Microbiology Chapter 13
The best known chronic infection involves herpes. chickenpox. hepatitis B. hepatitis A.
Hepatitis B
What are the two main criteria used in classifying viruses?
Host range and genome structure
Name three viruses that cause tumors in humans.
Human papillomavirus, Hepatitis B, and Epstein-Barr virus
Shape of virus that appears spherical when viewed, but the surface is 20 flat triangles like a soccer ball
Icosahedral
Consequences of lysogeny
Immunity to superinfection and lysogenic conversion.
Distinguish between acute and persistent viral infections at the cellular level.
In acute infections, the virions kill the host cells whereas in persistent infections, the virus does not kill the host cells.
Examples of diseases due to acute viral infections
Influenza, mumps, poliomyletis
Viruses replicate ONLY where?
Inside a living cell
What is unique to enveloped viruses?
Matrix protein found between the nucleocapsid and the envelope
Must all prion diseases result from eating infected food? Explain.
No- Mutations in the gene encoding the normal prion protein can result in the protein becomimg the abnormal prion protein.
What does a virion consist of?
Nucleic acid surrounded by a protein coat
Capsid + nucleic acid
Nucleocapsid
Multinucleated mass of cells due to the cytopathic effect of viral infections
Syncytium
Viruses that cause acute infection result in productive infections. True False
True
Genes that inhibit cell growth
Tumor suppressor genes
Why is it important to study viruses?
Viruses can be used as vectors for horizontal gene transfer in bacteria and there are many medically important viruses that cause diseases.
Site of viral replication that an infected cell forms in response to some viruses
inclusion body
in double stranded RNA viruses, the viral replicase uses the ___ strand of RNA to make a ___ strand of RNA that can be translated.
negative, positive
Filamentous phage only infect E. coli that have pili. only infect E. coli lacking pili. infect E. coli regardless of the presence of pili. do not infect E. coli.
only infect E. coli that have pili.
What is routinely used to quantitate phage particles in samples?
plaque assays
Quantitating viral titers of both phage and animal viruses frequently involves:
plaque formation
Normal tissue taken from animals and prepared immediately as media for viral growth is termed a(n) advantageous group. monolayer culture. plaque. primary culture.
primary culture.
A capsid is composed of
protein
Why are viruses difficult to study?
They require living organisms as hosts and are too small to be seen with a light microscope and can be visualized only with an electron microscope
Entry by endocytosis
1. Attachment- Entry to receptors triggers endocytosis 2. Endocytosis- Cytoplasmic membrane surrounds the virion, forming an endocytic vesicle 3. Release from vesicle- Envelope of virion fuses with the endosomal membrane 4. Uncoating- Nucleic acid separates form capsid
Entry by membrane fusion
1. Attachment- Spikes of virion attach to specific host cell receptors 2. Membrane fusion- Envelope of virion fuses with cytoplasmic membrane 3. Nucleocapsid released into cytoplasm- Viral envelope remains part of cytoplasmic membrane 4. Uncoating- Nucleic acid separates from capsid
How much smaller are viruses than the cell they infect?
100-1000 fold smaller
There are _______ major families of RNA containing viruses that infect vertebrates. 2 7 5 13
13
What happens in the integration process of a temperate phage?
A phage encoded enzyme (integrase) inserts the phage DNA into the host cell chromosome. The prophage replicates with the host chromosome prior to cell division. The prophage can remain integrated indefinitely or excised from the host chromosome and lytic infection begins.
What happens in the SOS repair system?
A protease is activated that destroys the repressor protein responsible for maintaining the integration of the prophage. The prophage is excised from the chromosome, allowing the phage to enter the lytic cycle.
Explain what it is meant by plaque-forming units?
A quantitative measure of the number of phage particles as measured by a plaque assay
Explain why a vaccine can prevent cervical cancer.
A vaccine against human papillomavirus prevents the infection of cells and the synthesis of virus proteins which appear to be responsible for the cancer.
