Ch. 19 Warm-up
1. Why do many scientists classify viruses
as non-living?
2. Draw the basic structure of a virus.
Label and define capsid, viral envelope
and nucleic acid.
3. Draw the lytic/lysogenic cycle.
4. What stage of the lytic-lysogenic cycle
is a virus virulent? Temperate?
5. What determines a host range?
VirusesChapter 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.
What is a virus?
• A virus is a submicroscopic
infectious particle composed of a
protein coat (capsid) and a nucleic
acid core (either DNA or RNA).
• Viruses are similar in size to a
large protein macromolecule,
generally smaller than 200 nm in
diameter.
Discovery of Viruses• Search for cause
of tobacco mosaic disease led to viruses
• Beijerinck proved that the disease was caused by a virus.
• The elusive virus was crystallized in 1935 by Wendell Stanley.
Bacteria vs. Viruses
Bacteria Virus
Prokaryotic cell
Most are free-living (some
parasitic)
Relatively large size
Antibiotics used to kill bacteria
Not a living cell (genes
packaged in protein shell)
Intracellular parasite
1/1000 size of bacteria
Vaccines used to prevent
viral infection
Antiviral treatment
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Viral Genomes • Viral genomes may consist
of either
– Double- or single-
stranded DNA, or
– Double- or single-
stranded RNA
• Depending on its type of
nucleic acid, a virus is called
a DNA virus or an RNA virus
• A capsid is the protein shell
that encloses the viral
genome
Flu virus magnified
100,000x
Viruses Very small (<ribosomes)
Components = nucleic acid + capsid
◦ Nucleic acid: DNA or RNA (double or
single-stranded)
◦ Capsid: protein shell
Some viruses also have viral envelopes
that surround capsid Limited host range
◦ Entry = attach to host cell membrane
receptors through capsid proteins or
glycoproteins on viral envelope (animal)
◦ Eg. human cold virus (rhinovirus) → upper
respiratory tract (mouth & nose)
Reproduce quickly within host cells
Can mutate easily
◦ RNA viruses: no error-checking mechanisms
Viral Capsids• Capsids are built from protein subunits called
capsomeres
• May be rod-shaped (helical viruses), polyhedral(icosahedral viruses) or more complex
• Some viruses have membranous envelopes that help them infect hosts (flu virus)
• Bacteriophages, also called phages, infect bacteria
• Viruses are obligate intracellular parasites,
which means they can reproduce only within a
host cell
• Each virus has a host range, a limited number
of host cells that it can infect
Viral Reproduction• Once a viral genome has
entered a cell, the cell begins
to manufacture viral proteins
using the host cell’s
materials (enzymes,
ribosomes, tRNAs, amino
acids, ATP, etc.)
• ** RNA viruses may have
codes for their own enzymes
however.
• Phages are the best
understood of all viruses
• Phages have two
reproductive mechanisms:
the lytic cycle and the
lysogenic cycle
Bacteriophage
Virus that infects bacterial cells
Viral Reproduction
Lytic Cycle:
◦ Use host machinery to replicate, assemble, and
release copies of virus
◦ Virulent phages: Cells die through lysis or apoptosis
Lysogenic (Latent) Cycle:
◦ DNA incorporated into host DNA and replicated
along with it
◦ Bacteriophage DNA = prophage
◦ Animal virus DNA = provirus
◦ UV radiation, chemicals: lysogenic → lytic cycle
◦ Temperate Phage: uses both methods of replication
The Lytic Cycle
• The lytic cycle culminates in the deathof the host cell by producing new phages and digests the host’s cell wall, releasing the progeny viruses
• A phage that reproduces only by the lytic cycle is called a virulent phage
• Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA
Lytic Cycle of T4
Phage
The Lysogenic Cycle• The lysogenic cycle replicates the
phage genome without destroying the
host
• The viral DNA molecule is incorporated
into the host cell’s chromosome and is
called a prophage.
• Every time the host divides, it copies the
phage DNA and passes the copies to
daughter cells
• Viruses that can be lysogenic or lytic
are called temperate phages.
Lytic Cycle vs. Lysogenic Cycle
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Viral Envelopes• Many viruses that infect animals
have a membranous envelope
• Viral glycoproteins (proteins with
carb attached) on the envelope
bind to specific receptor
molecules on the surface of a host
cell
• Some viral envelopes are formed
from the host cell’s plasma
membrane as the viral capsids exit
• Other viral membranes form from
the host’s nuclear envelope and
are then replaced by an envelope
made from Golgi apparatus
membrane
Animal Viruses• Classified as DNA or
RNA viruses, single or
double-stranded
• Many have envelopes
with glycoproteins
that are specific for
receptors.
• The glycoproteins are
made by the ER and
added to the host
cell’s membrane
which envelopes the
emerging viruses.
RNA Viruses• The broadest variety of RNA genomes
is found in viruses that infect animals
• Retroviruses use reverse transcriptase
to copy their RNA genome into DNA
(HIV is ex.)
• The viral DNA that is integrated into the
host genome is called a provirus
• Unlike a prophage, a provirus remains
a permanent resident of the host cell
Retrovirus RNA virus that uses
reverse transcriptase
(RNA → DNA)
Newly made viral
DNA inserted into
chromosome of host
(provirus)
Host transcribes
provirus to make
new virus parts
Example: HIV
(Human
Immunodeficiency
Virus)
HIV = Retrovirus
Evolution of Viruses• Since viruses can reproduce only
within cells, they probably evolved as bits of cellular nucleic acid
• Candidates for the source of viral genomes are plasmids and transposons (small mobile DNA segments)
• Mimivirus, a double-stranded DNA
virus, is the largest virus yet
discovered…. not any more…. Mega
Virus
Mimivirus and megavirus
Which came first,
the cell or the mimivirus?
Mimivirus was first
isolated in 1992 from
amoeba growing in a
water tower. Megavirus
was isolated from
infecting amoeba with
mimiviruses.
How fast can viruses evolve?
• When viruses face an obstacle to
infecting the cells they normally infect,
how long does it take for them to
evolve to successfully invade them
again? A new study has a frightening
answer: just a little more than two
weeks.
• how fast viruses evolve – lambda virus
Other Human Viruses
Herpes virus
Smallpox
Herpes Simplex Virus 1 (HSV-1)
Herpes Simplex Virus 2 (HSV-2)
Eradicated in 1979 due to
worldwide vaccination campaigns
Current Outbreaks Zika Virus
◦ Spread by Aedes mosquitoes (Aedes
aegypti)
◦ Major outbreak in Brazil and Latin
America
◦ Linked to birth defects (microcephaly)
Dengue Fever
Chikungunya
Zika Virus (as of Dec. 2015)
