Transcript Ch 19 Viruses

Virus Demo
Viruses
Chapter 19
What you must know:
The components of a virus.
 The differences between lytic and
lysogenic cycles.
 How viruses can introduce genetic
variation into host organisms.
 Mechanisms that introduce genetic
variation into viral populations.

Properties of a Virus
Bacteria vs. Viruses
Bacteria
Prokaryotic cell
 Most are free-living (some
parasitic)
 Relatively large size
 Antibiotics used to kill
bacteria

Virus
Not a living cell (genes
packaged in protein shell)
 Intracellular parasite
 1/1000 size of bacteria
 Vaccines used to prevent
viral infection
 Antiviral treatment

Parts of the Virus
Components = nucleic acid + capsid
◦ Nucleic acid: DNA or RNA (double or
single-stranded)
◦ Capsid: protein shell
 Accessory Structure: Some viruses also have
viral envelopes that surround capsid
 Found in many animal viruses
◦ Pick up bits and pieces as they
travel from host to host.
Introduces genetic variation
(Transduction).
Breaking all of the
rules
Mimivirus (Giant
virus) 400 nm
 1.2 million base
pair
 Codes 979
proteins – more
than any other
virus.
 This treasure chest
includes some
proteins never
found in any viral
genome

How are viruses classified?
What is the host range?
Classification
Obligate Intracellular Parasites
 Specific to their hosts (human, dog, some can
cross species)
 They can only attack specific cells, the
common cold is a virus that specifically
attacks cells of the respiratory track (hence
the coughing and sneezing and sniffling). HIV
specifically attacks white blood cells

Host Range
Limited host range
 Evolution: Recognition systems in viruses

◦ Entry = attach to host cell membrane receptors
through capsid proteins or glycoproteins on viral
envelope (animal)
◦ Eg. human cold virus (rhinovirus)  CAN ONLY
INFECT SPECIFIC TISSUE: upper respiratory tract
(mouth & nose)

Some Broad Range (West Nile – mosquitoes,
horses, humans, birds)
Simplified viral replicative cycle
Entry: Tail inject
genetic material,
Endocytosis, Fuse
with the
Membrane
A virus that infects a bacteria is
called…
Bacteriophage
Virus that infects bacterial cells
How do viruses reproduce?
E.Bola
Reproduction

Reproduce quickly within host cells

Lytic cycle - reproduction occurs, cells burst
Lysogenic cycle - reproduction does not
immediately occur (dormancy)
Lytic Cycle of T4
Phage
Maturation
Penetrate
Lytic Cycle
◦ Use host machinery to replicate, assemble, and
release copies of virus
◦ Virulent phages only go through the lytic cycle
◦ Cells die through lysis or apoptosis
Bacterial Defense

Restriction Enzymes – cut up the viral
DNA into pieces
◦ Bacterial DNA is methylated to prevent
attack of these restriction enzymes
Natural selection - favors bacteria with
surface proteins that don’t recognize the
phage
 Lysogeny/Dormancy

Lysogenic (Latent) Cycle:
◦ DNA incorporated into host DNA and replicated along
with it
Temperate Phage: Capable of both Lytic/Lyso cycles (lambda)
Prophage: Fuses with bacterial DNA and goes dormant
◦ Enviromental Signal: UV radiation, chemicals:
◦ Trigger lysogenic  lytic cycle
Lytic Cycle vs. Lysogenic Cycle
Expression of viral genes can change
host phenotypes

Secondary effects
◦
◦
◦
◦
Scarlet Fever
Botulism*
Diptheria**
Certain E.Coli strains
Release harmful toxins
* Paralysis neurotoxin
**Nerve inflammation, myocarditis, paralysis

Animal viruses have a membranous envelope

Host membrane
forms around exiting
virus
◦ Undercover, Covert
operations

Difficult for host
immune system to
detect virus
Plant Viruses

Horizontal Gene Transfer: Accidental,
Non-traditional reproduction (Wind,
Herbivore damage, Injury)

Vertical Gene Transfer: Generations
(Cutting/Asexual, Infected Seeds)
Retrovirus

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
RNA virus that uses reverse transcriptase (RNA 
DNA)
Newly made viral DNA inserted into chromosome of
animal cell (provirus)
Host transcribes provirus to make new virus parts
Examples:
◦ HIV (Human Immunodeficiency Virus)
 RNA and Enzymes
◦ Herpes
◦ Small pox
What is the difference between
a prophage and provirus?
How does HIV lead to AIDs?
NPR How Do Viruses Work
Mutations
 Can mutate easily
◦ Lack error-checking mechanisms
◦ Why there is no vaccine for common cold

Leads to genetic variation within virus
populations
Emerging Viruses

Endemic – regularly found in a certain
area
◦ Malaria of Africa

Epidemic – widespread outbreak
◦ 2003 SARS

Pandemic – Global epidemic
◦ Flu (waves) mutates quickly
◦ HIV/AIDS
Current Outbreaks

Zika Virus
◦ Spread by Aedes mosquitoes (Aedes aegypti)
“Arbovirus”
◦ Major outbreak in Brazil and Latin America
◦ Linked to birth defects (microcephaly)
Zika Virus (as of Dec. 2015)
As of Feb. 2016 (CDC.GOV)
Drugs for Prevention/Treatment

Vaccine: weakened virus or part of pathogen that
triggers immune system response to prevent
infection
◦ Ex. HPV, MMR, HepA, Flu shot

Antiviral Drugs: block viral replication/synthesis
of nucleic acids after infection
◦ Ex. Tamiflu (influenza), AZT (HIV)
Where did the first viruses come from?
Observation 1. Share cell properties
Observation 2. Infect all life forms
Theory: Arose after first cells
-Escaped bits of nucleic acids (plasmids)
-Infect humans and bacteria with same mechanisms
-Mix match of parts
*Small circular DNA molecules found in bacteria and
yeast
Viroids
Small, circular RNA
molecules that infect plants
 Cause errors in regulatory
systems that control plant
growth
 Cadang-Cadang: 10 mil:
Coconut palms in
Philippines

Prions
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Misfolded, infectious proteins that cause misfolding of
normal proteins
Mad cow disease (BSE)
Can be transmitted by food and cause neuro degeneration
Slow acting (10 years)
Can’t be denatured, no cure
Prion Neurodegenerative Diseases
Alzheimer’s Disease
 Parkinson’s Disease
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VIDEO: HIV LIFE CYCLE