What is a defective phage? A virus that has lost some of its genetic material, and therefore cannot replicate within a new target cell A virus that has lost some of its genetic material, and therefore cannot infect a new target cell A virus that lacks the ability to replicate independently of its host cell A virus that cannot attach to its host cell
A virus that has lost some of its genetic material, and therefore cannot replicate within a new target cell
What phages are single stranded DNA that look like long fibers?
Filamentous phages
Which transduction results from a packaging error during phage assembly?
Generalized
Phage heads that contain only bacterial genes in place of phage genes and cannot direct phage replication are called
Generalized transducing particles
How are generalized and specialized transduction different?
Generalized transduction can transfer any gene of the donor cell. Specialized transduction can only transfer bacterial genes adjacent to the phage DNA
What are the receptors made out of that viruses bind to in animals?
Glycoproteins
Why is it not surprising that AIDS patients frequently suffer a viral-induced tumor? HIV genomes integrate into the host cell chromosome. This integration might result in loss of control of the cell cycle (also known as cancer). The immune system's CD4+ T cells are directly responsible for elimination of tumor cells. HIV attacks them and eliminates them, making a person more susceptible to cancer. Products of an active HIV infection are highly mutagenic-so, as a person's illness progresses, these mutagenic compounds build up and are more capable of inducing a cancerous state. The therapies for HIV are highly mutagenic, which may lead to cancerous states in people taking the drug regimen.
HIV genomes integrate into the host cell chromosome. This integration might result in loss of control of the cell cycle (also known as cancer).
Influenza vaccines must be changed yearly because the amino acid sequence of the viral proteins change gradually over time. Based on this information, which is the most logical conclusion? The influenza virus:
Has an RNA genome.
All phages must have the ability to:
Have their nucleic acid enter the host cell and have their nucleic acid replicate in the host cell.
Shape of a virus that appears cylindrical and their capsomeres are arranged in a helix, like a spiral staircase
Helical
Which of the following can be used to quantify animal viruses?
Hemagglutination, direct count, quantal assay, plaque assay
Which is NOT required for synthesis of new virus particles in a host cell?
Homologous recombination
How are latent viral infections different from chronic ones?
In chronic infections, viral particles are produced continuously from infected cells, often without causing symptoms. In latent infections, the viral genome is silent but can be activated to once again cause disease.
Why would it be advantageous to a virus to interfere with the function of proto-oncogenes or tumor suppressor genes?
Interfering with the function of these genes would result in uncontrolled cell growth and thus more cells that could be invaded by the viruses, leading to increased virus production..
What is unique about Mimivirus?
Largest virus that was thought to be a bacteria because of its staining characteristics and its hairy appearance
Two types of persistent infections
Latent and chronic
Type of persistent infection where the viral genome remains silent within a host cell, yet can reactivate to cause a productive infection
Latent infection
The three types of relationships a bacteriophage can have with its host cell:
Latent infection where the host cell phenotype is changed, productive infection where the host cell does not lyse, and productive lytic infection.
Temperate phages can do all of these:
Lyse their host cells, change properties of their hosts, integrate their DNA into the host DNA, and become prophages.
Phages that exit the host at the end of the infection cycle by lysing the cell
Lytic/virulent phages
What two mechanisms may an enveloped virus use to enter a host cell?
Membrane fusion or endocytosis
Restriction enzymes cannot degrade what type of DNA?
Methylated
Which enzyme protects the host cell's own DNA from the action of another enzyme by adding a methyl group to the nucleobases recognized by the other enzyme?
Modification enzymes
Most enteric viruses are not enveloped. Why?
Naked viruses are more resistant to various chemicals which the enteric viruses are likely to encounter as they are transmitted by the fecal -oral route.
What are the two major categories of viral infections?
Persistent and acute
Type of infection that can continue with or without symptoms for years or even the life of the host
Persistent infection
What happens during the assembly (maturation) phase?
