VirusesVirus- an infectious

agent made up of a core of nucleic acid and a protein coat

viruses are NOT cells, they are non-living

a virus does NOT have a nucleus, a membrane, or cellular organelles.

viruses do have organized structural parts

•Viruses are tiny particles that are not alive•Viruses are about 50-100 times smaller than the smallest bacterium

Structure and Shape

BacteriophageBacteriophage - Virus that

infects a bacterium

All viruses have at least 2 parts: Core of nucleic acid (DNA/RNA)

Capsid - protective protein coat around the core of nucleic acid. The capsid protects the nucleic acid core from its environment.

Additional parts may include:

Envelope - an additional protective coating usually made of lipids, proteins, and carbohydrates.

**These are only found in viruses that infect animal cells (i.e. flu virus)!**

Additional parts may include:

Spiked projections that bind to the membrane of the cell being infected.

Examples of Viruses

Attachment to a host cell.

Host cell – A living cell that a virus enters and begins to take over (through replication)

A virus cannot live and replicate without a host cell

**living cells host viruses!**

Viral Attachment to Host Cell

*Fits like a jigsaw puzzle or two spaceships docking.

What happens when a virus attaches to a host?

Lytic Cycle

The viral replication process that immediately kills a host cell.

A virus attaches to a host cell.

The virus injects its DNA/RNA into the cell.

Lytic Cycle Cont.

The virus commands the host cell to make copies of the virus.

New viruses are assembled.

The cell bursts (lyses) and the viruses are released and ready to attack other host cells.

Viral replication- The Lytic Cycle

Lysogenic Cycle

A type of replication cycle in which a virus Does Not immediately kill a host cell.

Lysogenic Cycle

A virus attaches to a host cell and injects its DNA into the cell.

The viral DNA inserts itself into a host’s chromosome.

The host cell divides, replicating the viral DNA along with its own DNA.

A stimulus allows the viral DNA to separate from the cell’s chromosome and enter the Lytic cycle (ex. stress).

Lysogenic Cycle

Without a stimulus, the viral DNA stays in the cell’s chromosome.

A viral DNA segment that is inserted in a bacterial cell’s chromosome is called a provirus

Lysogenic Cycle

Special Types of Viruses

Retrovirus - a virus that contains an RNA core that replicates by first transcribing its RNA into DNA (ex. HIV). This is the reverse of what happens normally during replication.

Prion – somewhat like viruses, cause disease. Instead of nucleic acid they have a protein molecule that can cause disease in animals (ex. mad cow disease).

Special Types of Viruses

Viroid – made of a single strand of pure RNA only, no protein coat. These infect plants.

Vaccines

Vaccination - is the process of injecting a person with a harmless weakened or dead form of the virus.

Your immune system builds up antibodies for this virus and the next time you come in contact with it, your immune system is able to defend itself. Ex. chicken pox, small pox, polio, measles

Treating Viruses

Antibiotics – DO NOT GET RID OF VIRUSES! They only work on bacterial infections!

Antiviral drugs – can make disease symptoms milder but only work on certain viruses and do NOT get rid of the virus. They also must be taken IMMEDIATELY after symptoms begin in order to have a chance to work at all. Ex – Tamiflu for H1N1.

Treating Viruses

Viral suppression drugs – Keep a virus from entering the lytic cycle so it doesn’t replicate (i.e. you don’t have an outbreak). The virus is still in your body but is at really low levels, so low that it might not even show up on a test. These medications must be taken daily and you can still get an outbreak even while taking them. You can still pass on the virus whether or not you are having an outbreak. Ex – Valtrex for herpes

Important Information About Viruses

Herpes Simplex Virus – Includes oral herpes (Type 1) and genital herpes (Type 2). These viruses can be passed to ANY mucous membrane and you can pass oral herpes to the genital/anal region, and genital herpes to the oral and eye regions.

Important Information About Viruses

HIV/AIDS – HIV stands for Human Immunodeficiency Virus; AIDS stands for Acquired Immune Deficiency Syndrome. AIDS is NOT a virus. HIV is the virus that causes AIDS. HIV weakens your immune system so that your body cannot fight off diseases it should normally be able to. AIDS is stage 4 of HIV infection and is diagnosed after your immune system has been substantially damaged. People with HIV/AIDS do not die from it; they die from another infection due to their compromised immune system.

Virus Naming

Viruses are not named in the same way as organisms- Why? Some are named for the diseases they

cause Ex- rabies viruses, polioviruses

Adenovirus- affects the adenoid tissue in back of throat (common cold)

Some for the organ they infect

Virus Naming Cont.

Viruses have a genus and species name Genus name- Ends in virus

Some are given code numbers to distinguish between similar viruses.

T1, T2, ect. T stand for type

Questions

1. How does the viral nucleic acid enter the bacterial host cell?

The virus attaches to the host cell and injects the nucleic acid into the cell

2. Describe how new viruses are produced in the host cell.

After injecting the nucleic acid into the cell, the cell’s organelles are “tricked” into using the viral nucleic acid to make and assemble new viruses

Questions

3. Explain why viruses would cease to exist if they replicated via the lytic cycle only.

In the lytic cycle the virus kills the host cell very rapidly and spreads to other nearby cells, killing them shortly thereafter. It is possible for the virus to be killed off my the host organism’s immune system and not spread to another organism.

4. What is a provirus? What cycle would you find a provirus?

A provirus is a viral DNA segment that is inserted in a bacterial cell’s chromosome, this occurs in the lysogenic cycle

Questions

5. How does the lysogenic cycle change to the lytic cycle?

The lysogenic cycle enters the lytic cycle when a stimulus, usually stress related, triggers the provirus to enter the lytic cycle thereby causing the organism to show symptoms.

The End