A BRIEF INTRODUCTION TO VIRUSES

Viruses

Properties of viruses

• no membranes, cytoplasm, ribosomes, or other cellular components
• they cannot move or grow
• they can only reproduce inside a host cell
• 2 major parts - a protein coat, and DNA or RNA
• they are extremely tiny, smaller than cells and only visible with electron microscopes

Review the structure of DNA

RNA is similar to DNA

Instead of thymine, it has uracil
It has the sugar ribose, instead of deoxyribose
It is single stranded

Shape of a double helix, repeating units of nucleotides

Base pairs held together by hydrogen bonds (weak)

Adenine -----|-----Thymine
Guanine -----|---- Cytosine

The sides of the DNA made of alternating deoxyribose (5 ring sugar) and phosphates

Virus Structure

Virus has a covering that has a capsid and sometimes an envelope
Inner core contains a nucleic acid molecule (DNA or RNA) and various proteins

Viruses are usually very specific to their host and to the cells they can infect.

Viral Reproduction

See Video on How a Virus Invades Your Body (NPR)

Lytic cycle = reproduction occurs, cells burst
Lysogenic cycle = reproduction does not immediately occur (dormancy)

1. Attachment - specific proteins on cell surface attach to the virus (See Animation: Entry of Viruses)
2. Penetration - the virus is engulfed by the cell (Cell can enter Lysogenic or Lytic Cycle)
3. Biosynthesis - viral components are made (protein coat, capsid, DNA/RNA)
4. Maturation - assembly of viral components
5. Release - viruses leave host cell to infect new cells (often destroys host)
(See: Animation at McGraw-Lamba Phage Replication Cycle)

Types of Viruses

Bacteriophage - viruses that infect bacteria.

Retroviruses -- RNA viruses that have a DNA stage

How Are Viruses Named

Historically they have been named for a variety of factors, including

• the associated diseases (poliovirus, rabies) the type of disease caused (murine leukemia virus)
• the sites in the body affected or from which the virus was first isolated (rhinovirus, adenovirus)
• where they were first isolated (Ebola virus, Hantavirus)
• the animal that carries the virus (bird flu, swine flu)
• for the way people imagined they were contracted (dengue = ‘evil spirit’; influenza = ‘influence’ of bad air).

Newer Conventions

Example of an Influenza Virus Naming

Focus on Human Immunodeficiency Virus

- Causes the disease AIDS (Acquired Immune Deficiency Syndrome)

HIV Infection Cycle (animation) | HIV Life Cycle - drugs target specific viral processes
HIV Coloring Assignment *Make sure you understand the steps involved in infection and how drugs treat the disease.

Related to Viruses 

Viroids - even smaller than viruses, consist of RNA strands that lack a protein coat
Prions - "rogue protein", believed to be the cause of Mad Cow Disease, also may cause Kuru in cannibal tribes

Treatment of Viruses

Vaccines
Antiviral Drugs

WRITER;>
KABIR HUSSAIN BAIG