Honors Biology
Chapter 28
Viruses and Bacteria
John Regan
Wendy Vermillion
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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28.1 Viruses, viroids, and prions
• Biology of viruses-noncellular, obligate parasites – can only reproduce inside living host cell
Are viruses Alive or Not?
Can’t reproduce by themselves
Not made of cells
No respiration
Can be synthesized from chemicals in lab
do reproduce w/ help of cell regulate gene expression evolve do have genome(DNA/RNA) that directs reproduction
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28.1 Viruses, viroids, and prions
• Biology of viruses-noncellular, obligate parasites
– Viral structure
• 0.2 microns in size, much smaller than bacteria
• Many shapes- rods, blocks, polyhedrons
• 2 basic parts
– Outer capsid of protein subunits
– Inner core of nucleic acid
» Can be either double-stranded DNA. Single-stranded DNA, single-stranded RNA, or double-stranded RNA
• Some viruses have an envelope surrounding capsid
– Derived from host cell’s plasma membrane
– Viral spikes -extensions of envelop for adherence
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Viral structure
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Viruses, viroids, and prions cont’d.
• Viral reproduction
– Highly specific for host cells
• Spike and host cell membrane receptor fits like lock and key
– Life cycle of typical animal virus has 6 steps
• Attachment -spike binds to receptor
• Entrance -virus enters cell and uncoating occurs-remove capsid
• RNA replication -many copies of viral genome are made
• Biosynthesis -viral components synthesized
• Assembly -new virus particles assembled
• Budding -acquires an envelope with spikes
– These steps are illustrated on the following slide
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Life cycle of an animal virus
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Viruses, viroids, and prions cont’d
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• Latencytemporary dormancy
– Viral genome of some viruses can lie dormant within host cell genome
• Called a provirus – integrated viral genome
• Viral DNA is replicated along with the host genome
• Provirus not affected by antiviral drugs
– Can lie dormant for years
• Stress may stimulate provirus to enter biosynthesis stage
– Retroviruses
• Genome is in RNA
• Enzyme reverse transcriptase transcribes RNA into DNA
– This DNA is called cDNA (copy DNA)
• cDNA can incorporate into host cell genome
• HIV
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Viruses, viroids, and prions cont’d.
• Human viral diseases
– Influenza and colds
• Colds are caused by rhinoviruses
• Flu is caused by influenza viruses –fever, chill, nausea
• Why aren’t you immune to colds? >200 cold viruses
• Many antigenic forms of both
– Antigenic drift –small changes
No longer immune
Flu virus has 8 segments of RNA in genome
– Atigenic shift -re-assortment and combination of RNA from more than one viral form, causes large changes in surface spikes
– Explains why flu vaccinations must be given each year as new antigenic forms appear
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Influenza virus
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Viruses, viroids, and prions cont’d
.
• Measles
– Most contagious human disease
– Spread by respiratory route
– 7-12 day incubation period before flu-like symptoms and rash appear
• Major killer worldwide in non-vaccinated populations
• MMR vaccine protects against measles
• Herpes
– Chronic viral infection that can remain latent
• Herpes simplex type 1
-cold sores and fever blisters
• Herpes simplex type 2 -genital herpes
• Herpes zoster - chickenpox and shingles
• Chickenpox
– Blisters, can return later as shingles
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Chickenpox
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Viruses, viroids, and prions cont’d.
• Drug control of human viral diseases
– Difficult to develop drugs that affect viral reproduction without harming host cell
– Antiviral drugs against HIV have been developed
– Acyclovir is used against Herpes
– Antibiotics are not effective against viruses
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Viruses, viroids, and prions cont’d.
