Unit 2 Chapter 13 Objectives
What is a tumor?
An abnormal mass of tissue that results from excessive cell division that is uncontrolled and progressive.
Where do viruses multiply?
Inside host cells
What is the difference between latent and persistent viral infections?
Latent Viral Infections: can remain in host cells throughout life without causing disease. May be reactivated by immunosuppression, however, and cause disease. Persistent Viral Infections: progressive over a long period of time and are usually fatal. Different from latent viral infections in that the detectable virus builds slowly over a long period of time rather than appearing suddenly.
Define virus.
Obligate intracellular parasite composed of nucleic acid (either DNA or RNA) and a protein coat.
Explain the importance of reverse transcriptase and describe what occurs in the biosynthesis of a virus with reverse transcriptase.
Reverse transcriptase (RNA-dependent DNA polymerase) is used by retroviruses to transcribe DNA from RNA. Both viral RNA strands are sense strands, which are transcribed by reverse transcriptase to make complementary DNA strands. The original viral RNA is degraded and the DNA copies integrate into the host cell's genome.
List the characteristics of viruses.
• Single type of nucleic acid - DNA or RNA • Protein coat, or capsid, some have envelopes • Multiply inside of living cells using the host cell machinery • Direct the synthesis of structures to transfer viral nucleic acid to other cells
What is a plaque? What kind of information do you get from a plaque assay?
A clearing in a bacterial lawn resulting from lysis by phages. Theoretically each plaque in a plaque assay corresponds to a single virus in the initial suspension. This allows bacteriophages to be detected and counted.
What is the difference between an enveloped and a naked virus?
A naked or non-enveloped virus does not have an envelope and is protected by its capsid alone.
What are the three types of viruses based on the types of organisms they infect?
1. Animal virus 2. Plant virus 3. Bacterial virus (bacteriophage)
What are the steps in the multiplication of animal viruses? Give the proper sequence and describe what occurs in each step.
1. Attachment: (requires a receptor) - virion attaches to host cell. 2. Entry: Many viruses enter into eukaryotic cells by receptor-mediated endocytosis (when a virion attaches to the plasma membrane of a potential host cell, the host cell will enfold the virion and form a vesicle). 3. Uncoating: Separation of the viral nucleic acid from the protein coat may be accomplished by host or viral enzymes. 4. Biosynthesis: DNA Viruses: DNA of most DNA viruses is released into the nucleus of the host cell. Transcription and translation of early genes produces enzymes to reproduce viral DNA. Transcription and translation of late genes produces capsid proteins in the cytoplasm. RNA Viruses: RNA viruses multiply in the cytoplasm. RNA-dependent RNA polymerase synthesizes a double-stranded RNA. The sense strand (+ strand) can act as mRNA directly and as a template for antisense strand (- strand) synthesis. - Single strand + strand: The single stranded sense strand is transcribed to make antisense strands. The antisense strands serve as the template for making mRNA (sense strands), which code for viral proteins and serve as the viral genome that is packaged inside the capsid during assembly. - Single strand - strand: The single stranded antisense strand is transcribed to make sense strands, which serve as mRNA to code for viral proteins and also as a template to make more copies of the viral genome (single stranded antisense strands) which will be packaged inside the capsid during assembly. - Double strand +/-: transcription of the antisense strand makes more copies of the sense strand, which serves as mRNA. Transcription of the sense strand provides viral protiens (including RNA-directed RNA polymerase) and more copies of the antisense strand, which is packaged along with the complementary sense strands in the capsid during assembly. 5. Maturation or Assembly: Virus DNA/RNA and capsids are assembled into complete viruses. 6. Release: Complete viruses are released via either rupture or budding.
Give the medically important examples of RNA viruses from the list provided.
