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