A. General characteristics and components of viruses
B. Classification
C. Replication
D. Effects
E. Diseasses
F. Virus-like agents
G. Culturing
A. General characteristics of viruses
Acellular Too small to be seen with a light microscope Can't be grown on media Infectious agents of bacteria, fungi, plants and animals Obligate intracellular parasites To multiply, must be inside a host cell (animal, plant or bacteria) Use host's metabolic machinery to make new virions (viral particles) A cell infected with one virion may produce thousands of copies Often damages and kills cell
Acellular
Too small to be seen with a light microscope
Can't be grown on media
Infectious agents of bacteria, fungi, plants and animals
Obligate intracellular parasites
To multiply, must be inside a host cell (animal, plant or bacteria) Use host's metabolic machinery to make new virions (viral particles) A cell infected with one virion may produce thousands of copies Often damages and kills cell
To multiply, must be inside a host cell (animal, plant or bacteria)
Use host's metabolic machinery to make new virions (viral particles)
A cell infected with one virion may produce thousands of copies
Often damages and kills cell
Main categories
1. Type of nucleic acid DNA or RNA, not both 2. Enveloped or naked Enveloped viruses have a membrane that surrounds a capsid Naked viruses lack a membrane
1. Type of nucleic acid
DNA or RNA, not both
2. Enveloped or naked
Enveloped viruses have a membrane that surrounds a capsid Naked viruses lack a membrane
Enveloped viruses have a membrane that surrounds a capsid
Naked viruses lack a membrane
Components of viruses
1. Core: nucleic acid
2. Capsid: protein coat surrounding core
Protein fibers may extend from the capsid
(3.) Envelope (some viruses)
Lipids, proteins and carbohydrates Derived from membranes of host cell (e.g. endoplasmic reticulum, cell ot nuclear membranes) Spikes: proteins that help the virus attach to cell surface during infection
Lipids, proteins and carbohydrates
Derived from membranes of host cell (e.g. endoplasmic reticulum, cell ot nuclear membranes)
Spikes: proteins that help the virus attach to cell surface during infection
Sizes and shapes of viruses
Size: usually smaller than bacterial cell (~0.03 to 0.3 mm) Shape I. Symmetrical viruses i) Helical Ex. Tobacco mosaic virus ii) Polyhedral (many sided) Ex. Human adenovirus iii) Spherical (Enveloped) Ex. Herpesvirus II. Complex viruses Ex. Bacteriophages Have additional structures Tail and tail fibers attached to head (capsid) For attachment to host cell and to injection of phage DNA
Size: usually smaller than bacterial cell (~0.03 to 0.3 mm)
Shape
I. Symmetrical viruses i) Helical Ex. Tobacco mosaic virus ii) Polyhedral (many sided) Ex. Human adenovirus iii) Spherical (Enveloped) Ex. Herpesvirus II. Complex viruses Ex. Bacteriophages Have additional structures Tail and tail fibers attached to head (capsid) For attachment to host cell and to injection of phage DNA
I. Symmetrical viruses
i) Helical Ex. Tobacco mosaic virus ii) Polyhedral (many sided) Ex. Human adenovirus iii) Spherical (Enveloped) Ex. Herpesvirus
i) Helical Ex. Tobacco mosaic virus
ii) Polyhedral (many sided) Ex. Human adenovirus
iii) Spherical (Enveloped) Ex. Herpesvirus
II. Complex viruses
Ex. Bacteriophages
Have additional structures Tail and tail fibers attached to head (capsid) For attachment to host cell and to injection of phage DNA
Have additional structures
Tail and tail fibers attached to head (capsid)
For attachment to host cell and to injection of phage DNA
B. Classification of viruses
Important characteristics
Host cell (bacterial, fungal, plant or animal) Type of nucleic acid (DNA or RNA) Number of nucleic acid strands (single or double stranded) Capsid symmetry Enveloped or naked
Host cell (bacterial, fungal, plant or animal)
