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