1. Enterovirus, Rhinovirus, Hepatitis A Virus: single-stranded RNA + strand non-enveloped, Picornaviridae family, 28-30 nm, includes the polio-, coxsackie-, and echoviruses, can cause hand-foot-mouth virus, rhinoviruses are the most common cause of colds. 2. Hepatitis E Virus, Norovirus: single-stranded RNA + strand non-enveloped, Caliciviridae family, 35-40 nm, causes gastroenteritis. 3. Alphavirus, Rubivirus (rubella virus): single-stranded RNA + strand enveloped, Togaviridae family, 60-70 nm, transmitted by arthropods (Alphavirus), diseases include eastern equine encephalitis (EEE), western equine encephalitis (WEE), and chikungunya. Rubella virus is transmitted by the respiratory route. 4. Flavivirus, Pestivirus, Hepatitis C Virus: single-stranded RNA + strand enveloped, Flaviviridae family, 40-50 nm, can replicate in arthropods that transmit them, yellow fever, dengue, and St. Louis and West Nile encephalitis. 5. Coronavirus: single-stranded RNA + strand enveloped, Coronaviridae family, 80-160 nm, associated with upper respiratory tract infections and the common cold, includes SARS virus and MERS-CoV. 6. Vesiculovirus (vesicular stomatatis virus), Lyssavirus (rabies virus): - strand, single strand of RNA, Rhabdoviridae family, 70-180 nm, bullet-shaped viruses with spiked envelope, cause rabies and numerous animal diseases. 7. Filovirus: - strand, single strand of RNA, Filoviridae family, 80-14,000 nm, enveloped, helical viruses, includes Ebola and Marburg viruses. 8. Paramyxovirus, Morbillivirus (measles virus): - strand, single strand of RNA, Paramyxoviridae family, 150-300 nm, cause parainfluenza, mumps, and Newcastle disease in chickens. 9. Hepatitis D: - strand, single strand of RNA, Deltaviridae family, 32 nm, depends on coinfection with hepadnavirus. 10. Influenza Virus A, B, & C: - strand, multiple strands of RNA, Orthomyxoviridae family, 80-200 nm, envelope spikes can agglutinate red blood cells. 11. Bunyavirus (California Encephalitis Virus), Hantavirus: - strand, multiple strands of RNA, Bunyaviridae family, 90-120 nm, cause hemorrhagic fevers such as Korean hemorrhagic fever and Hantavirus pulmonary syndrome, associated with rodents. 12. Arenavirus: - strand, multiple strands of RNA, Arenaviridae family, 110-130 nm, helical capsids contain RNA-containing granules, cause lymphocytic choriomeningitis, Venezuelan hemorrhagic fever, and Lassa fever. 13. Oncoviruses, Lentivirus (HIV): single-stranded RNA, produce DNA, Retroviridae family, 100-120 nm, includes all RNA tumor viruses, oncoviruses cause leukemia and tumors in animals, Lentivirus causes AIDS. 14. Reovirus, Rotavirus: double-stranded RNA non-enveloped, Reoviridae family, 60-80 nm, generally mild respiratory infections transmitted by arthropods, includes Colorado tick fever.
What are the five morphological types of viruses?
1. Helical, non-enveloped 2. Helical, enveloped 3. Polyhedral, non-enveloped 4. Polyhedral, enveloped 5. Complex viruses
Give the medically important examples of DNA viruses from the list provided.
1. Human parvovirus B19: single-stranded DNA non-enveloped, Parvoviridae family, 18-25 nm, causes amnesia in immunocompromised patients. 2. Mastadenovirus: double-stranded DNA non-enveloped, Adenoviridae family, 70-90 nm, medium sized virus that causes respiratory infections in humans and sometimes tumors in animals. 3. Papillomavirus (human wart virus) & Polyomavirus: - double-stranded DNA non-enveloped, Papovaviridae family, 40-57 nm, small viruses that cause warts and cervical and anal cancer in humans. 4. Orthopoxvirus (vaccinia & smallpox viruses) & Molluscipoxvirus: double-stranded DNA enveloped, Poxviridae family, 200-350 nm, very large, complex, brick-shaped viruses that cause smallpox (variola), cowpox, and molluscum contagiosum (wartlike skin lesion). 5. Simplexvirus (HHV-1 and HHV-2), Varicellovirus (HHV-3), Lymphocryptovirus (HHV-4), Cytomegalovirus (HHV-5), Roseolovirus (HHV-6 and HHV-7), Rhadinovirus (HHV-8): double-stranded DNA enveloped, Herpesviridae family, 150-200 nm, medium-sized viruses that cause cold sores, chickenpox and shingles, infectious mononucleosis, CMV inclusion disease, measle-like rashes in infants, and Kaposi's sarcoma, primarily in AIDS patients. 6. Hepadnavirus (hepatitis B virus): double-stranded DNA enveloped, Hepadnaviridae family, 42 nm, causes hepatitis B and liver tumors, uses reverse transcriptase to produce its DNA from mRNA after protein synthesis.
How are animal viruses cultured in the laboratory?