Phage components are assembled into mature virions. once the phage head is formed, DNA is packed into it. The tail is then attached, followed by the addition of the tail spikes. Some other components self-assemble or proteins serve as scaffolds on which components associate.
What happens in the attachment phase?
Phage particles collide with host cell by chance, and phage attaches by protein to receptor
Discuss two methods used to quantitate animal viruses.
Plaque assays which rely on lysis of host cells and counting of virions using an electron microscope. The concentration of virions must be high enough to be seen
What are circular zones of clearing called?
Plaques
Discuss how prion proteins accumulate in nervous tissue.
Prion proteins accumulate because they are resistant to proteases which break down proteins and they also convert a normal cellular protein which is continually synthesized to the prion protein, thereby increasing the concentration of prion protein
Composed solely of protein
Prions
What is the function of the capsid?
Protects the nucleic acid from enzymes and toxic chemicals and carries any enzyme required to infect the host
What part of the E. coli T4 phage attaches to the host cell receptors? Spikes of the envelope Pili of the envelope Protein fibers at the end of the phage tail Capsid fragments around the nucleic acid
Protein fibers at the end of the phage tail
Genes that stimulate cell growth
Proto-oncogenes
Name for a silent viral genome
Provirus
What is the double stranded DNA in the m13 infection cycle referred to as?
RF- replicative form
Reverse transcriptase is an ___ enzyme
RNA-dependent DNA polymerase
In which stage of viral multiplication would you expect to see budding occur?
Release
What protein prevents expression of the gene required for excision and is essential for maintaining the lysogenic state?
Repressor
Which enzyme recognizes specific short nucleotides in foreign DNA and then cuts the DNA molecule at these sequences?
Restriction enzymes
Which mechanisms that defend against phage infection would also limit conjugation and DNA-mediated transformation?
Restriction-modification system and the CRISPR system
Viruses that have an RNA genome and use reverse transcriptase to synthesize a DNA copy of that genome are called ___
Retrovirus
What happens in the synthesis of the phage proteins and genome phase?
Some of the genes are transcribed and translated. The cell begins synthesizing the proteins. Phage DNA is replicated, other virion components are made, and the host DNA is degraded.
Type of transduction that results from an excision mistake made by a temperate phage during the transition from a lysogenic to a lytic cycle
Specialized
What happens during the release phase?
The bacterial cell lyses and many new infectious virions are released. Lysozyme digests the host cell wall from within, causing the cell to lyse and release the phage. The phage particles infect cell sin environment and replication is repeated.
What happens during specialized transduction?
The excised DNA replicates and becomes incorporated into phage heads during assembly. The defective phage particles are released as the cell lyses.
How might syncytia formation of infected cells benefit viruses?
The fusion of infected cells with uninfected cells would allow viruses to infect additional cells and not be subject to the defense mechanisms of the body.
Most temperate phages integrate into the host chromosome, whereas some replicate as plasmids. Which kind of relationship would you think would be more likely to maintain the phage in the host cell? Why?
The integrated state is more stable since plasmids are more easily lost in the course of bacterial cell division.
What happens in a lysogenic infection?
The phage DNA lives in the cell without causing damage. The prophage (integrated DNA) is replicated along with the host cell chromosome. When the cell divides, the prophage is passed on to the cell's progeny.
What happens when the modification enzyme methylates the incoming phage DNA before the restriction enzyme has acted?
The phage DNA will not be degraded, allowing it to replicate and lyse the host cell.
What occurs during the T4 phage's infection cycle? (include 5 steps)
The phage takes over the bacterial cell, directing that cell to synthesize new phage particles. Attachment, genome entry, synthesis of phage proteins and genome, assembly (maturation), and release.
What does phage induction allow?
The phage to escape from a damaged host
Why can viruses not replicate independently of living cells?
