Chapter 13 Viruses
Provide an example of a latent viral infection
Cold sores, shingles
Viral Multiplication
For a virus to multiply: It must invade a host cell It must take over the host's metabolic machinery One-step growth curve
Describe the lysogenic cycle of bacteriophage lambda.
The phage genome also enters the cell through attachment and penetration. A prime example of a phage with this type of life cycle is the lambda phage. During the lysogenic cycle, instead of killing the host, the phage genome integrates into the bacterial chromosome and becomes part of the host.
Which of the following is a viral benefit of lysogeny?
The prophage gets replicated every time the cell replicates.
Mature viruses are made during which stage?
assembly
The normal function of the PrP protein in mammals is believed to be:
assisting in normal synaptic development and function
__________ viruses are released by accumulating in high numbers in the host cell until the cell lyses.
Naked
How do normal prion proteins (PrP) differ from the infectious prion proteins?
Normal PrP have alpha-helices; infectious PrP have beta-pleated sheets
Why are the beta-pleated multimers of PrP potentially pathogenic?
The multimers are more stable and resistant to protease.
How Can Antiviral Medications Selectively Interfere with Viral Infection Cycles? Select strategies that are likely to interfere with viral infection and replication but will NOT damage the normal functioning of host cells.
-blocking host cell receptors that serve as sites for viruses to attach -inhibiting viral assembly and maturation processes -inhibiting the release of viruses from the host cell -inhibiting viral-encoded DNA polymerase -encoded DNA polymerase-blocking attachment sites on the virus surface
Arrange the following steps in the order in which they occur during the reproductive cycle of the type of virus that causes the common cold.
1. using the cell's machinery to synthesize new viral proteins and nucleic acid. 2. attaching to receptors on the host cell. 3. releasing their nucleic acid from the capsid into the host cell. 4. releasing the new viruses from the host cell. 5. assembling new viruses
Which of the following viral genomes can be used directly as messenger RNA?
+RNA
What Are the Implications of the Attachment Process?
-A particular virus may be able to infect humans but not mice. -A particular virus may be able to infect a liver cell but not a lung cell. -Some people are naturally (innately) resistant to certain viral infections. -Viral infection of a cell may be prevented by blocking the attachment sites on a virus with antibody. -Viral infection of a cell may be prevented by blocking the receptor sites on a host cell.
Which of the following is a component of temperate bacteriophage life cycle?
lysis of the host prophage lysogeny
Which of the following viral attachment mechanisms involves host receptor proteins?
naked virus attachment enveloped virus membrane fusion enveloped virus-induced phagocytosis
Which phase involves the injection of bacteriophage DNA?
penetration
From which phrase is the term "prions" derived?
Proteinaceous infectious particles
Transformed cells
have increased growth, loss of contact inhibition, tumor-specific transplant antigens, and T antigens happens when cells take in foreign DNA and express the genes encoded on that DNA. The expression of the genes gives the cell new characteristics, literally transforming the cell.
Describe how bacteriophages are cultured
•Viruses must be grown in living cells •Bacteriophages are grown in bacteria -Bacteriophages form plaques, which are clearings on a lawn of bacteria on the surface of agar §Each plaque corresponds to a single virus; can be expressed as plaque-forming units (PFU)
lytic cycle steps
-Phage causes lysis and death of the host cell 1. attachment 2. penetration 3. replication 4. assembly 5. lysis
How do prions replicate?
They cause normal proteins to misfold into infectious proteins.
How are prions different from other infectious agents?
They lack nucleic acid.
In what year did Stanley Prusiner win the Nobel Prize for his discovery of prions?
1997, found in 1982
Which of the following types of macromolecules are found in all viruses? Select all that apply. Nucleic acids Lipids Proteins Polysaccharides
Nucleic acids and Proteins All viruses have a protein coat, called a capsid, which surrounds some type of nucleic acid, either DNA or RNA. Some viruses also acquire a lipid bilayer envelope as they exit from the host cell.
Which statement concerning viral structure is true?
Spikes are found on some viruses. They are very consistent in structure and can be used for identification.
Lytic bacteriophages destroy their hosts.
True
Differentiate a virus from a bacterium
Viruses are intracellular parasites that can pass through bacteriological filters and are sensitive to interferons. They have no plasma membrane, ribosomes, do not perform binary fission, only contain one nucleic acid, and are not sensitive to antibiotics. All bacteria have plasma membranes, use binary fission, possess both nucleic acids, have ribosomes, and are sensitive to antibiotics but not to interferons.
dsDNA viral capsomeres __________________.
are synthesized in the cytoplasm. are assembled in the nucleus.
