Prokaryotes, Viruses, and Protistans

Chapter 20

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

The Prokaryotes

• Only two groups

• Archaebacteria and Eubacteria

• Arose before the eukaryotes

Prokaryotic Characteristics

• No membrane-bound nucleus

• Single chromosome

• Cell wall in most species

• Prokaryotic fission

• Metabolic diversity

Prokaryotic Body Plan

bacterial flagellum

pilus

capsulecell wall

plasma membrane

DNA

ribosomes in cytoplasm

Bacterial Shapes

coccus bacillus

spirillum

Archaebacteria

Methanogens

Extreme halophiles

Extreme thermophiles

Eubacteria

• Includes most familiar bacteria

• Have fatty acids in plasma membrane

• Most have cell wall; always includes

peptidoglycan

• Classification based largely on

metabolism

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

Prokaryotic Fission

Conjugationnicked plasmid

in donor cellconjugation tubeto recipient cell

Virus

• Noncellular infectious agent

• Consists of protein wrapped around a nucleic acid core

• Cannot reproduce itself; can only be reproduced using a host cell

Viral Body Plans

• Genetic material is DNA or RNA

• Coat is protein

Complex virus (bacteriophage)Polyhedral virusHelical virus

Enveloped Virus (HIV)

lipid envelope; proteins span the envelope, line its inner surface, spike out above it

viral coat (proteins)

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

Lytic Pathway

Virus injectsgenetic material

Production ofviral components

Assembly

Lysis Assembly

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

Replication of an

EnvelopedVirus

DNA replication

Transcriptionof viral genes

Translation

Proteins

Assembly

Viroids

• Smaller than viruses

• Strands or circles of RNA

• No protein-coding genes

• No protein coat

• Cause many plant diseases

Prions

• Small proteins• Linked to human diseases

– Kuru– Creutzfeldt-Jakob disease (CJD)

• Animal diseases– Scrapie in sheep– Bovine spongiform encephalopathy

(mad cow disease)

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

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

Cryptosporidium(Giardia)

• Motile infective stage (sporozoite)

invades intestinal epithelium

• Causes cramps, watery diarrhea

• Commonly transmitted by water

contaminated with cysts

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

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

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