Prokaryotes and
Viruses
Chapter 21
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
DNA
capsule
Figure 21.3
Page 348
bacterial flagellum
pilus
plasma
membrane
cell wall
ribosomes in
cytoplasm
cytoplasm
Bacterial Shapes
coccus
bacillus
spirillum
In-text figure
Page 348
Metabolic Diversity
 Photoautotrophs
 Chemoautotrophs
 Chemoheterotrophs
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 - 1
bacterial
chromosome
Bacterium before
DNA replication
DNA replication
begins
Figure 21.7
Page 350
Prokaryotic Fission - 2
parent DNA
molecule
DNA replication
completed
DNA copy
Membrane growth
moves DNA
molecules apart
Figure 21.7
Page 350
Prokaryotic Fission - 3
New membrane and
cell-wall material
deposited
Cytoplasm
divided in two
Figure 21.7
Page 350
Conjugati
on
Transfer of plasmid
Figure 21.8
Page 351
nicked plasmid
in donor cell
conjugation tube
to recipient cell
Prokaryotic Classification
EUBACTERIA
(Bacteria)
ARCHAEBACTERIA
(Archaea)
EUKARYOTES
(Eukarya)
•Traditionally classified by numerical taxonomy
•Now increased use of comparative biochemistry
Figure 21.9
Page 351
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
Eubacterial Diversity
 Photoautotrophic
 Aerobic (Cyanobacteria)
 Anaerobic (Green bacteria)
 Chemoautotrophic
 Important in nitrogen cycle
 Chemoheterotrophic
 Largest group
Some Pathogenic
Eubacteria
 Most are chemoheterotrophs
 E. coli strains
 Clostridium botulinum
 Clostridium tetanus
 Borrelia burgdorferi
 Rickettsia rickettsii
Bacterial Behavior
 Bacteria move toward nutrient-rich
regions
 Aerobes move toward oxygen;
anaerobes avoid it
 Photosynthetic types move toward light
 Magnetotactic bacteria swim downward
 Myobacteria show collective behavior
Virus
 Noncellular infectious agent
 Protein wrapped around a nucleic acid
core
 Cannot reproduce itself; can only be
reproduced using a host cell
Viral Body
Plans
Complex virus
(bacteriophage)
 Genetic material is
DNA or RNA
 Coat is protein
Helical virus
Polyhedral virus
Fig. 21.18
Page 356
Enveloped Virus (HIV)
viral protein
lipid envelope
(derived from host)
viral RNA
reverse transcriptase
viral coat (proteins)
Fig. 21.18
Page 356
Viral Multiplication Basic Steps
 Attach to host cell
 Enter host (virus or just genetic material)
 Direct host to make viral genetic material
and protein
 Assemble viral nucleic acids and proteins
 Release new viral particles
Lytic
Pathway
Lysis of host cell is induced;
infectious particles escape.
Tail fibers and other parts
are added to coats.
Virus particles bind to wall of suitable host.
Viral genetic material enters cell cytoplasm.
Viral protein molecules are assembled
into coats; DNA is packaged inside.
Viral DNA directs host
machinery to produce viral
proteins and viral DNA.
Stepped
Art
Fig. 21.20
Lysogenic
Pathway
Viral DNA usually becomes
integrated into the bacterial
chromosome.
Prior to prokaryotic fission, the
chromosome and integrated
viral DNA are replicated.
Viral DNA is excised from
chromosome and cell
enters lytic pathway.
After binary fission, each
daughter cell will have
recombinant DNA.
Stepped
Art
Fig 21.20
Replicatio
n of an
Enveloped
Virus
DNA virus particle
plasma
membrane
of host cell
Replication
of viral DNA
Transcription
of viral DNA
Translation
viral DNA
some
proteins for
viral coat
nuclear
envelope
other
proteins
for viral
envelope
Figure
21.21
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)
Nature of Disease
 Contagious disease pathogens must
directly contact a new host
 Epidemic
 Pandemic (AIDS)
 Sporadic
 Endemic
Evolution and Disease
 Host and pathogen are coevolving
 If a pathogen kills too quickly, it might
disappear along with the individual host
 Most dangerous if pathogen
 Is overwhelming in numbers
 Is in a novel host
 Is a mutant strain
New Threats
 Emerging Pathogens
 Ebola virus
 Monkeypox virus
 Drug-resistant strains
 Food poisoning
 E. coli
 Salmonella