BackBIO-55 Exam 2 Study Guide: Eukaryotes, Fungi, Protists, Helminths, Viruses, Microbial Genetics, and Bacterial Growth
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Eukaryotic Cells
Origins and Organization of Eukaryotic Cells
Eukaryotic cells are complex cells with internal compartmentalization and specialized organelles. Their origin is explained by two main theories:
Endosymbiosis: A process where one cell engulfs another, which survives inside as a symbiont. This relationship becomes permanent and is inherited by offspring. Mitochondria, chloroplasts, cilia, flagella, and centrioles are believed to have originated this way.
Membrane Infolding: The cell's own membrane folds inward, creating internal compartments such as the endoplasmic reticulum (ER), lysosomes, nucleus, and Golgi apparatus.
Eukaryotic cells are organized into three main areas: external structures, the cell boundary, and the internal cytoplasm containing organelles.
External Structures & Locomotion
Flagella: Used for locomotion; thicker than prokaryotic flagella, with a "9+2" microtubule arrangement. Moves the cell by pushing or pulling. Example: Trichomonas vaginalis.
Cilia: Shorter, more numerous than flagella; used for movement, feeding, and filtering. Found in some protozoa and animal cells.
Glycocalyx: Outermost boundary made of polysaccharides; functions in adherence, protection, and signal reception.
The Cell Boundary
Structure | Presence/Absence | Composition | Key Function |
|---|---|---|---|
Cell Wall | Present in fungi, most algae; absent in protozoa, few algae, animal cells | Fungi: chitin/cellulose; Algae: cellulose, pectin, mannans, silicon dioxide, calcium carbonate | Provides structural support and shape |
Cell/Cytoplasmic Membrane | Present in all eukaryotes | Phospholipid bilayer with proteins; contains sterols (e.g., ergosterol in fungi) | Selective barrier for transport, adhesion, secretion, and signal transduction |
Internal Organelles and the Endomembrane System
Nucleus: Double-membraned organelle with nuclear pores; contains chromosomes (DNA + histones) and nucleolus (site of rRNA synthesis).
Endomembrane System: Includes nuclear envelope, ER, and Golgi apparatus.
Rough ER (RER): Studded with ribosomes; synthesizes secretory proteins.
Smooth ER (SER): Lacks ribosomes; synthesizes lipids, processes nutrients.
Golgi Apparatus: Stack of cisternae; modifies, stores, and packages proteins/lipids.
Ribosomes: Sites of protein synthesis; larger than prokaryotic ribosomes; found free or on RER.
Vesicles:
Lysosomes: Contain digestive enzymes; involved in intracellular digestion and defense.
Vacuoles: Storage and digestion compartments.
Energy-Converting Organelles
Mitochondria: Site of ATP production via aerobic respiration; inner membrane forms cristae; contains its own DNA and ribosomes; divides independently.
Chloroplasts: Site of photosynthesis in algae and plants; inner membrane forms thylakoids (stacked into grana).
Cytoskeleton: Network of microfilaments and microtubules; provides structure, anchors organelles, and enables movement.
The Kingdom Fungi
General Characteristics and Forms
Cell Structure: Cell wall made of chitin; membrane contains ergosterol.
Growth Forms:
Yeast: Unicellular, round/oval, reproduce by budding; chains of buds form pseudohyphae.
Molds: Multicellular, filamentous; composed of hyphae (threadlike cells); mass of hyphae is a mycelium.
Dimorphic Fungi: Can exist as yeast or mold depending on environment; most pathogenic fungi are yeast in tissues.
Hyphae Types: Septate (with cross-walls) or non-septate (continuous); vegetative (nutrient absorption) or reproductive (spore production).
Reproduction and Classification
Asexual Reproduction: By fragmentation or spores (mitosis).
Sporangiospores: Formed inside a sporangium (sac).
Conidia: Free spores, not enclosed.
Sexual Spores: Formed by union of different hyphae strains; basis for classification.
Phylum | Sexual Spore Type | Example |
|---|---|---|
Zygomycota | Zygospores | Bread mold |
Ascomycota (Sac Fungi) | Ascospores (in ascus) | Yeasts, truffles, morels |
Basidiomycota (Club Fungi) | Basidiospores (on basidium) | Mushrooms, toadstools |
Imperfect Fungi (Deuteromycetes) | N/A (only asexual observed) | N/A |
Fungal Impacts and Infections (Mycoses)
Adverse Impacts: Mycoses (infections), allergies, toxin production (e.g., aflatoxin), crop/food destruction.
