VIRUSES
Chapter 13
VIRUSES
•Latin for poison
•Can infect bacteria, fungi, plants & animals
•1892 - Iwanoski - tobacco mosaic virus (TMV)
–Filtrate of diseased plant contained infectious material
–Filtrate did not contain bacteria smaller than bacteria
•1898 - Beijernick - continued TMV experiments
–Last plant contaminated as severely as the first
–Infectious material is replicating inside plant
GENERAL CHARACTERISTICS
•Obligate intracellular parasites
–Can not replicate outside of host
•Very small (<0.3 m), filterable
–Requires electron microscope to see
•Contain one type of nucleic acid, either DNA or RNA
•Nucleic acid surrounded by a protein coat called the capsid
•Have very little enzymes of their own
–Use host cell enzymes for replication and packaging
–Molecular, nonliving entities (acellular that is not cells)
•May have lipid envelopes
–Susceptible to disinfectants
•Many have a range of host cells able to infect
•Due to specific receptors on cell surfaces
STRUCTURE
•VIRION = complete particle
–Nucleic acid surrounded by capsid +/- lipid envelope
–SIZE: 20 - 300 nm (0.02 - 0.3 m)
•Pox virus = largest virus
•GENOME: contain a single type of nucleic acid
–Contain either DNA or RNA
–NEVER BOTH RNA and DNA
–Amount varies from a few genes to ~250 genes
•CLASSIFICATION
–Based on nucleic acid content then
–Capsid
–Envelope
1. NUCLEIC ACID
•DNA or RNA
–Can be single stranded, double stranded, linear or circular; continuous or segmented
•ssRNA genomes: have a polarity (+ or -)
+ RNA: can go in the cell and directly function as mRNA
- RNA: must first be transcribed into a +RNA strand which then serves as the mRNA
•Retrovirus or non-retrovirus
–Retrovirus: RNA transcribed into DNA inside the host
2. CAPSID
•CAPSID: protein coat surrounding the nucleic acid
•Each capsid is composed of units called CAPSOMERS
–May be of one protein of several different proteins
•CAPSID SHAPE or SYMMETRY
–HELICAL
–POLYHEDRAL
–COMPLEX
HELICAL SYMMETRY
•Look like a long rod
•Many copies of the same protein wrapped in a helix
•Nucleic acid surrounded by a hollow, helical, cylindrical capsid
–Tobacco mosaic virus (TMV)
–Rabies virus
POLYHEDRAL SYMMETRY
•Many sides
•ICOSAHEDRAL
–Regular polygon with 20 sides with 12 corners
–Each side = equilateral triangle
•Polio virus
•Adenovirus
•Herpes virus
3. ENVELOPE
•Surrounds the nucleocapsid of some animal viruses
–Enveloped viruses
•Made up of host cell lipids and viral proteins
–Lipid bilayer membrane surrounding the capsid
•May contain viral glycoproteins called PEPLOMERS or SPIKES
–May be used for attachment to host cell
•May help virus get away from the cell
•Keeps segments of a segmented genome together
•Antigenic – host immune system can mount an attack
TAXONOMY of VIRUSES
•AT FIRST: classified by host
–Plant viruses
–Animal viruses
–Bacteriophage
•THEN: classified by disease caused
–Respiratory diseases
–Enteric viruses
•NOW: classified according to structure
–NUCLEIC ACID content
–CAPSID SYMMETRY
–ENVELOPED or NONENVELOPED
VIRAL TAXONOMY
“FAMILY” - viridae
“SPECIES” - group of viruses
Have same nucleic acid and infect same host cells
DNA FAMILIES of VIRUSES #1
•1. PARVOVIRIDAE
–ssDNA, icosahedral, naked
•Gastroenteritis, fetal death
•2. ADENOVIRIDAE
–dsDNA, icosahedral, naked
–Respiratory diseases
DNA FAMILIES of VIRUSES #2
•3. PAPOVAVIRIDAE
–dsDNA, icosahedral, naked
–PA - papilloma = warts
–PO - polyoma = tumors
–VA - vacuolating = causes vacuoles in host cell
•HPV: human papilloma virus – more than 60 types, benign warts, plantar warts, laryngeal warts, and cervical warts.
