Viruses

What Is a Virus?

Viruses are particles of nucleic acid (DNA/RNA), and a protein coat.

All viruses enter living cells and use the infected cell to reproduce more viruses.

What is the structure of a virus?

Virus Structure

A virus is made of a core of DNA or RNA surrounded by a protein coat, called a capsid.

Capsid proteins bind to receptors on the cell surface and “trick” the cell into allowing it inside.

Most viruses are highly specific to the cells they infect. – Bacteriophages are

viruses that infect bacteria.

How do viruses cause infection?

Once the virus is inside the host cell, two different processes may occur:

1. Some viruses replicate immediately, killing the host cell (lytic infection)

2. Others replicate, but do not kill the host cell immediately (lysogenic infection)

Lytic Infection

In a lytic infection, a virus enters a cell, makes copies of itself, and causes the cell to burst.

Lytic Infection (Step I)

Lytic infection (Step II)

Lytic Infection (Step III)

Examples (Lytic)

• SARS Common cold Influenza Rabies Tobacco mosaic virus

• Animation

Lysogenic Infection

In a lysogenic infection, begins the same way as a lytic infection, but then integrates its DNA into the DNA of the host cell. This circle of DNA is called a prophage.

The viral genetic information replicates along with the host cell's DNA.

As the cell goes through mitosis the viral DNA remains dormant.

If put under stress the infected cell may switch from they lysogenic cycle to the lytic cycle and the virus is released again.

Lysogenic Infection (Step I)

Lysogenic Infection (Step II)

Lysogenic Infection (Step III)

Lysogenic Infection (Step IV)

Examples (Lysogenic)

• herpes • Diphtheria• Cholera• Dysentery• Scarlet Fever

Retroviruses

Retroviruses contain RNA as their genetic information.

When retroviruses infect cells, they make a DNA copy of their RNA.

This DNA is inserted into the DNA of the host cell.

The virus that causes AIDS is a retrovirus.

Tracking the Spread of a Nasty Virus

Science Friday – 10 min sound clip on the spread of Norovirus.