Microbiology Lab Concepts and Molecular Genetics Study Guide

Lab Concepts in Microbiology

Streak Plating

Streak plating is a fundamental microbiological technique used to isolate pure colonies of microorganisms from a mixed sample.

• Purpose: To obtain isolated colonies for further study or identification by spreading cells over the surface of an agar plate.

• Common Errors: Not flaming the loop between quadrants, using a loop that is too hot (killing cells), or streaking too heavily (leading to confluent growth).

• Troubleshooting: Ensure proper sterilization of the loop, use gentle streaking motions, and avoid overlapping streaks between quadrants.

DNA Extraction

DNA extraction is the process of isolating DNA from cells or tissues for downstream applications such as PCR or sequencing.

• Centrifuge Safety and Balancing: Always balance tubes of equal mass opposite each other in the centrifuge to prevent mechanical failure or accidents.

• Centrifuge Purpose: To separate components of a mixture based on density by spinning at high speeds.

• Supernatant vs Pellet: The supernatant is the liquid above the solid residue (pellet) after centrifugation. The pellet contains the denser material collected at the bottom of the tube.

Key Materials and Functions in Strawberry DNA Extraction

• Strawberries: High DNA yield due to being octoploid (eight sets of chromosomes).

• Dish Soap (Detergent): Disrupts cell and nuclear membranes by dissolving phospholipids, releasing DNA.

• Salt (NaCl): Neutralizes DNA's negative charge, allowing DNA strands to aggregate and separate from proteins.

• Ice-Cold Ethanol/Isopropyl Alcohol (70-90%): Precipitates DNA, making it visible as white fibers because DNA is insoluble in cold alcohol.

• Plastic Bag/Mash: Mechanically breaks down cell walls to release cellular contents.

• Filter (Coffee filter/cheesecloth): Removes cell debris, allowing DNA-containing solution to pass through.

• Stirrer (Wooden skewer/hook): Used to spool and collect the precipitated DNA.

Comparison to Other Methods: Bead bashing physically disrupts cells; DNA wash removes contaminants; column filtration binds DNA to a matrix for purification.

Additional info: These principles are similar in bacterial DNA extraction, with adaptations for cell wall structure.

Microbial Metabolism (Ch. 5)

Catabolism and Anabolism

Microbial metabolism consists of all chemical reactions within a microorganism, divided into catabolic and anabolic pathways.

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy. These are exergonic and often involve hydrolytic reactions (addition of water to break bonds).

• Anabolism: The synthesis of complex molecules from simpler ones, consuming energy. These are endergonic and involve dehydration synthesis (removal of water to form bonds).

• ATP: Catabolic reactions release ATP; anabolic reactions consume ATP.

Enzymes and Their Role

• Enzymes: Biological catalysts (usually ending in -ase) that speed up chemical reactions without being consumed.

• Pathways: Enzymes are organized into metabolic pathways, each catalyzing a specific step.

• Substrate: The molecule upon which an enzyme acts.

• Active Site: The region on the enzyme where the substrate binds.

• Products: Common products include acids, gases, alcohols, and ATP.

Factors Affecting Enzyme Activity

• pH: Each enzyme has an optimal pH; deviations can denature the enzyme.

• Temperature: High temperatures can denature enzymes; low temperatures slow reaction rates.

• Substrate Concentration: Increasing substrate increases reaction rate until saturation.

• Inhibitors: Competitive inhibitors bind the active site; noncompetitive inhibitors bind elsewhere, altering enzyme function.

Microbial Genetics (Ch. 8)

Central Dogma and -Omics Analyses

The central dogma describes the flow of genetic information: DNA → RNA → Protein.

• Genomics: Study of the entire genome (DNA).

• Transcriptomics: Study of RNA transcripts produced by the genome.

• Proteomics: Study of the entire set of proteins expressed.

• Mutations: Changes at any step can affect subsequent steps and overall phenotype.

Gene Expression and Regulation

• Transcription: Synthesis of RNA from DNA; represents gene expression.

• Constitutively Expressed: Genes that are always transcribed and translated.

• Operons: Clusters of genes under control of a single promoter and operator.

• Repressible Operon: Normally on; can be turned off by a repressor (e.g., trp operon).

