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Prokaryotes & Viruses
Chapter 16
Early Earth
• Earth is about 4.6 billion years old
• Fossils resembling photosynthetic prokaryotes
have been found in dome shaped rocks called
stromatolites, date back to 3.5 billion year ago.
Origin of Organic Molecules
• In 1953, Miller & Urey
designed an experiment to
simulate conditions on early
Earth
• Under many different
conditions this basic set up
has produced all 20 amino
acids, several sugars, lipids,
the nitrogenous bases found
in DNA & RNA and ATP
• Solutions of amino acids and
dropped them onto the
surface of hot sand, clay or
rocks has resulted in the
formation of polypeptides
The RNA World
• A characteristic of life is the process of inheritance, which
is based on molecules that can copy themselves.
DNA  RNA  Protein
• How did this information flow originate?
• A popular hypothesis is that genes were originally short
strands of RNA capable of replicating without enzymes
–Scientists have observed RNA molecules copying themselves in
solutions containing nucleotides without enzymes or cells present
– During the “RNA world”, RNA might have stored genetic
information in addition to directing protein synthesis
Formation of Pre-Cells
• Experiments have
shown that
polypeptides can form
microscopic fluid-filled
spheres.
• If certain kinds of lipids
are in the solution
selectively permeable
membranes will form.
• These “molecular
packages” are referred
to as pre-cells
Hypothetical 4-Stage Sequence for Origin of Life
1. Small organic molecules
formed from simpler
inorganic molecules
2. These small molecules
joined into more complex
ones
3. Molecules that could copy
themselves provided a basis
for inheritance of molecular
information
4. These molecules became
packaged within membranes
and separated from their
surroundings
Archaea v. Bacteria
• “archaea” is derived from
the Greek work for ancient
• Exist in harsh habitats
resembling conditions of
early Earth
• They are referred to as
“Extremophiles”
– Thermophiles
– Halophiles
– anaerobic
• Believed to be as closely
related to eukaryotes as
they are to bacteria
• Contain different
information in their
nucleic acids
• RNA polymerases differ
• Lack introns
• Susceptible to antibiotics
that do not affect archaea
• Contain peptidoglycan in
their cell walls
Phylogenic Tree of the Three Domains
Structure & Function of Bacteria: Shape
• Cocci – spherical
• Bacilli – rod-shaped
• Spirilla – spiral shaped
Structure & Function of Bacteria: Cell Wall
• Gram + (purple stain):
thick layer of
peptidoglycan
• Gram – (pink stain):
thinner layer of
peptidglycan with outer
membrane
Structure & Function of Bacteria: Motility
• Flagellum
• Pilli
• Slime secretion
Bacterial Reproduction
Binary fission: DNA is copied and moved to opposite
ends of the cell as the cell divides; occurs almost
continuously.
•Rapid
reproduction
rate, many can
divide within 20
minute
•Results in a
colony of cells
that are clones
(unless
mutations occur)
Genetic Variation
• Despite reproducing
asexually, bacteria are able
to obtain genetic variation
through conjugation
• Two bacterial cells
temporarily join and
exchange plasmid DNA;
does not need to occur
between cells of the same
species/strain
Endospores
• Allow bacteria to survive periods of very harsh conditions by
going into a dormant endospore form
• After copying DNA, one copy is surrounded by a thick protective
coat and the outer cell disintegrates
• Often able to survive for years in this state, when conditions are
more favorable, the endospores will absorb water and grow
again.
Modes of Nutrition
The Oxygen Revolution
• The evolution of photosynthetic cyanobacteria
resulted in the presence of free oxygen in oceans,
lakes & the atmosphere
• This oxygen was toxic to many existing organisms
because it attacks the bonds of organic molecules &
many went extinct
• Some were not exposed to oxygen and remained
anaerobic; their descendants still exist in similar
environments today
• A small number were able to use the oxygen in the
extraction of energy from food, their descendents are
the wide variety of aerobic organisms in existence
today
Beneficial Uses of Bacteria
• Chemical recycling:
– Decomposers: replenish soil nutrients and release CO2 back to the
atmosphere
– Nitrogen fixing bacteria: convert nitrogen gas in the atmosphere to an
organic form usable by other organisms for nucleotide and amino acid
formation
• Bioremediation:
– Sewage treatment: decompose organic matter in sewage sludge
– Oil spill clean-up: genetically modified digest oil
– Clean old mining sites: detoxify by extracting lead & mercury
• Medicine/Pharmaceuticals:
– Produce desired gene products: insulin, HGH
– Probiotics: restore beneficical intestinal bacteria
• Food:
– Cheese
– Yogurt
Viruses & Bacteriophage:
The Boundary of Life
• All living things share 8 characteristics. Viruses do not meet
all of these characteristics.
