Viruses AP Biology Unit 2 Images taken without permission from and

“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 (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

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 self- assemble Release: New viruses exit cell (may kill cell in the process)

A Closer Look at Biosynthesis Image obtained without permission from

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 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

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