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Viruses and Bacteria
Chapter 18
Viruses
Characteristics
• Non-living no respiration, growth, or development
• ½ - 1/100 the size of bacteria• Can’t reproduce on their own need a host
cell
Structure
• Capsid outer protein covering– Large viruses may have a second covering
called an envelope
• Core of nucleic acid DNA or RNA– Most have DNA
Attachment and Replication
Attachment
• Protein capsid attaches to specific proteins on host cell membrane
• Species specific• Some are cell-type specific
Injection
• Nucleic acid core injected into host cell• Takes over host cell’s genetic material
Lytic cycle (active stage)
• Host cell translates viral genes• New viruses produced• Host cell bursts (lyses) release of new
viruses
http://wwportfoli.com/flash/phage.htmGo Here For Full Cycle of the Virus
Lysogenic cycle (inactive stage)
• Viral genes incorporated into host genome provirus
• When host cell replicates, viral genes are replicated
• Can last many years• Eventually is activated lytic cycle• Herpes simplex I, hepatitis B, chicken pox
Like this…..
Types of viruses
Bacteriophages
• Infect bacteria
Retroviruses
• Contain RNA core• Carry enzyme converting RNA to DNA
provirus• HIV
Marberg viruses
• Most deadly known viruses• Attack human connective tissue• Central Africa• 50% of cases are lethal• Ebola can be 90% lethal
Ebola-Hemorrhagic fever
Plant viruses
• Not all are lethal• Tobacco mosaic virus first identified virus• Some cause striking color patterns in
flowers
Tumor viruses
• Cause cancer• HPV (human papilloma virus) cervical
cancer• Hepatitis B virus liver cancer
Origins of viruses
• May have originated in host cells
Bacteria
Archaebacteria
• Most primitive• Live in extremes no free oxygen
Methanogens
• Produce methane• Marshes, sewage plants, digestive tracts
Halophiles
• Water with high salinity• Great Salt Lake, Dead Sea
Thermoacidophiles
• Hot, acidic water• Sulfur springs, deep oceanic hydrothermal
vents
Eubacteria live almost anywhere
• Some are photosynthetic• Some are chemosynthetic
Bacterium structure
• Cell wall prevents lysis• Circular DNA plasmids• Small ribosomes, cytoplasm• Asexual reproduction binary fission• Sexual reproduction conjugation
Importance of bacteria
• Nitrogen-fixing bacteria in nodules of plant roots– Convert nitrogen gas into usable nitrates– Natural fertilizer
• Decomposers recycle nutrients throughout the environment
• Foods yogurt, cheese, vinegar
Importance cont.
• Medicine antibiotics, insulin• Cause disease 50% of all human diseases• Enter through air, water, food, cuts in skin• Have genes for antibiotic resistance that can
be passed from cell to cell