This is the image of a bacterium being attacked by viral

particles. You can see the contrast in size

easily.

In ancient Rome, ‘virus’ meant “poison” or “venomous”.

This name was passed down when these extremely small

particles were discovered in the late 1800’s.

Find a lower-case ‘i’ on your notes page….

• Viruses are extremely small non-cellular, infectious agents composed of genetic material (DNA or RNA)

enclosed in a protein coat.

• You will see Bacteriophages (bottom left) in many examples....

Bacteriophage Viruses Flu Viruses

• A virus has an inner core of nucleic acid, either RNA or DNA

and an outer protein coat called a capsid.

Capsid

Nucleic acid

Viral Structure

• Some larger viruses, (human flu viruses), have

an additional layer surrounding their capsids,

called an envelope (usually made of membrane

remnants of previously infected cells).

Capsid

Nucleic acid

Envelope

Vertebrate immune systems can detect certain viral proteins,

HOWEVERthe genes for viral proteins tend to mutate at very high frequencies, so

viruses can often elude immune fighters

For instance Influenza mutates often, that is why a new vaccine is created each year.

* viruses do not metabolize, grow, nor are they composed of cells. Also, they don’t possess a way to reproduce without a host.

However, THAT is true of many parasites.

All Living Things, as a species…:

• Are made of cells, and possess a system of organization

• Require, can metabolize, and use energy.

• Grow and develop.

• Reproduce.

• Can adapt to changing environments.

• Viruses are PATHOGENS- infectious, disease-causing agents that invade target organisms and

multiply inside them or on them.• Disease follows when the pathogens take over metabolic activities of other cells- damaging body

tissues and interfering with their normal functioning.

Nope, not if it can’t get in!

So…how does it get into the host cell?It works like a lock and key. If the key doesn’t match a lock, you can’t

open the door

The “lock” of the cell is called a receptor sitereceptor site.If the shape of the virus does not match the host’s receptor

site, the virus can not inject it’s nucleic acid or enter the cell.

Virus

Can a Virus Infect Any Cell?

INJECTION ENDOCYTOSIS

Steps of the Lytic Cycle:Steps of the Lytic Cycle:1. Virus attaches to host2. Virus injects nucleic acid3. Viral nucleic acid takes over and re-programs host’s DNA 4. New viruses form5. Lysis – cell bursts and releases new viruses

Lytic CycleLytic Cycle: Faster cycle that quickly destroys the host.

• A lysongenic cycle begins in the same way as a lytic cycle.

Lysogenic CycleLysogenic Cycle: Slow cycle in which the viral DNA hides in the host cell

Virus attaches to a host cell Viral nucleic acid enters the host

Viral nucleic acid integrates itself into the host’s DNA

Lysogenic CycleLysogenic Cycle: Slow cycle in which the viral DNA hides in the host cell

This part can go on for years….

The host cell will divide, and each time it replicates it’s DNA, it also makes a copy of

the viral nucleic acid.

At any time, this provirus can enter the Lytic cycle….

RNA Viruses