Microbiology Exam 2 Study Guide: Chapters 7–13

Chapter 7: Microbial Genetics and DNA Structure

Structure and Function of DNA

DNA (deoxyribonucleic acid) is the hereditary material in all living organisms. It encodes genetic information and directs cellular activities.

• Five Nitrogenous Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G), and Uracil (U) (Uracil is found in RNA).

• Base Pairing: DNA bases pair via hydrogen bonds: A with T, and C with G.

• Phosphodiester Bonds: Nucleotides are linked by phosphodiester bonds, forming the sugar-phosphate backbone.

• Double Helix: The structure is called a double helix due to its two intertwined strands.

Plasmids

Plasmids are small, circular DNA molecules found in bacteria, separate from chromosomal DNA. They often carry genes for antibiotic resistance or other functions.

• Function: Plasmids can replicate independently and transfer genes between cells.

Eukaryotic vs. Prokaryotic Chromosomes

Chromosomes are structures containing DNA. Eukaryotes have multiple, linear chromosomes; prokaryotes typically have a single, circular chromosome.

• Eukaryotic Chromosomes: Located in the nucleus, associated with histone proteins.

• Prokaryotic Chromosomes: Located in the nucleoid region, not membrane-bound.

DNA Replication

DNA replication is the process by which DNA is copied before cell division.

• Origin of Replication: Replication begins at specific sites called origins.

• Enzymes: DNA polymerase synthesizes new DNA; helicase unwinds the helix.

• Leading vs. Lagging Strand: The leading strand is synthesized continuously; the lagging strand is synthesized in fragments (Okazaki fragments).

Genotype and Phenotype

The genotype is the genetic makeup of an organism; the phenotype is the observable traits.

• Gene Expression: Genes are transcribed and translated to produce proteins, determining phenotype.

Central Dogma of Genetics

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

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into proteins by ribosomes.

Mutations

Mutations are changes in the DNA sequence. They can be spontaneous or induced by mutagens (e.g., UV light).

• Types: Point mutations, insertions, deletions, frameshift mutations.

• Effects: Mutations can alter protein function or be silent.

UV Light and DNA Damage

Ultraviolet (UV) light can cause thymine dimers, leading to mutations and cell death.

Horizontal Gene Transfer

• Transformation: Uptake of naked DNA from the environment.

• Transduction: Transfer of DNA via bacteriophages (viruses).

• Conjugation: Direct transfer of DNA between bacteria via a pilus.

Chapter 9: Microbial Control and Antimicrobial Agents

Sterilization, Disinfection, and Antisepsis

These are methods to control microbial growth.

• Sterilization: Complete destruction of all microbes, including spores.

• Disinfection: Elimination of most pathogens (not spores) from inanimate objects.

• Antisepsis: Reduction of microbes on living tissue.

Degerming, Sanitation, Pasteurization

• Degerming: Removal of microbes from a limited area (e.g., skin before injection).

• Sanitation: Lowering microbial counts to safe levels.

• Pasteurization: Heat treatment to kill pathogens in food and beverages.

Microbial Death Rate

The microbial death rate is the rate at which microbes are killed by an agent.

• Formula: $D = -kN$ (where D = death rate, k = rate constant, N = number of organisms)

Physical and Chemical Methods of Control

• Physical Methods: Heat, filtration, radiation.

• Chemical Methods: Disinfectants, antiseptics, antibiotics.

Antimicrobial Chemicals

• Methods of Action: Disrupt cell walls, membranes, proteins, or nucleic acids.

• Limitation: Many disinfectants do not kill spores.

Microbial Resistance

• Most Resistant: Prions, bacterial endospores.

• Least Resistant: Enveloped viruses, Gram-positive bacteria.

Chapter 10: Antimicrobial Drugs and Resistance

Antimicrobial Drug Mechanisms

Antimicrobial drugs target specific microbial structures or functions.

• Broad-spectrum vs. Narrow-spectrum: Broad-spectrum drugs affect many types of microbes; narrow-spectrum drugs target specific groups.

• Side Effects: Toxicity, allergic reactions, disruption of normal flora.

Kirby-Bauer Test, Etest, MIC, MBC

These are laboratory methods to assess antimicrobial effectiveness.

• Kirby-Bauer Test: Disk diffusion method to measure zones of inhibition.

• Etest: Uses strips with a gradient of antibiotic concentration.

• MIC (Minimum Inhibitory Concentration): Lowest concentration that inhibits growth.

• MBC (Minimum Bactericidal Concentration): Lowest concentration that kills bacteria.

Antimicrobial Resistance

• Plasmids: Can carry resistance genes.

• Cross-resistance: Resistance to multiple drugs due to shared mechanisms.

• Synergism: Combined effect of drugs is greater than individual effects.

Chapter 11: Prokaryotic Diversity and Structure

Prokaryotic Cell Morphology and Arrangement

Prokaryotes exhibit various shapes (cocci, bacilli, spirilla) and arrangements (chains, clusters).

Bacterial Endospores

Endospores are highly resistant, dormant structures formed by some bacteria (e.g., Bacillus, Clostridium).

Archaea and Extremophiles

• Halophiles: Thrive in high salt concentrations.

• Thermophiles: Thrive in high temperatures.

Phylum Characteristics

Major bacterial phyla include:

• Bacillota

• Actinomycetota

• Pseudomonadota

• Chlamydiota

• Spirochaetota

• Bacteroidota

Chapter 12: Eukaryotic Microbes and Reproduction

Eukaryotic Reproduction

Eukaryotes reproduce sexually and asexually. Sexual reproduction involves meiosis and fertilization.

• Mitosis: Cell division producing identical daughter cells.

• Meiosis: Cell division producing gametes with half the chromosome number.

• Chromosomes: Structures containing DNA; centromeres and spindle fibers are involved in cell division.

• Haploid vs. Diploid: Haploid cells have one set of chromosomes; diploid cells have two sets.

Fungi

• Characteristics: Eukaryotic, cell walls of chitin, heterotrophic.

• Dimorphism: Ability to exist in yeast or mold forms.

• Beneficial Roles: Decomposition, food production, antibiotics.

Diseases Transmitted by Arthropods

• Vectors: Fleas, lice, ticks, mosquitoes, kissing bugs.

• Emerging Diseases: New cases in previously unaffected locations.

Chapter 13: Viruses and Prions

Virus Structure and Function

Viruses are acellular entities that require host cells for replication.

• Bacteriophage: Virus that infects bacteria.

• Capsid: Protein shell enclosing viral genetic material.

• Envelope: Lipid membrane surrounding some viruses.

• Naked vs. Enveloped Viruses: Naked viruses lack an envelope; enveloped viruses have one.

Viral Replication Cycles

• Lytic Cycle: Virus replicates and lyses host cell.

• Lysogenic Cycle: Viral DNA integrates into host genome and replicates with it.

• Attachment, Entry, Uncoating: Steps in viral infection.

• Budding: Release of enveloped viruses from host cell.

Baltimore Classification

Viruses are classified based on their genome type and replication method.

• Seven Classes: dsDNA, ssDNA, dsRNA, ssRNA (+), ssRNA (−), retroviruses, and others.

• Viral Protein Synthesis: Order of gene expression depends on genome type.

Prions

Prions are infectious proteins causing neurodegenerative diseases.

• Replication: Prions induce misfolding of normal proteins.

• Diseases: Creutzfeldt-Jakob disease, mad cow disease.

Additional info:

• For detailed examples and disease fact sheets, refer to your textbook and class handouts.