Viruses

1. Viruses Pgs. 33 - 35

2. What is it? • A virus is a microscopic particle that invades a cell and often destroys it. • Many diseases are caused by viruses, including the common cold, the flu and acquired immune deficiency syndrome (AIDS) • They are so tiny 5 billion can fit in a drop of blood. • There are billions of different types of viruses.

3. Are they Living? • Viruses contain protein and nucleic acids like living things. • However, they do not eat, grow, breathe, or perform other biologic functions. • They cannot live on their own, they require a host to exist. • They invade cells and make the cell produce viruses instead of new cells

4. Classifying Viruses • Viruses can be grouped by the type of disease, life cycle, or genetic material they contain. • The shape can be used to classify them as well. • Examples are: • Crystals - polio • Spheres - Influenza • Cylinders – tobacco mosaic • Spacecraft – attack bacteria

5. Destructive Guest • The one function that viruses share with living thing is that they reproduce. • They infect living cells and turn them into virus factories in a cycle called the lytic cycle.

7. Lytic Cycle • Step 1 – the virus finds a host cell. • Step 2 – The virus enters the cell or in some cases, the virus’s genes are injected into the cell. • Step 3 – once the virus’s genes are inside, they take over the direction of the host cell, turning it into a virus factory. • Step 4 – The new viruses break out of the host cell ready to find a new host.

8. Clever Little Virus • Some viruses do not go into the lytic cycle. • They invade a host cell, but do not reproduce right away. • The host cell will reproduce with copies of the virus’s genes. • This is the lysogenic cycle. • The viruses can be inactive for long periods before they launch into the lyctic cycle.