DNA FAMILIES of VIRUSES #3
•4. POXVIRIDAE
–dsDNA, complex, enveloped
–Largest of all of the viruses
•Smallpox virus – variola, only disease eradicated by vaccination efforts.
•Cowpox virus - vaccinia
•5. HEPADNAVIRIDAE
–Circular dsDNA, icosahedral, enveloped
–Unusual replication cycle
•Hepatitis B virus
DNA FAMILIES of VIRUSES #4
•6. HERPESVIRIDAE
–dsDNA, icosahedral, enveloped
–Very large viruses, become latent
–Ubiquitous in animals
•Large number of these have been discovered
–Epstein-Barr virus (EBV) - infectious mononucleosis
–Cytomegalovirus (CMV)
–Varicella-zoster virus (VZV) - chickenpox & shingles
–Herpes simplex viruses (HSV)
•HSV-1: fever blisters (oral herpes)
•HSV-2: genital herpes
RNA FAMILIES of VIRUSES #1
•PICORNAVIRIDAE
+ve RNA, icosahedral, naked
–PICO = very small
•Polio virus – polio
•Coxsackieviruses
•Enteroviruses (“intestine”)
–Hepatitis A virus (HAV) - acute hepatitis
•Rhinoviruses - common cold
RNA FAMILIES of VIRUSES #2
•TOGAVIRIDAE
+ve RNA, icosahedral, enveloped
–TOGA = cloaked or envleoped
–Transmitted by arthropods (mosquitoes)
–Arboviruses
•Encephalitis viruses
–Rubiviruses - respiratory transmission
•Rubella = German measles
RNA FAMILIES of VIRUSES #3
•FLAVIVIRIDAE
+ve RNA, polyhedral, enveloped
•Flaviviruses – includes Yellow fever
•Hepatitis C virus (HCV)
•RETROVIRIDAE
Diploid +ve RNA*, icosahedral, enveloped
–*Only diploid virus - 2 identical copies of ssRNA
•Each strand ~10kb
–Reverse transcriptase - makes a DNA intermediate to insert into host chromosome
•RNA tumor viruses (oncoviruses)
•Leukemia viruses (HTLV-1)
•Immunodeficiency viruses (HIV)
RNA FAMILIES of VIRUSES #4
•PARAMYXOVIRIDAE (“near” + “mucus”)
-ve RNA, helical, enveloped
•Paramyxovirus - parainfluenza
•Mumps virus
•Rubeola virus – measles
•RHABDOVIRIDAE (“rod”)
-ve RNA, helical, enveloped
–Shaped like bullets with spiked envelope
–Rod with one flat & one curved end
•Rabies virus
RNA FAMILIES of VIRUSES #5
•ORTHOMYXOVIRIDAE (“straight”)
Segmented –ve RNA, helical, enveloped
–Influenzaviruses
•Influenza virus A - 1 virus causing disease
–Can infect swine, birds and horses
•Influenza virus B
•Influenza virus C
INFLUENZA VIRUS
•Orthomyxovirus: segmented ssRNA genome
–Has 8 nucleocapsids
–Each contains one ssRNA segment
•Envelope contains 2 different PEPLOMERS
–H = hemagglutinin peplomers
•Attachment to host cell
–N = neuraminidase peplomers
•Helps virus get away from the cell
RNA FAMILIES of VIRUSES #5
•FILOVIRIDAE
-ve RNA, helical, envleoped
–Long, thread-like
•Ebola virus - hemorrhagic fever - FATAL
•Marburg agent - hemorrhagic fever
•BUNYVIRIDAE
Segmented (3) –ve RNA, helical, enveloped
–Associated with rodents
•Hanta virus - hemorrhagic fever and hantavirus pulmonary syndrome (HPS)
RNA FAMILIES of VIRUSES #6
•ARENAVIRIDAE
Segmented (2) -ve RNA, enveloped
•Carried by rodents
•Causes respiratory diseases
•Arenaviruses – Lassa fever
•REOVIRIDAE
Segmented dsRNA*, icosahedral, naked
–*dsRNA = exception
–R = Respiratory diseases
–E = Enteric diseases
–O = Orphan - not associated with any disease
•Rota virus – severe diarrhea in young children
•Colorado tick fever virus
CULTIVATION OF VIRUSES
•Obligate intracellular parasites depends on the virus and it’s host cell type
•BACTERIOPHAGE
–Bacteria: suspension or solid media
