components of the ribosome.

• Enzymes: Aminoacyl-tRNA synthetases attach amino acids to their corresponding tRNAs.

Polysome: A complex formed when multiple ribosomes simultaneously translate a single mRNA molecule, increasing the efficiency of protein synthesis.

Folding and Secretion of Functional Proteins

• Newly synthesized polypeptides must fold into their correct three-dimensional structures to become functional proteins.

• Chaperone proteins assist in the folding process.

• Some proteins are secreted outside the cell via specialized secretion systems.

Chapters 8 and 28: Antimicrobial Drugs and Persistence

Antibiotics: Definition, Production, and Targets

• Antibiotics: Chemical substances produced by microorganisms (mainly fungi and bacteria) that inhibit or kill other microorganisms.

• They target essential processes such as cell wall synthesis, protein synthesis, nucleic acid synthesis, and metabolic pathways.

• B-lactams: A class of antibiotics (e.g., penicillins) produced by fungi (eukaryotes) that inhibit cell wall biosynthesis by targeting the enzyme transpeptidase.

Antibiotic Resistance Strategies

• Enzymatic degradation or modification of the antibiotic (e.g., beta-lactamases).

• Alteration of target sites (e.g., mutations in ribosomal proteins).

• Efflux pumps that expel antibiotics from the cell.

• Reduced permeability to antibiotics.

Microbial Persistence

• Persistence refers to a phenomenon where a subpopulation of bacteria survives antibiotic treatment without being genetically resistant, often by entering a dormant state.

Chapter 7: Metabolic Regulation

Gene Expression: Constitutive vs. Regulated

• Constitutively expressed genes: Genes that are continuously expressed at a fixed rate.

• Regulated genes: Genes whose expression is controlled in response to environmental or cellular signals.

Levels of Regulation

• Transcriptional regulation: Control of gene expression at the level of mRNA synthesis.

• Post-translational regulation: Control of protein activity after translation (e.g., by modification or degradation).

DNA Binding Proteins

• Two main types: Repressors (inhibit transcription) and Activators (enhance transcription).

• They bind to specific DNA sequences (operators or activator binding sites) to regulate gene expression.

Negative Regulation

• Operator: A DNA segment where repressor proteins bind to block transcription.

• Repressors may require a co-repressor (to activate repression) or an inducer (to relieve repression).

• Arginine operon: Repressed by arginine (co-repressor) binding to the repressor, which then binds the operator to block transcription.

• Induction: The process by which an inducer molecule inactivates a repressor, allowing gene expression.

• Lac operon: Induced by allolactose (inducer), which inactivates the repressor, allowing transcription of lactose catabolism genes.

Positive Regulation

• Activators bind to DNA and increase transcription, often by facilitating RNA polymerase binding.

• Maltose operon: Activated by maltose, which binds the activator protein, enabling it to bind DNA and promote transcription.

Dual Regulation of the Lac Operon

• The lac operon is regulated both negatively (by the lac repressor) and positively (by the cAMP-CRP complex, which responds to glucose levels).

Quorum Sensing

• A cell-density-dependent regulatory system where bacteria produce, release, and detect signaling molecules (autoinducers) to coordinate gene expression in a population.

• Plays roles in biofilm formation, virulence, and bioluminescence.

Chapter 5: Viruses and Their Replication

Virus Structure and Genomes

• Viruses: Acellular infectious agents composed of nucleic acid (DNA or RNA) enclosed in a protein coat (capsid); some have lipid envelopes.

• Genomes can be DNA or RNA, single- or double-stranded, linear or circular.

• Some viruses carry enzymes (e.g., reverse transcriptase) or other proteins within the virion.

Plaques and Permissive Cells

• Plaque: A clear area on a bacterial lawn caused by virus-induced cell lysis.

• Permissive cell: A host cell that supports the complete replication cycle of a virus; specificity is determined by viral attachment proteins and host receptors.

Viral Life Cycle: Virulent/Lytic Bacteriophage (e.g., T4)

• Attachment → Penetration → Synthesis of viral components → Assembly → Release (lysis).

Animal vs. Prokaryotic Viruses

• Animal viruses often enter cells by endocytosis or membrane fusion and may have more complex replication cycles.

• Consequences of animal virus infection include lysis, persistent infection, latent infection, or transformation (oncogenesis).

Chapter 9: Bacterial Genetics

Genotype vs. Phenotype

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable characteristics resulting from the genotype.

Prototroph vs. Auxotroph

• Prototroph: A microorganism that can synthesize all its required metabolites from basic nutrients.

• Auxotroph: A mutant that requires a specific additional nutrient that the wild-type does not.

Types of Mutations

• Point mutations, insertions, deletions, and frameshifts.

Horizontal Gene Transfer in Bacteria

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA via bacteriophages; generalized (any gene) vs. specialized (specific genes near prophage site).

• Conjugation: Direct transfer of DNA between bacteria via cell-to-cell contact, often involving the F plasmid.

• For transferred DNA to be retained, it must be integrated into the genome or maintained as a plasmid.

CRISPR System

• A prokaryotic adaptive immune system that uses Cas proteins (endonucleases) and spacer RNAs to recognize and cut foreign DNA.

