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Chapters 19 & 27 – Viruses and Prokaryotes - Coggle Diagram
Chapters 19 & 27 – Viruses and Prokaryotes
What is a Virus?
Not living – cannot reproduce or carry out metabolism outside a host
structure
Nucleic acid (DNA or RNA, single or double-stranded)
Capsid: protein shell
Some have envelope (from host membrane, contains viral glycoproteins)
Viral Genomes
DNA viruses vs RNA viruses
Genome can be single or double stranded
Circular or linear chromosomes
Small size: 3–100s of genes
Viral Reproduction (Infecting Cells)
General Steps
Entry and uncoating of viral genome
Replication of viral nucleic acid
Transcription and translation of viral proteins
Assembly of new viruses
Exit from host (lysis or budding)
Host Range
Some viruses infect only specific species or cell types
Determined by protein receptors on host cell
Bacteriophages (Phages)
Viruses that infect bacteria
Complex structure: head, tail, tail fibers
Lytic Cycle
Virus destroys host cell
Produces many new phages
Host lyses and dies
Lysogenic Cycle
Viral DNA integrates into host genome (prophage)
Passed to daughter cells
Can switch to lytic cycle
Animal Viruses
DNA viruses: often replicate in nucleus
RNA viruses: usually replicate in cytoplasm
Retroviruses (e.g., HIV)
Use reverse transcriptase
RNA → DNA → integration into host genome (provirus)
Viral Diseases
Acute: Influenza, COVID-19
Chronic: HIV, hepatitis
Cancer-causing: HPV (cervical cancer), HTLV (leukemia)
Defense Against Viruses
Bacteria: restriction enzymes, CRISPR-Cas system
Eukaryotes: immune system, antibodies
Vaccines: stimulate immune memory
Antiviral drugs: target specific viral steps (e.g., reverse transcriptase inhibitors)
Prokaryotic structures
No nucleus, no membrane-bound organelles
Cell wall
Bacteria: peptidoglycan
Archaea: various polysaccharides, no peptidoglycan
Capsule: sticky outer layer
Fimbriae: attachment pili
Flagella: for movement
Nucleoid: DNA region (circular chromosome)
Plasmids: small, circular DNA, independent replication
Cell Wall Types (Gram Staining)
Gram-positive: thick peptidoglycan wall → stains purple
Gram-negative: thin wall + outer membrane → stains pink
Often more resistant to antibiotics
Reproduction and Adaptation
Binary fission: asexual reproduction (every ~1–3 hours)
High mutation rate + fast reproduction = rapid evolution
Genetic Recombination
Transformation: uptake of foreign DNA from surroundings
Transduction: gene transfer via phages
Conjugation: DNA transferred via sex pilus
F factor enables conjugation
Can carry antibiotic resistance genes (R plasmids)
Metabolic Diversity
Obligate aerobes: require O₂
Obligate anaerobes: poisoned by O₂
Facultative anaerobes: use O₂ when available, can switch
Nutritional Modes
Photoautotrophs: light + CO₂ (e.g., cyanobacteria)
Chemoautotrophs: chemicals + CO₂ (e.g., deep-sea vent bacteria)
Photoheterotrophs: light + organic carbon
Chemoheterotrophs: organic compounds for both energy and carbon (most bacteria)
Archaea vs Bacteria
Archaea: extremophiles
Thermophiles: heat lovers
Halophiles: salt lovers
Methanogens: produce methane (anaerobic)
Archaea more closely related to eukaryotes than bacteria (based on molecular data)
Ecological Roles of Prokaryotes
Decomposers: recycle nutrients
Nitrogen-fixing bacteria: convert N₂ → NH₃
Symbiosis
Mutualism: both benefit (e.g., gut bacteria)
Commensalism: one benefits, other unaffected
Parasitism: one benefits, one harmed (pathogens)
Bacteria and Humans
Beneficial roles
Biotechnology, digestion, vitamin production
Harmful roles
Cause diseases (e.g., tuberculosis, cholera)
Antibiotic resistance increasing due to overuse and gene transfer