•PLANT VIRUSES
–Whole plant or plant cell culture
•ANIMAL VIRUSES
–Whole animal, embryonated eggs or animal cell culture
CELL CULTURE
•Most common way of culturing animal viruses
•Three cell lines can be used
–Primary cell lines : derived from tissues by enzymes , generally die after a couple of generations
–Diploid cell lines: derived from human embryos, multiply for about 50 – 100 generations and then die.
–Continuous cell lines: Immortal lines are derived from transformed or cancerous cells. Can multiply indefinitely in culture and are immortal. These are commonly used in propagation of viruses.
VIRAL REPLICATION
•Viral genes code for capsid (structural) proteins and some enzymes needed for replication
•Virus uses host cell enzymes needed for protein synthesis, energy production & ribosomes
•Viruses ONLY replicate inside a cell
•Viruses invade host cell and controls metabolic and replicative “machinery”
–Cell now primarily makes viral proteins and nucleic acid for new viral particles
VIRUS - HOST INTERACTIONS
•LYTIC CYCLE
–Host cell is lysed after viral replication
•T4 bacteriophage
•LYSOGENIC or LATENT CYCLE
–Host cell stays alive after replication
–Viral genome is integrated into host’s genome
•Lambda bacteriophage
•TRANSFORMING INTERACTION
–Viral genome is integrated into host’s genome
–Integration alters host cell growth & morphology
–Cancer-like cell
THE VIRAL REPLICATIVE CYCLE
•Attachment or absorption
•Penetration
•Uncoating
•Synthesis
•Maturation or assembly
•Release
•All the steps are similar for DNA and RNA viruses except for nucleic acid synthesis.
–Nucleic acid synthesis is different
1. ATTACHMENT OR ABSORPTION
•Viral particle attaches to host cell via specific receptors on on the surface of the host cell
–Some proteins and glycoproteins on host cell membranes can function as receptors for some viruses
•Viral particles have attachments sites made of spikes or fibers that bind these receptors
–Naked viruses - nucleocapsid
–Enveloped viruses - envelope
2. PENETRATION
•Entry of viral nucleic acid into host cell
–Can be achieved by endocytosis
–Naked and enveloped can enter this way
•Can be achieved via fusion of envelope with host cell plasma membrane
–Then viral nucleocapsid enters the cytoplasm
•Uncoating - separation of nucleic acid from the protein capsid coat
–Can be due to host lysosomal enzymes or configurational changes in the capsid
3. SYNTHESIS
•Virus takes over host’s biosynthetic “machinery”
–Period of time when new viral nucleic acid molecules, capsid proteins and other viral components are produced
–Synthesis of host DNA & proteins stops
•DNA Viruses
–All replicate in nucleus except Poxviruses, uses DNA dependent DNA polymerase
•RNA Viruses
–All replicate in cytoplasms except retroviruses and orthomyxoviruses, uses RNA dependent RNA polymerase
4. MATURATION or ASSEMBLY
•Assembly of capsomers into capsid
•Newly synthesized viral DNA is inserted into the newly made capsids to form new virions
5. RELEASE
•Naked viruses usually released from host cell by lysing the cell
–Host cell dies
•BURST TIME = length of time from attachment to release
•BURST SIZE = number of particles released from 1 cell
–50 - 200 virions are released
•Enveloped viruses usually bud out of the cell
