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Viruses
Chapter 25
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Adenovirus
Influenza Virus
Structure
Section 25.1
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Virology

Virus: nonliving particle composed
of nucleic acid & protein coat

Virology: study of viruses

Wendell Stanley (1900s)
crystallized tobacco mosaic virus
(caused withering of tobacco
leaves)
 Suggested viruses were
chemicals, not tiny cells
 Before his crystallization,
viruses were thought to be
primitive cells, perhaps
ancestors of bacteria

Virology now provides clues to
biochemistry of living organisms &
pharmaceutical companies use
viruses to develop new antiviral
medications
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
Characteristics
of
Viruses
Among smallest biological particles capable of causing disease

Range in size

Only have some characteristics of living things
 Able to replicate only by infecting cells and using organelles and enzymes
within cells

For our purposes: WE DO NOT CONSIDER VIRUSES TO BE ALIVE!
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Viral Structure
 Two
essential features:
Capsid: Nucleic acid & protein coat surrounding it
 Nucleic acid may be DNA or RNA (not both)


May be helical, closed loop, long strand
 Some
have envelope: membrane-like structure outside
capsid
Made mostly of lipids
 taken from host cell membrane during first stage of viral
replication
 Influenza, chickenpox, herpes simplex, HIV
 On surface are projections made of glycoprotein: proteincontaining sugar chains that virus uses to attach to host cell

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Viral Shape: Determined by capsid
or nucleic acid.


Icosahedron: geometric shape with
20 triangular faces

Formed by viral capsid

Polio
Helical: resembles coiled spring

Formed by nucleic acid

Rabies, measles, tobacco mosaic
virus
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
Grouping Viruses
Shape & Structure

Based on presence of capsid structure and envelope

According to whether they contain RNA or DNA & whether nucleic acid
is single or double-stranded
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Virus Types

DNA and RNA viruses differ in way they use host cells
machinery to produce new viruses

DNA virus may act in one of two ways:

Virus may directly produce RNA that then make viral proteins
OR

Will join with the host cell’s DNA to direct synthesis of new
viruses

RNA virus releases RNA into host cell’s cytoplasm and uses host
cell’s ribosomes to produce new viral proteins
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
Retroviruses – RNA viruses
Retroviruses contain reverse
transcriptase: uses RNA as
template to make DNA

Viral DNA is integrated
into host genome

DNA then makes RNA
transcript of itself

RNA transcript translated
into proteins which
become part of new
viruses
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Viroids and Prions

Simpler than viruses

Viroids: smallest known particles able to replicate

Consists of short, single-strand of RNA able to disrupt plant cell
metabolism and damage entire crops


Plants affected: potatoes, cucumbers, avocados, oranges
Prions: abnormal forms of proteins that clump together inside cell.

Clumping kills cells by blocking molecular traffic
 Found on surface of mammalian cells & in brain of hosts
 Have no nucleic acid, made of 250 AA
 Have been linked to certain diseases of brain:


Scrapie
Bovine spongioform encephalophathy (BSE) “mad cow disease”
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Viral Replication
Section 25-2
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
The Bacteriophage
Scientists gained better
understanding of viral replication
through working with
bacteriophages: viruses that infect
bacteria
 Most commonly studied
bacteriophage: T phages,
known to infect bacterium
found in human digestive tract,
Escherichia coli
 Parts:
 Icosahedral head
 Nucleic acid
 Contractile tail, including collar
& sheath
 Base plate
 Tail fibers
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Bacteriophage Reproduction
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The Lytic Cycle

Virus invades host cell,
produces new viruses by
“hijacking” cell
machinery, destroys host
cell, releases newly
formed viruses

Virus that undergoes
lytic cycle: virulent:
because they cause
disease

Consists of 5 main
phases
 Attachment of virus
to host
 Entry of viral DNA
into host
 Replication of viral
DNA
 Assembly of new
viruses
 Release of new
viruses from lysed
host cell
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The Lysogenic Cycle

Some viruses infect cell without
causing immediate destruction
 Temperate virus: virus that uses
lysogenic cycle

Lysogenic cycle: viruses in this cycle
stay in host cell for extended period
of time
 Four main phases:
 Attachment of virus to host
 Injection of viral DNA into host
 Integration of viral DNA into host
genome, now called a prophage
 Multiplication of host cell with
viral DNA

Radiation or certain chemicals can
cause a prophage to become virulent
 Enters the lytic cycle and destroys
the host cell
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Lysogeny in HIV

When HIV (a retrovirus) infects
susceptible white blood cell:
 Attaches to receptor sites on
host cell’s surface and
enters cell by fusing with
cell membrane
 Viral RNA and reverse
transcriptase released into
cell’s cytoplasm
 Reverse transcriptase then
transcribes viral RNA into
DNA
 Viral DNA inserts itself into
host cell’s genome for
period of time (viral DNA
called provirus)
 Lysogenic cycle ends when
provirus is transcribed into
RNA and translated into viral
proteins
 Proteins are used to
assemble new HIV particles
which get released from cell
through cell lysis.
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Viral Evolution

Viruses depend on cells to replicate

Most scientists think they evolved from early cells


Probably began as naked pieces nucleic acid able to travel from
one cell to another
Most viruses destroyed upon entering body but some remain

How are few viruses able to avoid destruction?

Mutations: some viruses mutate at fast rate therefore its difficult
to make vaccines
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Viruses and Human Disease
Section 24-3
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
Common human viral diseases:
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
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Common cold
Chickenpox
Measles
Mumps
Polio
Rabies
Hepatitis
Viral infections can affect:

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

Brain
Liver,
Heart
Lungs
Skin
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Prevention and Treatment

Control of viral diseases accomplished in two ways:


Vaccination: preparation of pathogens or other materials that stimulate body’s
immune system to provide protection against that pathogen
 Inactivated virus: do not replicate in a host system
 Attenuated virus: have been genetically altered so they are incapable of
causing disease under normal circumstances
Antiviral drugs: Drugs that interfere with viral nucleic acid or capsid synthesis
 There are only a few compared to antibacterial drugs
 Acyclovir: used against herpes simplex and chickenpox
 Azidothymidine (AZT): inhibits reverse transcriptase of retroviruses
 Protease Inhibitors: interfers with synthesis of viral capsids during viral
replication
 Combination of AZT and protease inhibitors helpful in slowing
progression from HIV to AIDS
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Smallpox Eradication Program
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Once killed 40% people infected, leaving other 60% scarred
and often blind

DNA virus transmitted by nasal droplets emitted during
sneezing & coughing

Vaccination played important role in eradication

World Health Organization began smallpox eradication program
in 1967 and finished in 1980 declaring smallpox eradicated

Last naturally acquired case occurred in Somalia in 1977
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Other Anti-Viral Approaches

Important to control animals that spread viral diseases

Yellow fever completely eradicated in U.S. through mosquitocontrol programs
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Pets are vaccinated against rabies

Wildlife officials put out meat containing oral rabies vaccine to
control the spread from coyotes and wolves.
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Emerging Viruses

Viruses that exist in isolated
habitats, but infect humans
when these habitats are
developed

Tropical forest of
Democratic Republic of
Congo (Zaire): emerging
Ebola virus has been known
to exist
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When these forests are
cleared, humans may be
exposed to virus-infected
animals & if virus infects
humans, it may spread
rapidly