Beneficial Impacts: Decomposers, antibiotic sources, production of alcohol, acids, and food.
Mycoses Categories:
Superficial/Cutaneous: Affect skin, hair, nails; "tinea" or ringworm. Example: Athlete’s foot (tinea pedis), yeast infections (Candida albicans).
Systemic: Deep infections, often inhaled as spores; dangerous for immunocompromised. Examples: Histoplasmosis (Histoplasma capsulatum), Coccidioidomycosis (Coccidioides immitis).
Fungal Intoxications: Diseases from ingesting mycotoxins, not infection. Examples: Ergot poisoning (Claviceps purpurea), aflatoxin (Aspergillus flavus), ethanol (Saccharomyces cerevisiae).
The Protists
General Characteristics
Algae: Photosynthetic eukaryotes with chlorophyll a; unicellular or colonial; base of aquatic food webs; some (dinoflagellates) cause red tides and release toxins.
Protozoa: Unicellular, lack tissues, all heterotrophic; feed by engulfing organic matter; mostly free-living in moist environments.
Protozoan Structure: Most lack a cell wall; cytoplasm divided into ectoplasm (outer, for movement/feeding/protection) and endoplasm (inner, contains nucleus, mitochondria, vacuoles).
Protozoan Life Cycle and Classification
Stage | Description | Function in Disease |
|---|---|---|
Trophozoite | Motile, feeding stage; requires food and moisture | Active, damaging form in host |
Cyst | Dormant, protective stage; forms in harsh conditions | Resistant to heat, drying, chemicals; aids transmission |
Transmission: Cyst-formers (e.g., Entamoeba histolytica, Giardia lamblia) spread via contaminated food/water; non-cyst formers (e.g., Trichomonas vaginalis) require direct contact.
Group | Motility | Pathogen Examples |
|---|---|---|
Mastigophora (Flagellates) | Flagella (sometimes amoeboid motion) | Giardia lamblia, Trichomonas vaginalis, Trypanosoma brucei, T. cruzi |
Sarcodina (Amoebas) | Pseudopods | Entamoeba histolytica |
Ciliophora (Ciliates) | Cilia | Balantidium |
Apicomplexa (Sporozoa) | Non-motile (most of life cycle) | Plasmodium, Toxoplasma gondii, Cryptosporidium |
Parasitic Helminths
General Features and Classification
Definition: Multicellular animals (tapeworms, flukes, roundworms).
Structure: Organs for reproduction, digestion, movement; often have hooks/suckers for attachment.
Group | Key Characteristics | Subgroups/Examples |
|---|---|---|
Flatworms (Platyhelminthes) | Flat body, simple nervous system, most hermaphroditic, absorb nutrients directly | Cestodes (tapeworms): Taenia saginata; Trematodes (flukes): Schistosoma |
Roundworms (Nematodes) | Round body, complete digestive tract, protective cuticle, sexes separate | Ascaris lumbricoides, Enterobius vermicularis, Necator americanus |
Life Cycle: Egg, larva, adult stages; definitive host (adult matures, sexual reproduction), intermediate host (larval development); infection via contaminated food/water/soil or skin penetration.
Viruses
General Properties and Structure
Definition: Obligate intracellular parasites; infect all life forms.
Nature: Non-cellular, ultramicroscopic (20–450 nm), not alive by standard criteria.
Genome: DNA or RNA (never both).
Structure:
Covering: Capsid (protein shell of capsomers), sometimes an envelope (from host membrane, with spikes for attachment).
Core: Nucleic acid genome, sometimes pre-formed enzymes (polymerases, replicases, reverse transcriptase).
Capsid Types: Helical (cylindrical) or icosahedral (20-sided).
Complex Viruses: More elaborate (e.g., poxviruses, bacteriophages).
Viral Classification and Multiplication
Baltimore Classification: Seven classes based on nucleic acid type, strandedness, and replication method.
ICTV Taxonomy: Family names end in -viridae; genus names end in -virus.
Multiplication Cycle (Animal Viruses):
Adsorption: Virus binds to specific host receptors (spikes or capsid).
Penetration: Entry by fusion (enveloped) or endocytosis (naked/enveloped).
