Infectious DiseasesUnit 1 Lesson 4 plan

Do Now

• How big do you think a virus is in comparison to a bacteria and to a red blood cell?

http://www.cellsalive.com/howbig.htm

How big are microbes?

There are a 1000 microns in a millimeterand a 1000 nanometers in a micron.

http://www.cellsalive.com/howbig.htm

The structure of viruses from the inside out

The genome – where the genes are located.The capsid– a protein coat that covers the genome.The envelope - a protein coat that covers the capsid.

Viruses with just a genome and a capsidare called naked.

Viruses with a genome, capsid and an envelope are called enveloped.

The structure of viruses from the inside out

Naked:

A genome surrounded by a capsid.

Enveloped:

A genome surrounded by a capsid.The capsid is surrounded by an envelope.

envelope

The Genome• Is inside the capsid.• Can be DNA or RNA.• Codes for viral proteins.

Genome

Naked Enveloped

Genome

The Capsid

• Protects the genome.• Composed of protein.• Contains enzymes needed for replication.

Capsid

Naked Enveloped

Capsid

The Envelope

• Protects the capsid when present.• Composed of protein and lipid.• Contains proteins needed to attach to host cells.

No envelope

Naked EnvelopedEnvelope

Virus Life Cycle

• The virus attaches to its host cell.• The virus enters its host cell.• The virus hijacks the host cell so it can replicate.• The new viruses exit the host cell.

• How depends on whether the virus is naked or enveloped

Attachment

Both Naked and Enveloped viruses use their own receptors to recognize proteins on the surface of the host cell.

HIV has a receptor called gp120.

Gp120 binds to a protein on host T cells called CD4.

How NAKED viruses enter host cells

• The naked virus might punch a hole in the host cell membrane and enter through the hole (example polio virus).

• The naked virus might be taken up by endocytosis and then punch a hole in the endosome membrane (example adenovirus).

Once the genome is in the cellIt can begin to replicate

How ENVELOPED viruses enter host cells

• Because the enveloped virus alsohas a membrane it might fuse with the host cell membrane (example HIV).

• The enveloped virus might be taken up by endocytosis and then fuse with the endosome membrane (example H1N1).

Once the genome is in the cellIt can begin to replicate

Mature viruses

How NAKED viruses exit host cells

• The host cell fills with mature viruses and bursts. This is known as lysis.

Even bacteria can be infected by viruses! This is a naked virus that infects bacteria called Bacteriophage.

How ENVELOPED viruses exit host cells

• The new envelope proteins are collected in the host cell membrane. • The new capsid proteins and the genome collect below the envelope

proteins.• The new enveloped virus forms and buds off from the host cell.

The capsid pushes out and picks up envelope

The new virus buds off the membrane

Which virus structures allow them to infect their hosts?

• Videohttp://www.npr.org/templates/story/story.php?storyId=114075029

• Complete the worksheet

Wrap up

Thousands of new viruses can pop out of an infected cell.

• What problem would the virus encounter as this happens?

• How could a virus solve that problem?

How do ENVELOPED viruses like H1N1 stop re-entering host

cells they just infected?• Problem: enveloped viruses don’t kill their host cells. How do they stop re-

attaching and re-entering?

The N spike on their envelopeIs an enzyme that can destroy The host cell receptor

Homework

• Complete the Venn Diagram worksheet.