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Prokaryotes, Viruses, and Protistans
Chapter 20
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Microorganisms
• Single-celled organisms that are too
small to be seen without a microscope
• Bacteria are the smallest living
organisms
• Viruses are smaller but are not alive
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The Prokaryotes
• Only two groups
• Archaebacteria and Eubacteria
• Arose before the eukaryotes
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Prokaryotic Characteristics
• No membrane-bound nucleus
• Single chromosome
• Cell wall in most species
• Prokaryotic fission
• Metabolic diversity
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Prokaryotic Body Plan
bacterial flagellum
pilus
capsulecell wall
plasma membrane
DNA
ribosomes in cytoplasm
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Bacterial Shapes
coccus bacillus
spirillum
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Archaebacteria
Methanogens
Extreme halophiles
Extreme thermophiles
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Eubacteria
• Includes most familiar bacteria
• Have fatty acids in plasma membrane
• Most have cell wall; always includes
peptidoglycan
• Classification based largely on
metabolism
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Bacterial Genes
• Bacteria have a single chromosome
– Circular molecule of DNA
• Many bacteria also have plasmids
– Self-replicating circle of DNA that has a
few genes
– Can be passed from one cell to another
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Prokaryotic Fission
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Conjugationnicked plasmid
in donor cellconjugation tubeto recipient cell
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Virus
• Noncellular infectious agent
• Consists of protein wrapped around a nucleic acid core
• Cannot reproduce itself; can only be reproduced using a host cell
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Viral Body Plans
• Genetic material is DNA or RNA
• Coat is protein
Complex virus (bacteriophage)Polyhedral virusHelical virus
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Enveloped Virus (HIV)
lipid envelope; proteins span the envelope, line its inner surface, spike out above it
viral coat (proteins)
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Viral Multiplication - Basic Steps
• Virus attaches to host cell• Whole virus or genetic material enters host• Viral DNA or RNA directs host to make viral
genetic material and protein• Viral nucleic acids and proteins are
assembled• New viral particles are released from cell
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Lytic Pathway
Virus injectsgenetic material
Production ofviral components
Assembly
Lysis Assembly
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Lysogenic Pathway
Latent period extends the cycle
Viral DNA becomes part of host chromosome for a time
Viral materialintegrated
Viral materialpassed on
Stimulus may cause cellto enter lytic pathway
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Replication of an
EnvelopedVirus
DNA replication
Transcriptionof viral genes
Translation
Proteins
Assembly
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Viroids
• Smaller than viruses
• Strands or circles of RNA
• No protein-coding genes
• No protein coat
• Cause many plant diseases
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Prions
• Small proteins• Linked to human diseases
– Kuru– Creutzfeldt-Jakob disease (CJD)
• Animal diseases– Scrapie in sheep– Bovine spongiform encephalopathy
(mad cow disease)
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Protistans Differ from Prokaryotes
• Have a nucleus and organelles
• Have proteins associated with DNA
• Use microtubules in a cytoskeleton, spindle apparatus, and cilia and flagella
• May contain chloroplasts
• May divide by mitosis and meiosis
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Sporozoans
• Parasitic
• Complete part of the life cycle inside specific cells of a host organism
• Many have elaborate life cycles that require different hosts
• Many cause serious human disease
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Cryptosporidium(Giardia)
• Motile infective stage (sporozoite)
invades intestinal epithelium
• Causes cramps, watery diarrhea
• Commonly transmitted by water
contaminated with cysts
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Toxoplasma
• Cysts may be ingested with raw or undercooked meat
• Exposure to cysts from cat feces
• Symptoms are usually mild in people with normal immune function
• Infection during pregnancy can kill or damage the embryo
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Malaria
• Symptoms have been known for more than 2,000 years
• Most prevalent in tropical and subtropical parts of Africa
• Kills a million Africans each year
• Caused by four species of Plasmodium
• Transmitted by Anopheles mosquitoes
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Plasmodium Life Cycle
sporozites
merozoite
Male and femalegametocytes in blood
sporozites
Gametes formin mosquito gut, combine to form zygotes
Offspring enter blood, cause malarial symptoms