1. Live animal cultures have to be used for some animal viruses. 2. Embryonated eggs can serve as substitutes for some viruses - can inoculate membrane that best supports specific virus (allantoic, amniotic, chorioallantoic, or yolk sac). 3. Cell culture is a lot cheaper and easier to work with (contamination can be a problem however).
What are the major methods used for viral identification?
1. Serological methods (i.e. Western blotting) 2. Cytopathic effects - diagnostic inclusion bodies are associated with rabies virus, measles virus, vaccinia virus, smallpox virus, herpesvirus, and adenoviruses. 3. Molecular methods (include PCR and RFLPs)
What are the possible outcomes of lysogeny?
1. The lysogenic cells are immune to reinfection by the same phage (but the host cell is not immune to infection by other phage types.) 2. Phage conversion: the host cell may exhibit new properties. Ex.: Corynebacterium diphtheriae, which causes diphtheria, is a pathogen whose disease-producing properties are related to the synthesis of a toxin. The organism can produce toxin only when it carries a lysogenic phage, because the prophage carries the gene coding for the toxin. 3. Specialized transduction is possible - mediated by a lysogenic phage, which packages bacterial DNA along with its own DNA in the same capsid. When a prophage is excised from the host chromosome, adjacent genes from either side may remain attached to the phage DNA.
How are bacteriophages cultured in the laboratory?
Bacteriophages can be grown either in suspensions of bacteria in liquid media or in bacterial cultures on solid media. The use of solid media makes it possible to use the plaque method to detect and count viruses. A bacteriophage sample is mixed with host bacteria and melted agar. The agar containing the bacteriophages and host bacteria is then poured into a Petri plate containing a hardened layer of agar growth medium. The virus-bacteria mixture solidifies into a thin top layer that contains a layer of bacteria approximately one cell thick. Each virus infects a bacterium, multiplies, and releases several hundred new viruses. These newly produced viruses infect other bacteria in the immediate vicinity, and more new viruses are produced. Following several viral multiplication cycles, all the bacteria in the area surrounding the original virus are destroyed. This produces a number of clearings (plaques) visible against a lawn of bacterial growth on the surface of the agar. While the plaques form, uninfected bacteria elsewhere in the Petri plate multiply rapidly and produce a turbid background. Theoretically each plaque corresponds to a single virus in the initial suspension. Therefore, the concentrations of viral suspensions measured by the number of plaques are usually expressed in terms of plaque-forming units (PFU).
Describe viral multiplication in bacteriophages.
Bacteriophages can multiply by two alternative mechanisms: the lytic cycle or the lysogenic cycle. The lytic cycle ends with the lysis and death of the host cell, whereas the host cell remains alive in the lysogenic cycle.
What is the difference between a benign and a malignant tumor?
Benign: tumors that generally don't spread, can be removed, and aren't life threatening. Benign tumors tend to be more differentiated (the extent to which tumor cells resemble the cell of origin in both appearance and function), exhibit dysplasia (disorganized but non-neoplastic), and the cells aren't usually pleomorphic (variation in shape and size). Benign tumors generally grow slowly and are usually demarcated and often encapsulated. Malignant: invasive, generally aggressive tumors that are life threatening. Malignant tumors tend to exhibit anaplasia (undifferentiated, cells appear almost embryonic) and pleomorphism (variation in shape and size). Malignant tumors exhibit a correlation between rate of growth and degree of differentiation - may have periods of slow growth followed by rapid growth, and may spontaneously regress due to central necrosis and inability to provide nutrition. Malignant tumors are invasive (penetrate surrounding tissue) and often exhibit metastasis.
Describe the composition of a virus. Define virion, capsid, capsomere, and protomere.
Composition Viruses are composed of nucleic acid surrounded by a protein coat. Some viruses have an envelope composed of a phospholipid bilayer with viral glycoproteins. Virion: a complete, fully developed viral particle Capsid: protein coat Capsomere: subunit of the capsid Protomere: subunit of the capsomere
Give some examples of oncogenic viruses.
DNA Oncogenic Viruses Found among: 1. Adenoviruses 2. Herpesviruses - Epstein-Barr Virus (EBV): Burkitt's lymphoma, Nasopharyngeal carcinoma - Herpes simplex: association with cervical cancer 3. Poxviruses 4. Papovaviruses - Human papilloma viruses (especially HPV-16): cervical cancer 5. Hepadnaviruses - Hepatitis B (HBV): hepatocellular carcinoma RNA Oncogenic Viruses 1. Retroviruses - HIV - Human T-Lymphotropic Virus types 1 & 2 (HTLV-1 and HTLV-2) are associated with human leukemia and lymphomas.
