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
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)
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
