19-2 VirusesBy: Katelyn, Rose and Ashley

What is a Virus?

Viruses- particles of nucleic acid, protein and sometimes lipids

Most viruses are so small, they can only be seen through a powerful electron microscope.

A typical virus is composed of a core of DNA or RNA surrounded by a protein coat.

The more complex the virus is the more genes it contains

They enter a living cell and once inside, use machinery of infected cell to produce more viruses

Living characteristics of viruses:

They reproduce at a good rate, but only in living host cells.

They can mutate.

Nonliving characteristics of viruses

Nonliving characteristics of viruses They are acellular, that is, they

contain no cytoplasm or cellular organelles.

viruses don't grow and divide. Instead, new viral components are synthesized and assembled within the infected host cell.

They possess DNA or RNA but never both.

Virus Structures

What is a Capsid?

Capsid- a protein coat The capsid enable’s host cells to enter the

virus The capsid proteins “trick” the virus by

binding the surface of the cell allowing the capsid to come inside the cell

The cell transcribes and translates the viral genetic information into viral capsid proteins.

Sometimes it causes the host cell to make copies of the virus, and it causes the host cell to be destroyed

Bacteriophages- viruses that infect bacteria

Once the virus is inside the host cell two different process may occur

In a lytic infection, a virus enters a cell and replicates itself and causes the cell to burst.

In the lysogenic infection the virus replicates itself in a way that doesn’t kill the host cell immediately.

Lytic and Lysogenic Infections

Prophage and Viral Infections Prophage - viral DNA that is embedded

in the host cell’s DNA Prophage may remain part of the DNA

for many generations before being active

A virus may not always stay in the prophage

Eventually one factor will activate the DNA of a prophage causing it to remove itself form the host cell DNA and direct the synthesis of a new virus particles

What are Retroviruses?

Retroviruses- viruses that contain RNA as their genetic information

When retroviruses infect a cell they produce a DNA copy of their RNA

The DNA is then inserted into the DNA of the host cell

Viruses and Living Cells

In order for a virus to grow it must infect a living cell

Viruses are parasites After infecting living cells viruses can

reproduce Viruses are borderline of living and

none living things Viruses are smaller and simpler than

cells

Review :

1. What are the two ways the two ways viruses cause infections?

Answer:

1.Lysogenic and lytic

Review:

2. What are typical viruses composed of?

Answer:

Core of DNA or RNA surrounded by protein coat

Review:

What is the viruses protein coat called?

Answer:

capsid

Review:

How do viruses “reproduce”?

Answer:

They enter a living cell and once inside, use machinery of infected cell to produce more viruses