Uncoating: Release of viral genome from capsid.
Synthesis: Production of viral components (DNA viruses in nucleus, RNA viruses in cytoplasm).
Assembly: Construction of new virions.
Release: Budding/exocytosis (enveloped, gradual), or cell lysis (naked/complex, sudden).
Viral Damage and Bacteriophages
Cytopathic Effects (CPE): Visible cell damage (shape/size changes, syncytium formation).
Persistence: Chronic latent infections (e.g., herpes); oncogenic viruses cause cancer (e.g., papillomavirus, Epstein-Barr).
Bacteriophages: Infect bacteria; lytic cycle (cell lysis, virion release), lysogeny (prophage integrates into host genome, can cause lysogenic conversion).
Nonviral Infectious Agents: Prions (misfolded proteins, cause TSEs), viroids (small RNA, plant pathogens), satellite viruses (require helper virus).
Microbial Genetics
Fundamentals of Genetics and DNA Structure
Genetics: Study of heredity, gene expression, and variation.
Genome: Total genetic material (DNA) in a cell.
Gene: DNA sequence coding for a product.
Genotype: All genes present; Phenotype: Observable traits.
Genome Location: Bacteria: single circular chromosome; Eukaryotes: multiple linear chromosomes; Viruses: DNA or RNA, few genes.
DNA Structure: Nucleotide (deoxyribose, phosphate, base); A/G = purines, C/T = pyrimidines; A-T (2 H-bonds), G-C (3 H-bonds); antiparallel strands (5' to 3').
DNA Replication
Semiconservative: Each new DNA has one original and one new strand.
Direction: New strand synthesized 5' to 3'.
Leading strand: Continuous synthesis.
Lagging strand: Discontinuous, forms Okazaki fragments.
Enzyme | Function |
|---|---|
Helicase | Unwinds DNA at origin |
Primase | Synthesizes RNA primer |
DNA Polymerase III | Adds nucleotides (5'→3'), proofreads |
DNA Polymerase I | Removes primers, fills gaps, repairs |
Ligase | Joins DNA fragments |
Gyrase | Supercoils DNA |
Gene Expression: The Central Dogma
Transcription (DNA → RNA): RNA polymerase binds promoter, synthesizes RNA using DNA template; A-U, G-C base pairing. Eukaryotes process RNA (splicing, capping, tailing); prokaryotes do not.
RNA Types:
mRNA: Carries code to ribosomes (read in codons).
tRNA: Brings amino acids, has anticodon (not read in codons).
rRNA: Forms ribosomes, catalyzes protein synthesis.
Translation (RNA → Protein): Ribosomes read mRNA codons; tRNA brings amino acids; peptide bonds form. Start codon: AUG (methionine); stop codons: UAA, UGA, UAG. Prokaryotic ribosomes: 70S; eukaryotic: 80S.
Gene Regulation (Operons) and Mutation
Operons (Prokaryotes): Coordinate gene regulation.
Inducible (Catabolic): Normally OFF; turned ON by substrate (e.g., lac operon).
Repressible (Anabolic): Normally ON; turned OFF by product (e.g., arginine operon).
Mutations: Change in genotype/phenotype.
Spontaneous: Random errors.
Induced: Caused by mutagens (UV, X-rays).
Type | Description | Consequence |
|---|---|---|
Missense | Single amino acid change | Alters protein (e.g., sickle-cell anemia) |
Nonsense | Codon becomes STOP | Premature translation termination |
Frameshift | Insertion/deletion shifts reading frame | Alters entire amino acid sequence |
Genetic Recombination (Horizontal Gene Transfer)
Conjugation: Direct transfer via pilus; plasmid or chromosomal DNA.
Transformation: Uptake of free DNA from environment.
Transduction: Bacteriophage transfers DNA; can be generalized (random) or specialized (specific genes).
Bacterial Growth & Reproduction
Binary Fission and Exponential Growth
Binary Fission: Asexual reproduction; one cell divides into two identical daughter cells. Steps: DNA replication, chromosome separation, septum formation, cell separation. Example: E. coli divides every 20 minutes.
Exponential Growth: Population doubles each generation under ideal conditions.
Mathematical Representation: Final cell number () is calculated by: Where = initial number of cells, = number of generations.
Growth Curves: Arithmetic scale: J-shaped curve; logarithmic scale: straight line (slope = generation time).