They don't have any of the structures, such as ribosomes required for protein synthesis and the machinery required for harvesting energy.
Where does an enveloped virus get its envelope?
They obtain the bilayer fro the host cell
A virus' ability to infect only a single or limited amount of cells due to specificity is called what?
Tissue tropism
Phages can transfer DNA from one bacterial cell (donor) to another (recipient) through what process?
Transduction
A lysogenic cell contains viral DNA, a prophage, integrated into the host chromosome. True False
True
Filamentous virus is incapable of causing a lytic infection. True False
True
If a virus utilizes a lytic life cycle of reproduction, it will not induce tumors. True False
True
An abnormal growth of tissue resulting form a malfunction in the normally highly regulated process of cell growth
Tumor
Classification of viruses is based on all of these:
Type of nucleic acid, shape of virus, host infected, and strandedness of nucleic acid..
What is it called when viruses can only infect a single species
Viral host range
A viral particle
Virion
Consists of a small single-stranded RNA molecule that forms a closed ring
Viroid
Distinguish between a viroid and a prion in terms of structure and hosts.
Viroids consist of short segments of RNA and infect only plants. Prions consist only of protein and infect only animals, including humans.
Is it important to have fewer phages than bacterial host cells when doing a quantitative plaque assay? Explain.
Yes, because to measure the number of phage requires that only a single virion infect a bacterial cell. If there were fewer bacteria than phage particles, more than one virion would infect the same bacterium., but only a single plaque would result.
List 3 ways in which viruses can be transmitted from one organism to another.
Zoonotic- animal to human Enteric- fecal to oral Respiratory- coughing and sneezing
Acute infections result in:
a burst of virions being released from infected host cells
Twort and d'Herelle discovered:
a virus that destroys bacteria
Reassortment of gene segments that encode viral surface proteins that are recognized by the immune system can result in the loss of the immune system's ability to recognize and respond to the virus. This is called
antigenic shift
Filamentous phages
are extruded from the host cell.
The tail fibers on phages are associated with
attachment
The way most enveloped viruses are released in which the virus acquires an envelope
budding
Prions
cause disease in humans
Serially diluting viral samples, mixing each dilution with red blood cells, and then determining the highest dilution that shows maximum clumping of red blood cells
hemagglutination
The protection of the lysogens against infection by the same phage is provided by the repressor protein that will bind to the ___ on an incoming phage DNA, preventing expression of the phage genes.
operator
One group of animal viruses that are able to agglutinate red blood cells are the retrovirus. reovirus. coronavirus. orthomyxovirus.
orthomyxovirus.
The filamentous phages all contain double-stranded DNA. single-stranded DNA. double-stranded RNA. single-stranded RNA.
single-stranded DNA.
The protein projections on the surface of a virus that are involved in attachment to the host cell are called? hooks. pili. cilia. spikes. suckers.
spikes.
What is the lytic phage that has been intensely studied?
the T4 Phage
In acute viral infections, although the infected host cells may die, the host may survive because
the host's immune system may gradually eliminate the virus
What is the concentration of infectious phage particles in the original phage suspensions?
titer
Prions replicate by converting normal host proteins into prion proteins. responsible for "Mad Cow Disease" can cause a similar disease in humans. can be transmitted by consumption of dried or cooked food. that cause Spongiform Encephalopathy have the same amino acids but different folding properties from PrPc. All of the above
All of the above
Why is it virtually impossible to stamp out a disease caused by a zoonotic virus? You'd have to drive the vector organism extinct to do so. Many vector organisms have multiple stages of their life cycle that can carry a zoonotic virus, which complicates controlling the vector-borne transmission. Many viruses transmitted in this manner may utilize more than one vector organism. Many zoonotic viruses may be able to reside in more than one host organism, complicating control measures. All of the above are correct.
All of the above are correct.
Plant viruses may be transmitted by worms. contaminated seeds. humans. insects. All of the choices are correct.