• Inducible Operon: Normally off; can be turned on by an inducer (e.g., lac operon).

• Enzymes and Substrates: Induction often requires a substrate to activate gene expression; repression involves end-product inhibition.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Operator: DNA segment where a repressor protein can bind to block transcription.

Enzymes in Genetic Processes

• Enzymes are essential for DNA replication (e.g., DNA polymerase), transcription (RNA polymerase), and translation (ribosomes, tRNA synthetases).

RNA and Ribosomes

• Prokaryotic vs Eukaryotic RNA: Prokaryotes often have polycistronic mRNA (multiple genes per transcript); eukaryotes have monocistronic mRNA and undergo RNA processing (capping, polyadenylation, splicing).

• Ribosomes: Sites of protein synthesis; prokaryotic (70S) and eukaryotic (80S) ribosomes differ in size and structure.

• Amino Acids: Building blocks of proteins; sequence determined by mRNA codons.

• Codon: A sequence of three nucleotides in mRNA that specifies an amino acid. Not all codons are the same; some are redundant (degeneracy of the code).

Gene Transfer and Mobile Genetic Elements

• Vertical Gene Transfer: Transmission of genetic material from parent to offspring.

• Horizontal Gene Transfer: Transfer of genes between organisms in the same generation.

• Mobile Genetic Elements (MGEs): DNA segments that can move within or between genomes (e.g., plasmids, transposons).

• Plasmids: Small, circular DNA molecules; conjugative plasmids transfer via conjugation; R factors confer antibiotic resistance.

• Transposons: "Jumping genes" that move within the genome; can disrupt genes (cons: mutations) or spread beneficial traits (pros: adaptation).

• Phages: Viruses that infect bacteria; lytic phase destroys host, lysogenic phase integrates into host genome.

Mechanisms of Horizontal Gene Transfer

• Conjugation: Direct transfer of DNA via cell-to-cell contact (e.g., F plasmid in E. coli).

• Transformation: Uptake of free DNA from the environment (demonstrated by Griffith's experiment).

• Transduction: Transfer of DNA by bacteriophages.

Biotechnology and DNA Technology (Ch. 9)

Key Concepts

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

• Biotechnology: The use of living organisms or their products to modify or make products for specific uses.

• rDNA Technology: Recombinant DNA technology involves combining DNA from different sources to create genetically modified organisms or products (e.g., insulin, growth hormone).

• Recombination: The exchange of genetic material between different DNA molecules, resulting in new genetic combinations.

PCR (Polymerase Chain Reaction)

• Definition: A technique to amplify specific DNA sequences exponentially.

• Heating: Denatures double-stranded DNA.

• Cooling: Allows primers to anneal to target sequences.

• Extension: DNA polymerase synthesizes new DNA strands.

Cloning and DNA Insertion Methods

• Cloning: Producing identical copies of DNA fragments, cells, or organisms; useful for gene studies, protein production, or therapeutic applications.

• DNA Insertion Methods:

  • Transformation: Uptake of naked DNA by cells.

  • Electroporation: Electric pulses create pores in cell membranes for DNA entry.

  • Protoplast Fusion: Fusion of cells without cell walls to combine genetic material.

  • Microinjection: Direct injection of DNA into cells using a fine needle.

Sequencing Methods

• Amplicon Sequencing: Targets and sequences specific DNA regions (e.g., 16S rRNA gene for microbial identification).

• Shotgun Sequencing: Randomly fragments and sequences all DNA in a sample; useful for whole-genome or metagenomic studies.

• Nanotechnology: Used to enhance sequencing accuracy and throughput.

Summary Table: Comparison of DNA Insertion Methods

Method	Principle	Application
Transformation	Uptake of naked DNA by cells	Bacterial genetic engineering
Electroporation	Electric field creates membrane pores	Efficient DNA delivery in bacteria, yeast, and plant cells
Protoplast Fusion	Fusion of cells without cell walls	Hybrid cell creation, plant and fungal genetics
Microinjection	Direct injection of DNA into cells	Animal cell and embryo modification

Additional info: These molecular techniques are foundational for modern microbiology, biotechnology, and genetic engineering.