Type of nucleic acid (DNA or RNA)
Number of nucleic acid strands (single or double stranded)
Capsid symmetry
Enveloped or naked
Viruses have common names
Ex. herpes virus, poliovirus, ebola virus, influenza virus
Also, they are formally classified into Genera and Families
Ex. Common name: ebola virus Family: Filoviridae Genus: Filovirus
Ex. Common name: ebola virus
Family: Filoviridae Genus: Filovirus
Family: Filoviridae
Genus: Filovirus
C. Viral replication
Produces more virions so that the virus can infect new cells and propagate itself Replication cycle has 5 main steps 1. Adsorption (binding) to host cell 2. Penetration of cell 3. Biosynthesis of viral components DNA or RNA, proteins 4. Maturation --assembly of viral components into complete virions 5. Release from cell Budding through cell membrane, or Lysis of cell
Produces more virions so that the virus can infect new cells and propagate itself
Replication cycle has 5 main steps
1. Adsorption (binding) to host cell 2. Penetration of cell 3. Biosynthesis of viral components DNA or RNA, proteins 4. Maturation --assembly of viral components into complete virions 5. Release from cell Budding through cell membrane, or Lysis of cell
1. Adsorption (binding) to host cell
2. Penetration of cell
3. Biosynthesis of viral components
DNA or RNA, proteins
4. Maturation --assembly of viral components into complete virions
5. Release from cell
Budding through cell membrane, or Lysis of cell
Budding through cell membrane, or
Lysis of cell
Ex. humans, other primates, species of plants, insects
Ex. liver, neural, epithelial, haematophoetic (blood-forming), skeletal muscle, etc.
Ex. poliovirus: Only humans hepatitis B virus: liver cells
Ex. poliovirus: Only humans
hepatitis B virus: liver cells
Ex. rabies virus: humans, bats, foxes, raccoons
How do viruses recognize the cells that they are able to infect?
Different types of cells have proteins (receptors) in their membranes that are unique to the cell and provide specificity for adsorption of a virus that is able to infect it.
1. Enveloped Animal Virus
Viral envelope proteins (spikes) and cell membrane receptor proteins (receptors) interact specifically to bind the virus to the cell Cells lacking the specific receptor in their membrane can't bind the virus to the membrane Ex. Human immunodeficiency virus (HIV) has spikes that bind the virus to the CD4 membrane receptor of human helper T cells (a type of immune system cell)
Viral envelope proteins (spikes) and cell membrane receptor proteins (receptors) interact specifically to bind the virus to the cell
Cells lacking the specific receptor in their membrane can't bind the virus to the membrane
Ex. Human immunodeficiency virus (HIV) has spikes that bind the virus to the CD4 membrane receptor of human helper T cells (a type of immune system cell)
2. Naked viruses may use their capsid proteins to bind to a specific cell membrane receptor
D. Effects of viruses on cells
1. Disruption of normal cellular metabolism
Diverted to replication of viral nucleic acid and synthesis of viral proteins
2. Cytopathic effects on cells (visible effects)
Changes in shape Detachment from adjacent cells Swelling and lysis Fusion into giant multinucleted cells (syncytia)
Changes in shape
Detachment from adjacent cells
Swelling and lysis
Fusion into giant multinucleted cells (syncytia)
3. Teratogenesis
Teratogen -A drug or agent that induces defects in a developing embryo Some viruses pass across placenta into the womb and infect the embryo Ex. Cytomegaolvirus Death, mental retardation, damaged organs Herpes simplex viruses Damaged eyes Rubella virus Deafness, mental retardation, damaged organs
Teratogen -A drug or agent that induces defects in a developing embryo
Some viruses pass across placenta into the womb and infect the embryo
Ex.
