Viruses are composed of nucleic acids enclosed in a protein coat and are smaller than the smallest bacteria.

Capsid

Nucleic Acid

Most biologists consider viruses to be nonliving because:

They don’t

1. carry out respiration

2. grow

3. develop

What is a Virus?

* All viruses can do is replicate—make copies of

themselves—and they can’t even do that without

the help of Host cells.

A cell in which a virus replicates inside of is called the host cell.

What is a Host Cell?

Viruses, such as rabies viruses and polioviruses, were named after the diseases they cause.

• Other viruses were named for the organ or tissue they infect.

What are viruses named after?

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

(JUST LIKE A CELL HAS DNA…BOTH HAVE GENETIC MATERIAL)

and an outer protein coat called a capsid.

Capsid

Nucleic acid

Viral Structure

• Some relatively large viruses, such as human flu viruses, may have an additional layer, called an envelope, surrounding their capsids.

Capsid

Envelope

Viral Structure

Nucleic acid

Before a virus can replicate, it must enter a host cell.

A virus attaches to a host cell when one of its proteins interlocks with the receptor site on the host cell’s plasma membrane.

Virus

Polyhedral virus shape

Human Papilloma Virus

Envelope studded with projections

(flu and AIDS)

VIRUS SHAPES

Long narrow helical shape (Tobacco Mosaic

Virus)

Polyhedral-shaped head attached to a cylindrical tail

with leg-like fibers

VIRUS SHAPES

• Each virus has a specifically shaped attachment protein. Therefore, each virus can usually attach to only a few kinds of cells.

• In general, viruses are species specific, and some also are cell-type specific. For example, polio viruses normally infect only intestinal and nerve cells.

Viral Attachment to Host

• The species specific characteristic of viruses is significant for controlling the spread of viral diseases.

For example, smallpox was easier to eradicate because it only affects humans (unlike the flu and West Nile that affect several types of animals.)

• Once attached to the plasma membrane of the host cell, the virus enters the cell and takes

over its metabolism.

Viral Replication

Cells VS Viruses• Virus= non-living

• Viruses= need a host cell

• Both have DNA or RNA

• Cells= Use Energy

• Viruses= Cannot Use

Energy