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

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

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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