Transcript AP Bio Viruses and bacteria

Molecular Biology
of Prokaryotes
VIRUSES AND BACTERIA
Look at the relative size of these cells.
Viruses
Have a genome, but can only
reproduce in a host cell.
 Structure: infectious particles
consisting of nucleic acid enclosed in
a protein coat
 Genome can consist of doublestranded DNA, single-stranded DNA,
double-stranded RNA, or singlestranded RNA (depends on virus type)
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Capsids and Envelopes
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Capsids are the protein shell enclosing the
viral genome. Most complex in viruses
affecting bacteria (phages).
Depending on the virus type, the capsid
may be rod-shaped, polyhedral, or a more
complex shape.
Capsids are built from protein subunits
called capsomeres.
Viral envelopes surrounding viruses come
from host cell phospholipids and membrane
proteins.
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Viral Reproductive Cycles
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Viruses are obligate intracellular parasitesthey cannot reproduce without a host cell.
Viruses lack enzymes for metabolism.
No ribosomes for protein synthesis
Viruses can affect only a limited number of
host cells called the host range.
Viruses identify host cells by a “lock and
key” mechanism. Virus binds to receptor
molecules on the host cell surface.
Lytic vs. Lysogenic Cycles
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Lytic cycles result in the death of the host.
The bacterium lyses (breaks open) and
releases phages produced by the cell.
Virulent phages reproduce this way.
In lysogenic cycles, the phage genome is
replicated without destroying the host cell.
Viral genome becomes incorporated into
the bacterial chromosome as a prophage.
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LYTIC CYCLE
Animal Viruses
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Viruses are classified by the form of genetic
material they contain. (see chart in
textbook)
Nearly all animal viruses with RNA
genomes have an envelope, as do some
with DNA genomes.
Some enveloped viruses enter the host cell
by fusion of the envelope with the cell’s
plasma membrane, similar to endocytosis.
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Human Immunodeficiency
Virus (HIV)
Retrovirus that causes AIDS
 Reverse transcriptase transcribes an
RNA template into DNA, providing a
RNADNA information flow (opposite
the usual direction)
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This is hepatitis
Viral Diseases in Animals
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The damage done by a virus depends on
the ability of the affected tissue to
regenerate by cell division.
The immune system is the basis for
preventing viral infections by the use of
vaccines.
Vaccines are harmless variants or
derivatives of pathogenic microbes that
stimulate the immune system to defend
against the pathogen.
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Influenza virus
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Flu epidemic of 1915
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Child with small pox
Child with measles
Measles virus
Herpes simplex virus
Child with polio
Emerging Viruses
Major source is the mutation of
existing viruses
 Spread from one host species to
another
 Spread of disease from a small,
isolated population can lead to
widespread epidemics
 Examples: HIV, Ebola, SARS
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EBOLA
Why are emerging viruses so
dangerous?
 Viruses can evolve very quickly,
mutating rapidly.
 Viruses can sometimes jump between
species, especially if exposed to
populations without defenses against
them
 Medications that treat viral diseases
are difficult to develop, and often have
to target host cells.
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Plant Viruses
More than 2,000 types of viral
diseases affecting plants are known.
 Symptoms include: bleached or
brown spots on leaves or fruits,
stunted growth, damaged roots and
flowers
 Decrease crop and food production
 Most have RNA genomes
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Tobacco Mosaic Virus in squash
Horizontal vs. Vertical
transmission of plant viruses
Horizontal transmission: plant is
infected by an external source of the
virus. Plant is more vulnerable if it
has been damaged in some way.
 Vertical transmission: plant inherits
a viral infection from a parent. Occurs
through asexual reproduction
(cuttings) or sexual reproduction by
infected seeds.
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Electron
Micrograph
Of TMV
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TMV isolated
T
T
Viroids
Viroids are plant pathogens
composed of molecules of naked
circular DNA several hundred
nucleotides long.
 A single molecule can be an infectious
agent that spreads disease.
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Prions
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Prions are infectious proteins which appear
to cause a number of degenerative brain
diseases in animal species. Ex) Mad cow
disease and Creutzfeld-Jakob
Prions may be transmitted in food.
Prions have two disturbing characteristics:
1) they have long incubation periods, and
2) heating fails to deactivate them
PROKARYOTES
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The short generation span of bacteria
facilitates their adaptation to changing
environments.
Composed of one double-stranded circular
molecule of DNA, found in a nucleoid region
(no nucleus)
Transcription and translation occur
simultaneously.
Some bacteria contain extrachromosomal
DNA in plasmids.
Binary Fission
After replication of the bacterial
chromosome, bacteria divide by
binary fission.
 Bacteria divide rapidly. For example,
E.coli reproduce every 20 minutes.
 Binary fission is an asexual process,
so most bacteria in a colony are
identical to the parent cell.
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E. coli dividing
Mutation and Genetic Variation
Because bacteria can reproduce
rapidly, new mutations can quickly
increase a population’s genetic
variation.
 Genetic recombination of DNA from
two different bacterial cells increases
diversity in populations as well.
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Gene Transfer and Genetic
Recombination
New bacterial strains can arise by the
transfer of DNA from one cell to
another cell.
 In transformation, naked DNA enters
the cell from the surroundings.
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E. coli DNA
Gene transfer, cont.
In transduction, bacterial DNA is
carried from one cell to another by
phages.
 In conjugation, an F+ donor cell,
which contains F plasmid, transfers
plasmid DNA to an F- recipient cell.
 R plasmids transfer resistance to
antibiotics.
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Plasmids
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Conjugation: pili
and plasmids
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