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

“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 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 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