Viruses AP Biology Unit 2 Images taken without permission from and .
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Transcript of Viruses AP Biology Unit 2 Images taken without permission from and .
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VirusesViruses
AP BiologyUnit 2
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
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“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)
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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
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Basic Components of a Virus
• 3 basic components:– Nucleic Acid (DNA or
RNA)– Capsid (Protein Coat
surrounding nucleic acid)– Viral Envelope (derived
from host cell membranes) – NOT PRESENT IN ALL VIRUSES
Nucleic Acid Capsid
Viral Envelope
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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)
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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
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Viral Reproductive Cycle• Attachment: Virus binds to
host cell• Entry: Virus enters host cell• Biosynthesis: Viral genome and
proteins are synthesizes• Assembly: New viruses self-
assemble• Release: New viruses exit cell
(may kill cell in the process)
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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
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Bacteriophages
• Viruses that infect bacteria
• Inject their DNA into host cell
• Also called phages
Nucleic Acid
Capsid
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Lytic Cycle
• Results in the death of the infected bacteria
• Phage that only carry out the lytic cycle are known as virulent phage
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Steps of the Lytic Cycle
Phage DNA is injected in
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Question…
• Do you remember how bacteria protect themselves from bacteriophages (foreign DNA)?– They have restriction enzymes to cut them up!
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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
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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
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Temperate Phage: Lytic or Lysogenic?
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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
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Step 1: Glycoproteins on viral envelope bind to receptors on host cell
Through endocytosis
Step 2:
Step 3: Biosynthesis of viral components
Step 4: Assembly of Viruses
Step 5: New viruses bud out of host cell, gaining a viral envelope
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Important Concept
• Even though the drawing only shows one new virus coming out, in actuality many (tens of thousands) viruses will come out!
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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
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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”
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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.
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Animations of Lytic & Lysogenic Cycles
• Animation