Chapter 18. 1796 – Edward Jenner developed the smallpox vaccine 1897 – Beijerinck coined the...
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Transcript of Chapter 18. 1796 – Edward Jenner developed the smallpox vaccine 1897 – Beijerinck coined the...
The Genetics ofViruses
Chapter 18
1796 – Edward Jenner developed the smallpox vaccine
1897 – Beijerinck coined the term “virus” meaning poison
1935 – Wendell Stanley crystalized sap from tobacco leaves with TMV and found that viruses were made of nucleic acid and protein
Discovery of Viruses
Study of viruses Deadly viruses are virulent Viruses couldn’t be seen until the electron
microscope was invented
Virology
Viral Characteristics Non-living
◦ Non-cellular◦ Cannot grow or
reproduce on its own◦ No metabolism
Cause disease◦ AIDS, colds, flu,
measles, mono Cause cancer
◦ Cervical, leukemia
Genetic material◦ DNA or RNA
Capsid◦ Protein coat surrounding
nucleic acid Envelope
◦ Some; comes from host cell membrane
Mode of entry◦ Spikes, receptors, etc.
Viral Structure
Virus Types
Virus Specificity Host range:
◦ Broad: West Nile Mosquitoes, birds,
humans, horses
◦ Narrow: Measles Humans
Viruses are small
Among the most complex viruses
Attack bacterial cells Composed of a head,
tail, base plate, & tail fibers
Long DNA molecule is inside the head
Tail helps inject the viral DNA into host cell
Tail fibers used to attach to host
Bacteriophages or T-Phages
Viral replication that rapidly kills the host cell causing it to lyse or burst
Involves 5 steps 1. Adsorption --- phage attaches to cell membrane of
host 2. Injection --- nucleic acid (DNA) of virus injected into
host cell 3. Replication --- viral DNA inactivates host cell's DNA &
uses host's raw materials & ribosomes to make viral DNA, capsids, tails, etc.
4. Assembly --- new viral parts are combined to make new phages
5. Lysis --- enzymes weaken & destroy the cell membrane causing it to lyse releasing new viruses that infect other cells
Lytic Cycle
Lytic Cycle
1. Bacteriophage infects bacteria cell and injects its DNA
2. DNA becomes incorporated into bacterial chromosome
◦ Prophage
3. Bacteria reproduces, making more copies of viral DNA
Lytic cycle can be triggered
Lysogenic Cycle
Temperate Phages: Lytic & Lysogenic
• Two key variables in classifying viruses that infect animals:◦ DNA or RNA?◦ Single-stranded or
double-stranded?
Animal Viruses
Class/Family Envelope Examples/Disease
I. Double-stranded DNA (dsDNA)
Adenovirus No Respiratory diseases, animal tumors
Papovavirus No Papillomavirus (warts, cervical cancer): polyomavirus (animal tumors)
Herpesvirus Yes Herpes simplex I and II (cold sores, genital sores); varicella zoster (shingles, chicken pox); Epstein-Barr virus (mononucleosis, Burkitt’s lymphoma)
Poxvirus Yes Smallpox virus, cowpox virus
Class/Family Envelope Examples/Disease
II. Single-stranded DNA (ssDNA)
Parvovirus No B19 parvovirus (mild rash)
III. Double-stranded RNA (dsRNA)
Reovirus No Rotavirus (diarrhea), Colorado tick fever virus
Class/Family Envelope Examples/Disease
IV. Single-stranded RNA (ssRNA); serves as mRNA
Picornavirus No Rhinovirus (common cold); poliovirus, hepatitis A virus, and other enteric (intestinal) viruses
Coronavirus Yes Severe acute respiratory syndrome (SARS)
Flavivirus Yes Yellow fever virus, West Nile virus, hepatitis C virus
Togavirus Yes Rubella virus, equine encephalitis viruses
Class/Family Envelope Examples/Disease
V. ssRNA; template for mRNA synthesis
Filovirus Yes Ebola virus (hemorrhagic fever)
Orthomyxovirus Yes Influenza virus
Paramyxovirus Yes Measles virus; mumps virus
Rhabdovirus Yes Rabies virus
VI. ssRNA; template for DNA synthesis
Retrovirus Yes HIV (AIDS); RNA tumor viruses (leukemia)
Contain RNA Reverse transcriptase
enzyme uses the RNA to make DNA
Use the host cell's ribosomes & raw materials to make viral proteins
Cause some cancers & AIDS
Retroviruses
HIV Infection
Smallest particle able to replicate
Made of a short, single strand of RNA with no capsid
Cause disease in plants
Viroids
Infectious protein◦ protein particles that have
folded incorrectly No nucleic acid or
capsids Attacks the central
nervous system Cause animal diseases
in cows (Mad Cow disease), sheep, & humans
Prions
Interferon are proteins made by cells to fight viruses
Two types of viral vaccines exist --- inactivated & attenuated ◦ Inactivated virus vaccines don't replicate in the
host's system ◦ Attenuated viral vaccines have been genetically
altered so they can't cause disease Antiviral drugs (AZT, acyclovir, & azidothymidine)
interfere with viral DNA synthesis Protease Inhibitors interfere with viral capsid
production
Viral control