• Cas proteins are also used in eukaryotic gene editing technologies.

Chapter 11: Subviral Agents

Viroids and Prions

• Viroids: Small, circular RNA molecules that infect plants; lack protein coats.

• Affect mainly higher plants, causing various diseases.

• Prions: Infectious proteins that cause neurodegenerative diseases in animals; composed solely of misfolded protein.

• Prion diseases are not caused by mutations in the gene sequence but by abnormal protein folding.

Chapter 12: Genetic Engineering and Biotechnology

PCR and Applications

• PCR (Polymerase Chain Reaction): A technique to amplify specific DNA sequences.

• Applications include cloning, diagnostics, forensics, and research.

DNA Visualization

• DNA is visualized using gel electrophoresis and staining (e.g., ethidium bromide).

Molecular Cloning

• Cloning is necessary to produce large quantities of a gene or protein.

• Vector: A DNA molecule (e.g., plasmid) used to carry foreign DNA into a host cell.

• Selectable marker: A gene (e.g., antibiotic resistance) that allows identification of cells containing the vector.

• MCS (Multiple Cloning Site): A region with multiple restriction sites for inserting foreign DNA.

• Recombinant DNA: DNA molecules formed by laboratory methods of genetic recombination.

• Blue/white screening: A method to detect recombinant vectors using disruption of the lacZ gene (white colonies = recombinant).

• Three basic steps: (1) Isolation of DNA, (2) Insertion into vector, (3) Introduction into host cell.

• Problems with cloning eukaryotic genes in bacteria include intron processing and differences in post-translational modifications.

Somatotropin Production

• Somatotropin (growth hormone) is produced by inserting the human gene into bacteria, which then express the protein for medical use.

Chapter 14: Phototrophy

Photosynthesis: Requirements and ATP Generation

• Photosynthesis is the process by which light energy is converted into chemical energy (ATP and NADPH).

• Requirements: Light, pigments (chlorophylls), electron donors, and membranes with reaction centers.

• ATP is generated by photophosphorylation.

Types of Photosynthesis

• Oxygenic photosynthesis: Uses water as an electron donor, produces O2, found in cyanobacteria and plants, involves two photosystems.

• Anoxygenic photosynthesis: Uses other electron donors (e.g., H2S), does not produce O2, found in purple and green bacteria, involves one photosystem.

Photopigments

• Chlorophylls: Main light-absorbing pigments in oxygenic phototrophs.

• Bacteriochlorophylls: Found in anoxygenic phototrophs.

• Carotenoids: Accessory pigments that protect against photooxidative damage and broaden the spectrum of light absorption.

Autotrophy and the Calvin Cycle

• CO2 is fixed into organic molecules via the Calvin Cycle.

• Reducing power is provided by NADPH or NADH.

Alternative Electron Flow

• If CO2 is not fixed, electrons may be used for other reductive biosynthetic processes; NADH may not be produced.

Nitrogen Fixation

• Nitrogen fixation is an exergonic process (releases energy).

Chapter 14: Fermentation

Fermentation: Definition and Energy Generation

• Fermentation is an anaerobic process where ATP is generated by substrate-level phosphorylation.

• Fermentation products (e.g., ethanol, lactic acid) are produced to regenerate NAD+ from NADH.

• Yeast may ferment even in the presence of O2 (Crabtree effect) due to regulatory mechanisms.

Types of Fermentation

• Homolactic fermentation: Produces only lactic acid.

• Heterolactic fermentation: Produces lactic acid, ethanol, and CO2.

• Secondary fermentation: Further fermentation of primary products (e.g., propionic acid fermentation from lactate).

Chapter 22: Microbiology of the Built Environment

Microbial Degradation of Oil and Xenobiotics

• Key enzymes for oil degradation include oxygenases and hydrolases.

• Xenobiotic: A synthetic compound not naturally found in nature, often resistant to biodegradation.

Water Treatment

• Wastewater treatment removes organic matter, pathogens, and nutrients before release.

• BOD (Biochemical Oxygen Demand): A measure of the amount of oxygen required by microorganisms to decompose organic matter in water.

• Primary treatment: Physical removal of solids.

• Secondary treatment: Biological degradation of dissolved organic matter.

• Potable: Water that is safe to drink.

Chapter 23: Microbial Symbioses

Types of Symbiosis

• Symbiosis: Close association between different species.

• Mutualism: Both partners benefit.

• Parasitism: One benefits, the other is harmed.

• Commensalism: One benefits, the other is unaffected.

Legume-Rhizobia Symbiosis

• Legumes provide carbohydrates to Rhizobia; Rhizobia fix atmospheric nitrogen for the plant.

• Nitrogen fixation occurs in root nodules, protected from O2 by leghemoglobin.

• Nod factors are signaling molecules critical for nodule formation and host specificity.

The Rumen

• The rumen is a fermentation chamber in ruminant animals containing diverse bacteria (cellulolytic, methanogenic, etc.) that degrade plant material.

• Metabolisms include fermentation of cellulose to volatile fatty acids, which are absorbed by the host.