Viruses
Images taken without permission from
http://www.zo.utexas.edu/faculty/sjasper/images/ebola_virus.jpg and
http://www.bioitworld.hu/adat/200316/21662/sars_virus.jpg
AP Biology
Unit 2
“Famous” Viruses
• Influenza (the Flu virus)
– Actually many different strains (including
H1N1)
• Cold Virus (Rhinovirus)
– Actually many different strains
• HIV
• SARS (Coronavirus)
• Chicken Pox (Varicella Zoster)
Viruses are nonliving
• Viruses are considered
nonliving because
– They do not have cellular
structure
– They require another living cell
to replicate (copy) themselves –
infectious particles
• Viruses are very small in size
compared to even a bacterium
Basic Components of a Virus
• 3 basic components:
Nucleic Acid
– Nucleic Acid (DNA or
RNA)
– Capsid (Protein Coat
surrounding nucleic acid)
– Viral Envelope (derived
from host cell membranes) –
NOT PRESENT IN ALL
Viral Envelope
VIRUSES
Capsid
Question…
• Why might having a viral envelope help a
virus? (hint: what does it resemble/ what
was it derived from?)
– Helps it bind to the host cell
– May also help protect it (camouflage it)
Host Range
• Viral infection is limited to specific hosts
and specific cells within that host
– For example:
• Rabies – only infects warm-blooded animals
• Measles only infects humans
• Flu viruses infect the tissues in the upper
respiratory tract
• HIV only infects specific immune system
cells
Viral Reproductive Cycle
• Attachment: Virus binds to
host cell
• Entry: Virus enters host cell
• Biosynthesis: Viral genome and
proteins are synthesizes
• Assembly: New viruses selfassemble
• Release: New viruses exit cell
(may kill cell in the process)
A Closer
Look at
Biosynthesis
Image obtained without permission from http://bcs.whfreeman.com/thelifewire/default.asp?s=&n=&i=&v=&o=&ns=0&uid=0&rau=0
Bacteriophages
• Viruses that infect bacteria
• Inject their DNA into host cell
• Also called phages
Nucleic Acid
Capsid
Lytic Cycle
• Results in the death of the infected bacteria
• Phage that only carry out the lytic cycle are
known as virulent phage
Steps of the Lytic Cycle
Phage DNA
is injected in
Question…
• Do you remember how bacteria protect
themselves from bacteriophages (foreign
DNA)?
– They have restriction enzymes to cut them up!
Temperate Phage
• Temperate Phage are bacteriophage that can
go through the lytic cycle OR the lysogenic
cycle
• In the lysogenic cycle, the phage does not
cause the bacterial host to burst & die
Lysogenic Cycle
The lysogenic
cycle only
consists of this
half of the
diagram
The phage is
“dormant” – gets
copied with the
rest of the DNA
Temperate Phage: Lytic or Lysogenic?
Animal (Human) Viral Infection
• Many animal (human) viruses have a viral
envelope
• There are glycoproteins attached to the viral
envelope that help the virus bind to the host
cell
Step 1: Glycoproteins on
viral envelope bind to
Step 2:
receptors on host cell
Through
endocytosis
Step 3: Biosynthesis
of viral components
Step 4: Assembly of
Viruses
Step 5: New viruses
bud out of host cell,
gaining a viral envelope
Important Concept
• Even though the drawing only shows one
new virus coming out, in actuality many
(tens of thousands) viruses will come out!
RNA Viruses
• Some viruses have RNA as
their nucleic acid
• Also known as retroviruses
• Also have to have reverse
transcriptase included in
virus
– Creates a DNA template
from the RNA
• Ex. HIV
What about H1N1?
• H1N1 is a new strain of the influenza virus
– First detected in April 2009
• Why was it called “swine flu”?
– originally found that some of the genes in virus
were similar to genes in the flu virus in pigs
(swine)  later found to be very different
– Contains genes from flu viruses found in pigs
in Asia, Europe, birds and humans
• “quadruple assortment”
What about H1N1?
• Why are we so worried about this flu?
– Because it’s a new strain that hasn’t been seen
before
– Most people’s immune systems don’t have
immunity against this virus
– This will cause more infections than the
seasonal flu.
– Spreading quickly among young people.
Animations of Lytic & Lysogenic
Cycles
• Animation