• Viroids and prions
– Also acellular pathogens
– Viroids are naked RNA molecules which do not code for proteins
• Infect plant cells and cause disease
– Prions are rogue folded proteins-passed through ingestion of infected tissues
• Derived from normal proteins in the brain
• Scrapie in sheep
• Mad cow disease in cattle
• Creutzfeld-Jakob disease in people
• Kuru- human to human transmission through cannibalism
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Prion Video
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• Biology of bacteria
– 3 basic shapes-
Bacillus (bacilli) rod shaped
Coccus (cocci) round shaped
Spirillum (spirilla) spiral shaped
Bacilli and cocci may be solitary or in clusters (staph) or chains (strep)
– Bacterial structure
• Plasma membrane
• Many have a cell wall containing peptidoglycans
– Differential stains like the Gram stain interact with cell wall components
» Gram-positive bacteria -retain crystal violet
» Gram-negative bacteria -retain the counter stain safranin; high lipopolysaccarhide content causes crystal violet to be washed out
• Organelles as illustrated on the following slide
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Typical bacterial cell
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• May have capsule outside of cell wall –protect from dehydration or immune system
• Motile bacteria have flagella
• Pili – structure that binds to receptors of cell
• Single circular chromosome – in region called nucleoid
• Plasmids – extra circular DNA
Flagella, pili, capsule,
Cell membrane, cell wall
cytoplasm, ribosomes, nucleoid
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• Bacterial reproduction
– Binary fission -after a period of growth a bacterial cell can replicate its genome and divide in half asexually
– In harsh conditions, Gram-positive bacteria (and some Gram negatives) can form a resistant endospore
– No sexual reproduction, but three mechanisms of genetic recombination
• Conjugation ”male” cell passes DNA to “female” through a sex pillus
• Transformation
-bacterium takes up DNA from environment released by dead bacteria
• Transduction -viruses carry bacterial DNA from cell to cell
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Binary fission
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• Bacterial metabolism
– Most are heterotrophic
• Some are anaerobic and cannot use oxygen as final electron acceptor
– Sulfate reducers-transfer electrons to sulfate
– Denitrifying bacteria use nitrate
– Some are photosynthetic
• Cyanobacteria- have chlorophyll a and undergo photosynthesis
• Other types split hydrogen sulfide instead of water to produce sulfur as a by-product
– Some are chemosynthetic
• Reduce carbon dioxide to an organic compound using electrons from ammonia, hydrogen gas, hydrogen sulfide, or certain minerals such as iron
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Cyanobacteria
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Bacteria and archaea cont’d
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• Environmental and medical aspects of bacteria
– Bacteria are decomposers
• Break down organic matter and make inorganic nutrients available for plants
• Essential to biogeochemical cycles
• Intestine- aid in digestion
– Cyanobacteria produce oxygen from photosynthesis
• First colonizers of rocky habitats
• Many capable of both carbon and nitrogen fixation
• Form a symbiotic relationship with fungi in lichens
- Sewage treatment
- Nitrogen fixation
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• Environmental and medical aspects of bacteria, cont.
– Food processing –cheese, yogurt, buttermilk, saurkraut, vinegar
– Antibiotic production – mostly from soil bacteria
– Genetically engineered bacteria produce many products such as insulin and vaccines
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• Bacterial diseases in humans
– Streptococcus infections
• Cause more disease than any other genus
• Pharyngitis-most common and mild strep disease
• Streptococcus pneumoniae -important agent in bacterial pneumonia
• Streptococcus mutans - contributes to dental caries
• Streptococcus pyogenescauses the most disease of any strep
– Impetigo in infants- mild skin disease
– Rheumatic fever- from endotoxins
– Necrotizing fasciitis-”flesh-eating” bacteria
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• Bacterial diseases in humans
– Tuberculosis
• Highest mortality worldwide of any disease
• 1/3 of the world’s population is infected
• Caused by Mycobacterium tuberculosis
• Active lesions in lung cause tubercles from immune response
– Become calcified
– Food poisoning
• Salmonella-symptoms take several days to weeks to develop
• Staphlyococcus - acts quickly due to toxin production
• Clostridium botulinum - produces one of the most deadly toxins on
Earth; anaerobic, and can survive in improperly canned foods
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Tuberculosis
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• Bacterial diseases in humans
– Chlamydia infections
• Chlamydia trachomatis
– small intestinal parasite causing variety of diseases
» Blindness
» Leading STD in US
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• Drug control of human bacterial diseases
– Antibiotics interfere with metabolic processes unique to bacteria
• Inhibition of synthesis of bacterial proteins
• Should not harm human cells
– Problems related to antibiotic therapy
• Potentially fatal allergic reactions
• Killing off of normal flora
• Prevention of natural immunity
• Bacterial resistance
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• Biology of archae
– Very different from the bacteria
– Archaea are more closely related to Eukarya based on nucleic acid similarities
– Inhabit extreme environments
• Extreme thermophiles - live in extremely high temperatures
• Extreme thermoacidophiles - line in habitats with high temperature and low pH
• Halophiles - live in salty habitats
• Methanogens - anaerobic environments
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Extreme habitats
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• Archaeal structure and metabolism
– Structure
• 0.1-15 µm in size
• DNA genome is a single, closed circular molecule
• Gram-positive archaea have thick polysaccarhide cell wall
• Gram-negative archaea have a protein surface layer
• Cell membranes-single lipid layer which may be highly branched
• Chemical characteristics make them acid and heat tolerant
• Reproduce by binary fission
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• Archaeal structure and metabolism cont’d.
– Metabolism
• Some are heterotrophs, some are autotrophs
• Halophiles -unique photopigment resembling pigment in human retina
• Many are obligate anaerobes
– Methanogens reduce carbon dioxide to methane
» Found in swamps, lake sediments, hot springs, and digestive tracts of animals
– Methane can be used for fuel, also a greenhouse gas
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