Differentiate between the lytic cycle and lysogeny.
Lytic cycle 1. Attachment or adsorption: (requires a receptor) - phage attaches to host cell. 2. Penetration: T-evens release lysozyme to break down a portion of the cell wall. The tail sheath contracts and the tail core is driven through the hole in the wall to the plasma membrane. The viral genome is then injected into the bacterium. 3. Biosynthesis: Viral DNA and proteins are synthesized. Host protein synthesis is stopped by degradation of host DNA, interference with transcription, or repression of translation. 4. Maturation: Phage DNA and capsids are assembled into complete viruses. 5. Release: Phage lysozyme breaks down the cell wall and newly synthesized phage particles are released. Lysogeny A cycle in which the phage DNA recombines with the bacterial chromosome. The incorporated viral DNA is now a prophage. The prophage genes are regulated by a repressor coded for by the prophage, and the prophage is replicated each time the host DNA is replicated. Exposure to mutagens can lead to excision of the prophage and initiation of the lytic cycle.
How are enveloped ands non-enveloped viruses are released from host cells?
Non-eneveloped or naked viruses are released via rupture. Enveloped viruses are released by budding.
What is an oncogene? What is a tumor suppressor gene? Which is "activated" and which is "lost"?
Oncogene: A gene involved in tumor formation. Oncogenes were first identified as part of viral genomes, but it was later shown that the viral oncogenes were actually derived from animal cells. Oncogenes may be activated (to function abnormally or without normal controls) by mutagenic chemicals, radiation, and viruses. Activation events may include mutation, transduction, translocation, and amplification. Tumor Suppressor Gene: Tumor suppressor genes are normal genes that slow down cell division, repair DNA mistakes, or tell cells when to die. When tumor suppressor genes don't work properly or are lost, cells can grow out of control, which can lead to cancer.
What is a prophage?
Phage DNA inserted into a host's DNA.
What is the difference between primary, diploid and continuous cell lines?
Primary cell lines have a short lifespan in culture - a few generations before reaching senescence. Diploid cell lines are derived from embryos and can grow for up to 100 population doublings before senescence. Continuous cell lines are derived from transformed cells and grow indefinitely in culture.
What are prions? What kind of diseases do they cause?
Prions are infectious proteins that cause transmissible spongiform encephalopathies (TSEs). TSEs are characterized by large vacuoles in the brain.
What is the host range of virus? What determines the host range of a virus?
The host range of a virus is the specific types of cells a virus can infect in its host species. Host range is determined by attachment sites (receptors). Viruses are usually species specific but there have been a number of viruses demonstrated to cross species barriers (e.g. influenza).
What are viral envelopes? Where do viruses acquire them?
The outer covering of some viruses - the envelope is derived from the host cell plasma membrane when the virus buds out.
What are viral spikes? List two kinds and describe their function.
Viral glycoproteins that project from the envelope. 2 types: 1. H (hemagglutinin): allows the virus to attach to host cells (and red blood cells) 2. N (neuraminidase): an enzyme that allows the mature viral particles to escape from the host cell
Identify the site of viral protein synthesis.
Viral protein synthesis occurs within the host cell. Host protein synthesis is stopped by degradation of host DNA, interference with transcription, or repression of translation.
What is a viroid?
Viroids are naked (lacking a protein coat) pieces of RNA that cause some plant diseases. They are internally base paired, so they assume a folded conformation that protects them from enzymatic degradation. Viroids don't code for proteins and research indicates that they have similarities to introns, which suggests researchers may discover animal viroids in the future. The Hepatitis D virus consists of a single circular RNA (the delta factor) encased in a Hepatitis B capsid and is thought to be close to what an animal viroid may be like.
What kind of nucleic acid do viruses have?
Viruses have either DNA or RNA, never both.
What are cytopathic effects? Describe the cytopathic effects of viruses on host cells.
Visible effects on a host cell caused by a virus. Cytopathic effects can appear early or late in the course of the viral infection. They may be cytocidal (cell death) or non-cytocidal. Non-cytocidal effects include acidophilic or basophilic inclusion bodies in the nucleus, cytoplasm, or both, cell fusion, and transformation. Cytopathic effects can be so characteristic of individual viruses that they can often be used to identify viruses.
