Lesson 7-3

What are Viruses?

• A virus is a strand of DNA surrounded by a layer of protein that can infect and replicate in a host cell.

Characteristics of Viruses

• A virus:

is smaller than the smallest bacteria!

http://learn.genetics.utah.edu/content/begin/cells/scale/

Viruses contain proteins, nucleic acids & can

reproduce

But…..

Characteristics of Viruses (cont.)

• Viruses– Don’t eat, grow or breathe

– Do not have any organelles, or a cell membrane.

– They are NOT cells!

• Based on this info, do you think viruses are living organisms????

• No!

• A virus is not considered alive because it does not have all the characteristics of a living organism.

Characteristics of Viruses (cont.)

• Viruses can have different shapes, such as crystal, cylinder, sphere, and bacteriophage shapes.

Characteristics of Viruses (cont.)

• Bacteriophage viruses only infect bacteria

Protein coat

Characteristics of Viruses (cont.)

• Viruses must use organisms to carry on the processes that we usually associate with a living cell.

• Viruses must be inside a cell, called a host cell, to replicate.

Characteristics of Viruses (cont.)

• Even though viruses are not truly a living organism, they can reproduce.

• How?

• The Lytic Cycle

– A 4-step process

• http://www.youtube.com/watch?v=wVkCyU5aeeU

Lytic Cycle

• When a virus enters a cell, it can either be active or latent.

• Once it becomes active, a virus takes control of the host cell and replicates.

Characteristics of Viruses (cont.)

Lytic Cycle

Step 1- attachment Step 2 - injection

Step 3- Replication Step 4 - Escape

1. Virus attaches to a host cell

2. Virus injects DNA into the host cell

3. Host cell is then directed to make more viral DNA, and new protein coats (replication)

4. The host cell is often destroyed and the viruses escape and move on to infect other cells

Lytic Cycle

Characteristics of Viruses (cont.)

• As viruses replicate, their DNA frequently mutates, enabling them to adjust to changes in their host cell.

• Viruses cause many human diseases, such as chicken pox, influenza, HIV, and the common cold. Viruses can also affect other animals and plants.

• Since viruses are constantly changing, viral diseases can be difficult to treat.

• Antibiotics work only against bacteria, not viruses.

• One of the best ways to prevent a viral infection is to limit contact with an infected human or animal.

Treating and Preventing Viral Diseases

• When a virus infects a person, their body begins to make antibodies.

• An antibody is a protein that prevents an infection in your body.

• Antibodies bind to viruses and other pathogens and prevent them from attaching to a host cell.

Treating and Preventing Viral Diseases(cont.)

• Natural immunity develops when a mother passes antibodies on to her unborn baby.

Treating and Preventing Viral Diseases(cont.)

• A vaccine is a mixture containing material from one or more deactivated pathogens, such as viruses.

• When an organism is given a vaccine for a viral disease, the vaccine triggers the production of antibodies.

• After being vaccinated against a particular pathogen, the organism will not get as sick if exposed to the same pathogen again.

Treating and Preventing Viral Diseases(cont.)

• Scientists are researching new ways to treat and prevent viral diseases in humans, animals, and plants.

• Viruses have been used by some scientists to treat genetic disorders and cancer using gene transfer.

Research with Viruses