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Chapter 19
Viruses
Concept 19.1: Virus Structure
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Viruses are not cells.
They are infectious particles consisting of nucleic acid encased in a protein coat, and
possibly, a membranous envelope.
Viruses range in size from only 20nm in diameter to that barely resolvable with a light
microscope.
Host Range: “lock and key”; each virus can infect cells of a limited variety of hosts; ex. Measles can
infect humans only; West Nile Virus infects humans, birds, mosquitoes, horses
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The genome of viruses includes options other than the double-stranded DNA that
we have studied.
– Viral genomes may consist of
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double-stranded or single-stranded DNA,
double-stranded or single-stranded RNA,
– The
viral genome is usually organized as a single linear or circular molecule of
nucleic acid.
– The
smallest viruses have only four genes, while the largest have several
hundred.
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The capsid is a protein shell enclosing the viral genome.
Capsids are composed of a large number of protein subunits called capsomeres, but
with limited diversity.
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The capsid of the tobacco mosaic virus has over 1,000 copies of the same
protein.
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Some viruses have membranous envelopes that help them infect hosts
These viral envelopes surround the capsids of influenza viruses and many other
viruses found in animals
Viral envelopes, which are derived from the host cell’s membrane, contain a
combination of viral and host cell molecules
The most complex capsids are found in viruses that infect bacteria, called
bacteriophages or phages.
The phages that infect Escherichia coli have a 20-sided capsid head that encloses
their DNA and protein tail piece that attaches the phage to the host and injects the
phage DNA inside.
General Characteristics
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Alive
Alive or not?
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Viruses have characteristics of both living and nonliving.
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1. Nucleic acids
2. Reproduction
3. Respond to stimuli
4. Evolve
Nonliving
1. DNA or RNA not both
2. Obligate intracellular parasite
3. Can be crystallized
4. No growth or metabolism
5. No Cell structure
Concept 19.2: Virus Reproduction
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Lytic Cycle
– Attachment
= (adsorption) by chance collision, viruses do not move on their
own.
– Penetration
– chemical interaction causes the contraction of the tail and it
injects DNA into bacterial cell.
– Synthesis
– the bacterial cell is making many copies of viral DNA.
synthesis is taking place making capsids, proteins, and nuclease.
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Protein
Nuclease destroys bacterial DNA so host has only viral DNA
– Assembly
and maturation – viral parts come together, chemical reactions
bring and glue pieces together.
– Release
by lyses – Lysozyme and enzymes are used to lyses the bacterial
cells allowing viruses to escape.
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Burst time – average 20-40 min. to go from penetration to release.
Burst size – 50-200 viruses per cell burst.
Lysogenic Cycle
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Similar to Lytic Cycle with one major variation.
– Prior
to Synthesis, the Bacteriophage DNA is integrated into the bacterial DNA
becoming a prophage. As the cell goes through binary fission, it copies the
bacteriophage DNA with it.
– Excision
occurs when prophage DNA comes out of bacterial DNA, takes over
the cell, and the lytic phase of the cycle begins.
– Temperate phage: phage capable of undergoing both lytic and lysogenic
cycles
Animal Virus Reproduction
-Similar to lytic and lysogenic cycles with some differences
– Similarities: adsorption, synthesis, assembly
– Differences:
1. Entering the cell (Penetration): bind to receptors on the host cell; enter
through fusion or endocytosis
2. Leaving the cell: Naked viruses exit by lysis, while enveloped viruses
exit by exocytosis (also known as budding)
Viral Diseases
-Viruses cause diseases by:
1. releasing hydrolytic enzymes
2. producing toxins
3. presence of toxic molecules (membrane proteins)
Emerging viruses: new viruses that have never been seen before
e.g. West Nile in 1999; new to North America
e.g. SARS outbreak in China, 2002
Epidemics: general outbreak
Pandemics: global epidemic; e.g. Spanish flu of 1918 killed 40
million people; birds were the source of recombinant viruses- 2
strains mixed in one host
Treating viruses: NO antibiotics!!! Let the infection takes its course;
some antivirals are used for infections (HIV)
Vaccines: prevents infection; introduce the inactive virus and the
immune system responds