Cytomegaolvirus
Death, mental retardation, damaged organs
Herpes simplex viruses
Damaged eyes
Rubella virus
Deafness, mental retardation, damaged organs
4. Cancer
~15% of human cancer may be from viral infections Viruses may transform normal cells into cancer cells Ex. Hepatitis B virus can cause liver cancer Some types of human papilloma virus cause cervical cancer
~15% of human cancer may be from viral infections
Viruses may transform normal cells into cancer cells
Ex. Hepatitis B virus can cause liver cancer
Some types of human papilloma virus cause cervical cancer
E. Some important viral diseases
I. RNA viruses
II. DNA viruses
Erythema infectiosum a.k.a. Fifth disease (infant rash)
Parvo (puppies)
F. Virus-like agents (i.e. small, acellular, infectious)
1. Viroids
Much smaller than virus particles, composed only of RNA No capsid or envelope Cause some plant diseases, 1 human disease Tomato apical stunt disease Hepatitis D
Much smaller than virus particles, composed only of RNA
No capsid or envelope
Cause some plant diseases, 1 human disease
Tomato apical stunt disease Hepatitis D
Tomato apical stunt disease
Hepatitis D
2. Prions
Diseases Creutzfeldt-Jakob Disease --humans Scrapie --sheep Bovine spongioform encepahlopathy --mad cow disease Chronic wasting disease --deer and elk
Diseases
Creutzfeldt-Jakob Disease --humans
Scrapie --sheep
Bovine spongioform encepahlopathy --mad cow disease
Chronic wasting disease --deer and elk
G. Emerging viral diseases
Increased incidence of previously unknown or rare viral diseases
Ex. Human immunodeficiency virus (HIV) AIDS Ebolavirus Hemorrahagic fever Hantavirus Hemorrahagic fever
Human immunodeficiency virus (HIV) AIDS
Ebolavirus Hemorrahagic fever
Hantavirus Hemorrahagic fever
Factors that contribute to emergence of a new viral disease
1) Increased human population and migration to cities Virus is more easily spread 2) International travel Viruses that may be rare or absent in one region are spread by increased contact with people, plants or animals where the virus is common 3) Clearing of tropical forests Increases contact between humans and animal that are reservoirs for viruses Insects, monkeys, bats, rodents 4) Mutations that change the nucleotide sequence of a viral gene, allowing virus to cross species barriers Ex. monkeys -----> humans
1) Increased human population and migration to cities
Virus is more easily spread
2) International travel
Viruses that may be rare or absent in one region are spread by increased contact with people, plants or animals where the virus is common
3) Clearing of tropical forests
Increases contact between humans and animal that are reservoirs for viruses Insects, monkeys, bats, rodents
Increases contact between humans and animal that are reservoirs for viruses
Insects, monkeys, bats, rodents
4) Mutations that change the nucleotide sequence of a viral gene, allowing virus to cross species barriers
Ex. monkeys -----> humans
Ex. West Nile Virus
Illinois 2002: 835 human cases, 54 deaths. We were Number 1! Doh!
Polyhedral, enveloped, RNA viruses Spherical: 0.04 - 0.065 mm Other flaviviruses: yellowfever, St. Louis encephalitis, dengue and hepatitis C virus
Polyhedral, enveloped, RNA viruses
Spherical: 0.04 - 0.065 mm
Other flaviviruses:
yellowfever, St. Louis encephalitis, dengue and hepatitis C virus
H. Culturing viruses
For production of vaccines
1) Fertilized chicken eggs (flu vaccines) 2) Tissue culture Animal cells grown in petri dishes Medium requires special growth factors or serum
1) Fertilized chicken eggs (flu vaccines)
2) Tissue culture
Animal cells grown in petri dishes Medium requires special growth factors or serum
Animal cells grown in petri dishes
Medium requires special growth factors or serum
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SIUC / College of Science / Microbiology / Microbiology 201 http://www.micro.siu.edu/micr201/chapter6N.html Last updated: Feb. 22, 2007 /jdh