–As they leave the cell, they acquire their envelope from the host cell plasma membrane
RETROVIRUSES
•GENOME = RNA
•REPLICATE through DNA intermediate
–Before integrates into host genome must make a dsDNA copy of the viral RNA genome
•Utilize an enzyme = REVERSE TRANSCRIPTASE
–RNA dependent DNA polymerase
–RNA ------> DNA ----> integrates
RETROVIRAL REPLICATION
•Attachment to host by viral peplomers
•Penetration by fusion with host’s membrane
•Biosynthesis
–RNA ---> dsDNA
–DNA enters nucleus ---> integrates
–DNA remains in the host’s genome
–Integrated DNA ---> RNA ---> proteins
•Assembly
•Release ---> budding
•RNA ---> DNA ---> RNA
VIRAL LATENCY
•Many viruses can remain latent in the host cell
–Period of time when there is no replication of the viral genome
•No apparent disease occurs during this time
•Virus can be activated by different stimuli
–Now causes disease
•HSV-1
–Remains latent in nerve cells
–Stimulation cold sores or blisters
•Chicken pox virus
–Remains latent but after activation shingles
VIRUSES & CANCER
•HTLV - I and HTLV - 2
–Cause cancer in humans
–Human T-cell Lymphotrophic Virus
•Both retroviruses cause leukemia
•Other viruses cause cancer in animals
–EBV : Epstein-Barr virus associated with African Burkitt’s Lymphoma and nasopharyngeal carcinoma
–HBV: Hepatitis B virus associated with hepatocellular carcinoma ( liver cancer)
–HPV: human papilloma virus associated with cervical cancer
ONCOGENES
•ONCO = tumor
•ONCOGENE = tumor gene
–Originally derived from normal cellular genes
•ONCOGENIC VIRUS = a virus that promotes tumor formation
•“-OMA” refers to tumor
•SARCOMA VIRUS - promotes tumor formation
PROTOONCOGENES
•Normal cellular genes
–Usually involved in regulating cell growth & division
•Cellular oncogenes = altered or overexpressed protooncogenes
•Both cellular & viral oncogenes promote tumor formation cancer
OTHER “CANCER” VIRUSES
•HBV - Hepatitis B virus
–Can cause liver cancer (hepatocellular carcinoma)
–DNA virus - Hepadnaviridae
•EBV - Epstein Barr virus
–African Burkitt’s lymphoma and nasopharyngeal carcinoma
–DNA virus - Herpesviridae
•HPV - Human papilloma virus
–Certain strains associated with cervical cancer
–DNA virus - Papovaviridae
CARCINOGENESIS
•CARCINOGEN = substance that promotes tumor formation
•CARCINOGENESIS = process of tumor formation
•COCARCINOGENESIS - more than one factor is involved in the formation of tumors
•HBV + excessive alcohol consumption
–Promotes liver cancer?
MECHANISMS of VIRAL DISEASES
•Some promote tumor formation
•Some virus lyse the host ---> host cell death
•Some break up the host chromosomes
•Some stimulate the fusion of several host cells ----> POLYKARYOCYTE
•Some promote changes on cell surface
•Some cause shrinking of nucleus
•Some stimulate the formation of vacuoles
•Some stimulate APOPTOSIS (programmed cell death)
PRIONS
•PROteinaceous INfectious particles
•First identified by Stanley Prusiner in 1982
•DISEASES: kuru, Creuztfeld-Jacob disease, bovine spongiform encephalopathy (Mad Cow disease), Scrapie (found in sheep)
–All are neurological diseases, spongy appearance of the brain
–All are fatal (slow viral infection)
•INFECTIVE PARTICLE = PROTEIN ONLY
–IS INFECTIOUS
–Goes to the brain
–Converts the normal protein to an infectious protein