2.2 Viruses, Viroids and Prions
Viruses
What is a virus?
Viruses Put to Work
- small, non-living, infectious particles
- contains genetic material in the form of DNA or RNA, which is sourrounded by a capsid
- viruses do not create waste, do not use or produce energy, and cannot grow or reproduce on their own
Examples
(Swine Flue Virus Particles)
- viruses are responsible for many diseases
- common diseases, such as the common cold and chicken pox, produce mild symptoms
- severe diseases, such as AIDS and rabies, produce more serious symptoms and even death
- some viruses even play a role in cancer
Significance of Viruses
- Viruses can be beneficial in an ecosystem.
- genes are introduced to a cell to replace a defective gene.
- Viruses can be used as tools to insert drugs or DNA into cells.
- Viruses can enter specific cells to insert DNA into the chromosomes of the cells that they infect, this is done by placing drugs inside virus capsules.
- Virus Capsule: Where the DNA and RNA makeup is stored
- Gene Therapy can be used to treat diseases
- Gene therapy is slowly making successes in fighting parkinsons disease, cancer and hereditary blindness
- viruses are significant because of their ability to spread
- epidemic- a large rapidly spreading outbreak of a disease in a certain area
- pandemic- when an epidemic spreads all over the world
- viruses control the population of organisms
Infectious Cycles
Infectious cycle- process by which a virus infects, replicates and destroys the host cell
- viruses do not carry out life functions like living cells
- become active when their genetic material enters and takes control of living cell
Phage Infection - any of a group of viruses that infect bacteria
Vaccinations and Human Health
- vaccines-mixtures containing weakened forms or parts of virus
- when injected, trigger response from immune system without infection
- exposure creates form of chemical "memory" which allows immune system to react quickly if the person ever comes in contact with real virus
HIV Infected Cells
Viroids and Prions
- lysis- rupturing of cell; can occur when newly made viruses released from host cell
- when assembly complete, lysis occurs as host cell ruptures
- this releases approx. 100-200 new viruses into surroundings
- host cell is destroyed
- whole cycle can take less than hour
- in this state viral DNA stays in dormant state called lysogeny for years
- bacterium grows and divides, making copies of the virus DNA in chromosome
- when cell environment changes, viral DNA becomes active, sperates from bacteria, and enters lytic cycle
Transduction
Transmission
- air
- direct physical contact
- biting insects or animals
- exchange of body fluids
- enter through body injuries
- rare occasion when viral DNA separates from bacteria, taking a small piece of bacteria DNA with it, incorporating it into viral DNA
- the virus then carries this DNA and inserts it into other bacteria when infecting it
Viroids
- Viroids are plant pathogens
- They are circular RNA (ribonucleic) particles that are much simpler than a virus
- Like viruses, they can reproduce only within a host cell
- Unlike viruses, they do not manufacture any proteins
- They only make a single RNA molecule
- Viroid-infected plants are responsible for crop failures that lose millions of dollars in agricultural revenue each year
- plants they infect include potatoes, cucumbers, tomatoes and avocados
- There have not been any human diseases known to be caused by viroids
Prions
- infectious particles smaller than viruses that contain no nucleic acids (no DNA or RNA)
- Prions: proteinacious infectious particles
- cause a number or rare diseases in animals
- Abnormally shaped
- Can be found in the brain and nervous tissues.
- Interaction of normal and infectious prions can affect normal ones
- If an infectious tissue were to be eaten by another mammal the prion would enter the normal mammals blood stream and go to it's brain affecting that mammal
Classifying Viruses
Characteristics
- Vary in structure, but all consist of an RNA or DNA molecule surrounded by a capsid
- Some viruses surrounded by an envelope, which is created when a virus leaves the host cell and the host cell membrane wraps around virus
- Capsid vs. Envelope: capsid is a protein coat of the genetic material (DNA or RNA) of the virus and a envelope is a protective covering of the protein capsid
Origin
- There have been several hypothesis...
- 1) Originated as small infectious cells that over time lost their cytoplasm and their ability to reproduce outside a living cell
- 2) Originated as escaped fragments of DNA(deoxyribonucleic acid) or RNA(ribonucleic acid) that were once a formed part of living cells
- 3) Virus-like particles existed before the first cell.
Classfication and Phylogeny
- Viruses are non-living, but reproduce
- Orders, Families, Genera and Species
- Classified by: size, shape, and type of genetic material
- 4000 virus species have been classified
- Viruses: can infect only single host species or few closely related hosts
- Species of Virus: infect only one organ system, or a single tissue or cell type in their host (ie. HIV)
- Bacteriophages: Do not enter a host, but inject their DNA into bacterium, and protein capsule remains outside the cell.