Induction is?
the trigger of prophage excision from the host chromosome.
What is the function of the enzyme reverse transcriptase?
to make DNA copies of the RNA virus of retroviruses
Oncogenes
transform normal cells into cancerous cells
bacteriophages (phages)
viruses that infect bacteria
Which of the following statements regarding latent viral infections is true?
Latent infections can persist for years in an individual without causing any symptoms.
Differentiate persistent viral infections from latent viral infections.
Latent virus Remains in asymptomatic host cell for long periods May reactivate due to changes in immunity Persistent viral infection Occurs gradually over a long period; is generally fatal -Subacute sclerosing panencephalitis (measles virus)
General Characteristics of Viruses
Obligatory intracellular parasites Require living host cells to multiply Contain DNA or RNA Contain a protein coat No ribosomes No ATP-generating mechanism
lysogenic cycle steps
Phage DNA is incorporated in the host DNA -Phage conversion -Specialized transduction 1. attachment 2. penetration 3. integration 4. replication 5. assembly 6. lysis
Why could it be argued that a virus is not alive?
A virus requires a host cell in order to reproduce.
Which of the following statements is true?
All viruses follow the same general life cycle.
Which of the following is NOT a sign or symptom of the common cold? A) Fever B) Runny nose C) Coughing D) Diarrhea
D) Diarrhea As a viral infection of the upper respiratory system, a common cold often results in fever, a runny nose, and a cough.
Describe the chemical and physical structure of both an enveloped and a nonenveloped virus.
ENVELOPED: In some viruses, the capsid is covered by an envelope, which chemically, usually consists of some combination of lipids, proteins, and carbs. In most cases, the envelop contains proteins determined by the viral nucleic acid and materials derived from normal host cell componants. Structurally, the enveloped virus consists of the nucleic acid>>Capsomeres>> Envelop>>Spikes(on some enveloped viruses). NONENVELOPED: Are viruses whose capsids are not covered by an envelope. The capsid protect the viruses from nuclease enzymes in biological fluids and promotes the viruses attachment to susceptible host cells.
Discuss how a protein can be infectious.
Prions are proteinaceous infectious particles and cause neurological diseases in humans and animals. Prion diseases are due to an altered protein. The cause can be a mutation in the normal gene for PrP (host glycoprotein) or contact with an altered protein prion proteins cause plaques and holes in neural tissue resulting in progressive loss of function and eventual death
How does the number of infectious prions increase?
Prions transform normal proteins into the misfolded beta-pleated sheet configuration; therefore, prions multiply by conversion.
RNA oncogenic viruses
Retroviridae Viral RNA is transcribed to DNA (using reverse transcriptase), which can integrate into host DNA HTLV-1 and HTLV-2 cause adult T cell leukemia and lymphoma
Which of the following are the main causative agents of the common cold?
Rhinoviruses and coronaviruses Common colds are viral infections; they are caused most commonly by rhinoviruses, coronaviruses, and some previously unclassified viruses.
Which of the following statements concerning viruses is true?
The "host range" for a virus is determined by the presence or absence of particular components on the surface of a host cell that are required for the virus to attach.
Which statement is INCORRECT concerning animal viruses?
The genome of animal viruses is always single-stranded.
host range
The spectrum of host cells a virus can infect • Most viruses infect only specific types of cells in one host • Determined by specific host attachment sites and cellular factors
Which of the following statements best describes why antibiotics are ineffective treatments for the common cold? Viruses contain enzymes that deactivate antibiotics before they can reach their targets. Viruses do not possess targets for antibiotics. Viruses are constantly mutating and changing their antibiotic targets. Viruses contain drug pumps that pump antibiotics out of the virus before the antibiotic can reach its target.
Viruses do not possess targets for antibiotics.
Define oncogene
a gene that in certain circumstances can transform a cell into a tumor cell.
oncogenic viruses
become integrated into the host cell's DNA and induce tumors
In which stage is the host chromosome broken into small fragments?
biosynthesis
The stage in which viral proteins are produced is the ___________________.
biosynthesis stage.
Discuss the relationship between DNA- and RNA-containing viruses and cancer.
~ 10% of cancers are caused by viruses. Several DNA viruses are retroviruses and oncogenic. The genetic material of oncogenic viruses become integrated into the host cells DNA. DNA viruses include HPV and Hep. B. The RNA viruses only the retroviruses seem to oncogenic. HTLV-1&2 have been found to cause cancer. Activated oncogenes transform normal cells into cancerous cells Transformed cells have increased growth, loss of contact inhibition, tumor-specific transplant antigens, and T antigens The genetic material of oncogenic viruses becomes integrated into the host cell's DNA
Describe the lytic cycle of T-even bacteriophages.
