Download Prokaryotes

What are prokaryotic cells?
• Single-celled bacteria and
archaeans
• No nucleus or membrane-bound
organelles
• Smallest, most widely
distributed, numerous, and
metabolically diverse organisms
– Autotrophs and
heterotrophs
• Spheres (cocci), rods (bacilli),
spirals (spirilla)
How does the structure look?
• Typical surface structures
– 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:
– 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
– Some are pathogens (cause
disease in a host)
Bacterial Diversity: Cyanobacteria
• Oxygen-releasing
photoautotrophs
– Chloroplasts probably
evolved from ancient
cyanobacteria by
endosymbiosis
Bacterial diversity: proteobacteria
• The most diverse bacterial group
– Include autotrophs and
heterotrophs, free-living
species, beneficial symbionts,
and pathogens
– Example: Thiomargarita
namibiensis
– Bacterial are divided into two
classes or groups
• Gram Positive
• Gram negative
What are
gram-positive bacteria ?
• Have thick walls
– Endospores resist heat, boiling,
irradiation, acids and
disinfectants
– Some are human pathogens
• Chlamydias
What are spirochetes?
• Spring-shaped
– Live on their own or in hosts
– Some are pathogens
What are the archaeans?
• Archaeans are prokaryotic, but like
eukaryotic cells in certain features
– 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 infections
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
– 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
– Lytic pathway
– Lysogenic pathway
Lytic Pathway
• New virus particles are
released by lysis
– 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?
Prions are infectious misfolded
versions of normal proteins
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