All of the choices are correct.
Spongiform encephalopathy occurs in? humans. cattle. sheep. All of the choices are correct.
All of the choices are correct.
Types of bacteria defenses against phages
Altering receptor sites, restriction modification systems, and the CRISPR system
Antigenic variation that occurs as mutations accumulate in genes encoding key viral surface proteins that are recognized by the immune system
Antigenic drift
When a new subtype of a new virus is formed by reassortment between different strains of a virus or even between different viruses
Antigenic shift
Acute infections of animals:
Are a result of productive infection
How does m13 initiate infection?
Attaches to protein on f pilus. its single stranded DNA genome enters the cytoplasm of the bacterial cell, where the host DNA polymerase synthesizes the complementary strand
Replication of temperate phages and lytic phages have which steps in common?
Attachment, genome entry, assembly, biosynthesis of viral components, release
Phases of animal virus replication
Attachment, penetration and uncoating, synthesis of viral proteins and replication of the genome, assembly and maturation, release
What happens in the CRIPSR system?
Bacterial cells that survive infection retain spacer DNA (small segments of phage DNA) that are inserted the CRISPR system (chromosomal region). the spacer DNA remember past phage infections, allowing the cell and its progeny to recognize and block infections of the same phage.
What happens in the genome entry phage?
Bacteriophage injects its genome into a cell. The phage "squats" and injects its DNA into the cell wall and membrane while the capsid remains outside the cell
Why is lysogenic conversion medically important?
Because the phage can carry genes which are responsible for the pathogenicity of the organism.
Why are virally encoded enzymes medically important?
Because they represent potential sites for the action of antimicrobial medicines. The medicines would not likely have any harmful effects on the host cells.
How can a productive phage infection not kill a host cell?
By integrating their genome into the host cells' genome and by being extruded through the cell envelope which is then repaired.
What can cover a phage receptor?
Capsules and slime layers
Type of persistent infection characterized by the continuous production of low levels of viral particles
Chronic infection
Which viral infection is characterized by continuous, low-level production of new virus particles by animal host cells?
Chronic viral infections
Shape of a virus that is complicated and can vary
Complex
What did Stanley do to TMV?
Crystalized the virus, which could still cause disease
Morphological alterations in infected cells caused by a virus propagated in cell culture
Cytopathic effects
The replication strategies of animal viruses can be divided into 3 categories:
DNA viruses, RNA viruses, and reverse transcribing viruses
If the infecting phage lacks some critical pieces of DNA necessary for replication, it is called incomplete. mutated. vegetative. defective.
Defective
What type of agar is used in plaque assay?
Double layer agar
Lipid bilayer outside of a capsid
Envelope
Are enveloped or naked viruses more susceptible to disinfectant and why?
Enveloped because the disinfectant damages the envelope, making the virus non-infectious
Bacteriophage, unlike animal viruses, often have special viral-specific enzymes carried in the capsid which enter the host cell at the same time as the nucleic acid. True False
False
Capsids are made of a number of capsomeres that are covalently bonded to one another. True False
False
Electron microscopy is useful for counting viruses and distinguishing between infective and non-infective virions. True False
False
The first viral proteins include:
1. A nuclease that degrades the host cell's DNA 2. Proteins that modify a subunit of the host cell's RNA polymerase so that it no longer recognizes bacterial promoters
Type of infection characterized by sudden onset of symptoms of a relatively short duration
Acute infection
Viruses spread by arthropods that are biological vectors
Arbovirus
Identical protein subunits of a capsid
Capsomeres
Give an example of a virulent phage and of a temperate phage.
T4 is a virulent phage and lambda is a temperate phage.
Which is more likely to be a specialized transducing phage—a lytic or temperate phage?
Temperate phage
What is burst size?
The number of particles released during the release phase of the infection cycle.