•Attachment: phage attaches by the tail fibers to the host cell •Penetration: phage lysozyme opens the cell wall; tail sheath contracts to force the tail core and DNA into the cell •Biosynthesis: production of phage DNA and proteins •Maturation: assembly of phage particles •Release: phage lysozyme breaks the cell wall
General Morphology
•Helical viruses—hollow, cylindrical capsid •Polyhedral viruses—many-sided •Enveloped viruses •Complex viruses—complicated structures
Animal Viruses Steps
Attachment: viruses attach to the cell membrane Entry by receptor(penetrating)-mediated endocytosis or fusion Uncoating by viral or host enzymes Biosynthesis: production of nucleic acid and proteins Maturation: nucleic acid and capsid proteins assemble Release by budding (enveloped viruses) or rupture
Describe how animal viruses are cultured.
•In living animals •In embryonated eggs -Virus injected into the egg -Viral growth is signaled by changes or death of the embryo•In cell cultures -Tissues are treated with enzymes to separate cells -Virally infected cells are detected via their deterioration, known as the cytopathic effect (CPE) -Continuous cell lines are used
Prions
•Proteinaceous infectious particles •Inherited and transmissible by ingestion, transplant, and surgical instruments In Humans -Spongiform encephalopathies "Mad cow disease" -Creutzfeldt-Jakob disease (CJD) -Gerstmann-Sträussler-Scheinker syndrome -Fatal familial insomnia -Kuru In animals -Sheep scrapie -Chronic wasting disease -Bovine spongiform encephalopathy -transmissible mink encephalopathy •PrPC: normal cellular prion protein, on the cell surface • PrPSc: scrapie protein; accumulates in brain cells, forming plaques
Viral Structure
•Virion—complete, fully developed viral particle -Nucleic acid—DNA or RNA can be single- or double-stranded; linear or circular -Capsid—protein coat made of capsomeres (subunits) -Envelope—lipid, protein, and carbohydrate coating on some viruses -Spikes—projections from outer surface *sensitive to heat, detergents, and dessication *enveloped viruses are easier to kill
What Happens in Each Step of the Viral Replication Process for a DNA-containing Virus?
Attachment: requires a physical and chemical interaction between the surface of the virus and host cell surface Entry and uncoating: -this process disassembles the viral capsid and releases the viral DNA -may involve fusion of the viral envelope with the host cell plasma membrane -may involve receptor-mediated endocytosis Biosynthesis: -Transcription of viral genes by host cell RNA polymerase occurs during this stage -New copies of viral DNA genomes are made during this stage -Translation of viral capsid proteins and other viral proteins occurs during this stage -The viral DNA must enter the host cell nucleus prior to this stage Maturation: -Capsids are assembled from capsomeres -Viral genomic DNA is packed into newly assembled capsids Release: -Nonenveloped viruses often lyse the cell during this process -Enveloped viruses accomplish this via a budding process
Compare and contrast the multiplication cycle of DNA- and RNA-containing animal viruses.
DNA •viruses replicate their DNA in the nucleus of the host using viral enzymes •Synthesize capsid in the cytoplasm using host cell enzymes RNA •Virus multiplies in the host cell's cytoplasm using-dependent RNA polymerase -ssRNA; + (sense) strand §Viral RNA serves as mRNA for protein synthesis -ssRNA; - (antisense) strand §Viral RNA is transcribed to a + strand to serve as mRNA for protein synthesis -dsRNA—double-stranded RNA RNA Viruses That Use DNA •Single-stranded RNA, produce DNA -Use reverse transcriptase to produce DNA from the viral genome §Viral DNA integrates into the host chromosome as a provirus -Retroviridae §Lentivirus (HIV) §Oncoviruses
Which of the following is true regarding cultivation and isolation of animal viruses?
Diploid cell culture lines, developed from human embryos, are widely used for culturing viruses that require a human host.
Which of the following is true concerning a lysogenic viral replication cycle?
During lysogeny, the viral genome integrates into the host DNA, becoming a physical part of the chromosome.
A virus will co-opt the host cell machinery to ________________________.
produce viral proteins replicate the genetic material
Which of the following is related to the lysogenic cycle?
prophage
DNA oncogenic viruses
• Adenoviridae • Herpesviridae Epstein-Barr virus • Poxviridae • Papovaviridae Human papillomavirus • Hepadnaviridae Hepatitis B virus
