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