• Viruses attack eukaryotic cells and Bacteriophages attack
prokaryotic cells.
• Viruses and phages are capable of reproducing at a very
rapid rate, but only in a host cell.
• Viruses are responsible for many diseases and can be found
everywhere.
Viral Structure
Viruses & Disease
• Method of causing disease is very different from that of
bacteria
• Therefore methods for treatment & prevention differ from
methods for bacteria
• Antibiotics will not work on viruses because they target
specific not found in viruses or host cells
– Damage or prevent formation of bacterial cell wall
• Some examples of viral diseases include:
Influenza (RNA)
Common cold (RNA)
Measles (RNA)
Mumps (RNA)
AIDS (RNA)
Polio (RNA)
Hepatitis (DNA)
Herpes (DNA)
Smallpox (DNA)
Rabies (RNA)
Viral Infection
• Viruses and bacteriophages
invade cells and use the host
cell's machinery to synthesize
more of their own
macromolecules.
• Once inside the host the
bacteriophage or virus may go
into two reproductive cycles:
– Lytic cycle: destroying the
host cell during
reproduction.
– Lysogenic Cycle – a parasitic
type of partnership with the
cell
Lytic Cycle & Lysogenic Cycle
Viruses are host specific – a protein on the surface of the virus
has a shape that matches a molecule in the plasma membrane
of its host, allowing the virus to lock onto the host cell.
Proviruses
A provirus is a DNA virus that has been inserted into a host
cell chromosome.
Retroviruses & HIV
• Retroviruses reverse the
normal DNA to RNA to
protein flow
– In RNA viruses, the RNA
nucleotide will be
translated
• Reverse transcriptase
catalyzes the synthesis of
DNA from an RNA template,
the DNA intermingles with
host DNA as a provirus
making it difficult to detect
Prions
• Proteins that cause several diseases of
the brain: Mad cow disease, Kuro,
Creutzfeldt-Jacob disease (CJD) &
Scrapie (in sheep)
• Only infectious agent that do not
contain genetic material
• Normal form play important roles in
brain function such as helping nerve
cells communicate
• Abnormal prions destroy the brain
• Three ways to acquire abnormal
prions:
– Infection with abnormal prions
– Inherited genes that give rise to abnormal
prions
– Spontaneous genetic mutations that give
rise to abnormal prions
Viroids
• Small strands of RNA rather than strands of protein.
• Smaller than the strands of genetic info in viruses and contain no
protein coat.
• Replicated using host cell machinery, like viruses
• Cause plant diseases: potato spindle tuber, avocado sunblotch,
chrysanthemum stunt, and chrysanthemum chlorotic mottle
Immune Response
HIV doesn’t target just any cell, it goes right for the cells that want to kill
it. “Helper" T cells are HIV's primary target. These cells help direct the
immune system's response to various pathogens.
HIV undermines the body's ability to protect against disease by
depleting T cells thus destroying the immune system. The virus
can infect 10 billion cells a day, yet only about 1.8 billion can be
replaced daily.
Defense Against Viral Diseases
From HIV to AIDS
• During first few years (7-10) after HIV infection, person is
usually asymptomatic.
• During the symptomatic phase, the body has insufficient
numbers of T-Cells (from normal 800-1200 /mm3 to 200/ mm3 )
to mount an immune response against infections.
– Chronic diarrhea, minor mouth infections, night sweats, headache
& fatigue are common
• At the point when the body is unable to fight off infections,
a person is said to have the disease AIDS. (Generally when
count drops below 200 /mm3 )
• It is not the virus or the disease that ultimately kills a
person; it is the inability to fight off something as minor as
the common cold.
AIDS: The Global Epidemic
• Around 2.6 million
people became infected
with HIV in 2009.
• Sub-Saharan Africa has
been the hardest hit by
the epidemic. In 2009
over two-thirds of AIDS
deaths were in this
region