The diseases arboviruses can cause
West Nile encephalitis, La Crosse encephalitis, yellow fever, and dengue fever
Outward signs of plant viral infections
Yellowing of leaves, tumors may appear, stunt of growth
Could the same type of virus cause both an acute and a persistent infection? Explain
Yes, the initial infection might be acute but the virius can later become latent by becoming integrated into the host cell genome. An example is the virus that causes AIDS
Animal viruses that do not have an envelope can only enter their host cell by:
endocytosis
Viruses of the fecal-oral route are described as
enteric
The viral envelope closely resembles the prokaryotic cell wall. capsomere. eukaryotic cell membrane. cytoplasm.
eukaryotic cell membrane.
To maintain the lysogenic state, a repressor protein prevents expression of the gene required for
excision
The protein coat of a virus is called a capsid. protects the nucleic acid. is involved in the recognition of host cell receptors by non-enveloped viruses. is called a capsomere. is called a capsid, protects the nucleic acid, AND is involved in the recognition of host cell receptors by non-enveloped viruses.
is called a capsid, protects the nucleic acid, AND is involved in the recognition of host cell receptors by non-enveloped viruses.
Herpes (cold sores) and varicella (shingles) are examples of
latent viruses
The integration of phage DNA into the bacterial chromosome occurs because of similar RNA nucleotides in both. the phage's ability to synthesize enzymes to enter the bacterium AND similar RNA nucleotides in both. the similarity in enzyme metabolism. the phage's ability to synthesize enzymes to enter the bacterium. the phage's ability to synthesize an enzyme that integrates its DNA into the host's chromosome.
the phage's ability to synthesize an enzyme that integrates its DNA into the host's chromosome.
Retrovirus may lead to latent infections because
they can integrate a DNA copy of their genome into the host chromosome
Infective and non-infective viruses may be distinguished by growth on MacConkey's agar. True False
False
Physical rupture of a cell
lysis
Infected cell
lysogen
The clustered, regularly interspaced short palindromic repeats (CRISPR) system in bacterial cells has been called the "immune" system of bacteria. CRISPR protect bacteria from a repeat infection from the same phage because bacterial cells modify the attachment sites for the phages so that new infections cannot take place. recognize proteins on the surface of the phage and secrete enzymes that digest the phage. recognize proteins on the surface of the phage and secrete proteins that block the binding of the phage. integrate fragments from the phage DNA in their own chromosomes and target for destruction any DNA that contains the same fragments in the future.
integrate fragments from the phage DNA in their own chromosomes and target for destruction any DNA that contains the same fragments in the future.
What is the change in the phenotype of a lysogen as a consequence of the specific prophage it carries
lysogenic conversion
Temperate phages have the option of:
lytic infection or incorporating their DNA into the host cell genome.
Carriers may have a persistent infection AND may be a source of infection. may be a source of infection. have been cured of the infection. usually show symptoms of the disease. may have a persistent infection.
may have a persistent infection AND may be a source of infection.
Assembly of the T4 phage may involve some self-assembly. may involve the use of scaffolds. is completely self-assembly. is completely dependent on scaffolds. may involve some self-assembly AND may involve the use of scaffolds
may involve some self-assembly AND may involve the use of scaffolds
Viruses are incapable of
metabolism, replication, or motility
Dimitri Iwanowksy and Martinus Beijerinck found that:
mosaic disease was caused by an unusual agent. Beijerinck called it "filterable virus"
Viruses increase the number of bacteria. probably keep the numbers of bacteria in check. have no effect on the number of bacteria. probably keep the numbers of bacteria in check AND are active in passing DNA from one bacterium to another. are active in passing DNA from one bacterium to another.
probably keep the numbers of bacteria in check AND are active in passing DNA from one bacterium to another.
Lytic phages result in the formation of new viral particles and are called
productive infections
What 2 enzymes do restriction modification systems use?
restriction and modification enzymes
What protects bacteria from phage infection by quickly degrading incoming foreign DNA?
restriction modification systems