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What are prokaryotic cells?
Single-celled bacteria and archaeans
No nucleus or membrane-bound organelles
Smallest, most widely distributed, numerous, and metabolically diverse organisms
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Autotrophs and heterotrophs
Spheres (cocci), rods (bacilli), spirals (spirilla)
How does the structure look?
Typical surface structures
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Cell wall
Outermost protective capsule or slime layer
One or more flagella
Pili
How do Prokaryotes Reproduce?
Only bacteria and archaeans reproduce by prokaryotic fission:
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Replication of single, circular prokaryotic chromosome
Division of parent cell into two genetically equivalent daughter cells
Horizontal gene transfers can move genes between prokaryotes
Conjugation moves a plasmid and some chromosomal genes into another cell through a
sex pilus
What are the bacteria?
The most common and diverse prokaryotes
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Some are pathogens (cause disease in a host)
Bacterial Diversity: Cyanobacteria
Oxygen-releasing photoautotrophs
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Chloroplasts probably evolved from ancient cyanobacteria by endosymbiosis
Bacterial diversity: proteobacteria
The most diverse bacterial group
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Include autotrophs and heterotrophs, free-living species, beneficial symbionts,
and pathogens
Example: Thiomargarita namibiensis
Bacterial are divided into two classes or groups
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Gram Positive
Gram negative
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What are gram-positive bacteria ?
Have thick walls
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Endospores resist heat, boiling, irradiation, acids and disinfectants
Some are human pathogens
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Chlamydias
What are spirochetes?
Spring-shaped
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Live on their own or in hosts
Some are pathogens
What are the archaeans?
Archaeans are prokaryotic, but like eukaryotic cells in certain features
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Halophiles (salt lovers), extreme thermophiles, and methanogens (methane
makers)
Comparisons of structure, function, and genetic sequences put archaeans in a
separate domain, between
eukaryotes and bacteria
Archaeans are more diverse and widely distributed than previously thought
What are the viruses?
Viruses are noncellular infectious particles that cannot reproduce on their own
Viruses infect a host cell; their genes and enzymes take over the host’s mechanisms
of replication and
protein synthesis
Virus Structure
A virus particle consists of a core of DNA or RNA and a protein coat
In some viruses, the coat is enveloped in some of an infected cell’s plasma membrane
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Outer envelope forms as each new virus particle is released by budding or lysis
In bacteriophages and other complex viruses, the coat has a sheath and other
structures
Viral Multiplication Pathways
Multiplication pathways vary greatly
Two are common among bacteriophages
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Lytic pathway
Lysogenic pathway
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Lytic Pathway
New virus particles are released by lysis
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Multiplication is rapid
Lysogenic Pathway
Virus enters a latent state that extends the cycle
– Host cell is not killed outright
Viral nucleic acids integrate into host chromosome
– All host cell’s descendants inherit genetic material
– May be reactivated many generations later, causing cell to enter lytic pathway
What are prion infections?
How do pathogens evolve?
Pathogens evolve to not kill a host before they can infect other host individuals
Use of antibiotics favors antibiotic-resistant bacteria
Genes that convey drug resistance can arise by mutation, may spread among members
of the same
or different species by conjugation
Diseases can be fatal
– If an individual becomes host to multiple pathogens
– If an individual has no coevolved defenses
– If a pathogen mutates into a different form that can breach current defenses
• Two deadly emerging pathogens
– Ebola and the H5N1 strain of bird flu