Microbiology lab Ch.13

Explain why HIV becomes resistant so quickly when a single drug is used therapeutically.

HIV uses reverse transcriptase to make a cDNA version of its RNA genome. This polymerase is "'sloppy," with a high rate of error and lack of a proofreading capability. This leads to a high rate of mutation of the viral genome and increased chance for drug resistance

Why are viroids resistant to nucleases?

Having a circular RNA "genome," they are resistant to most exonucleases (that digest the free ends of RNA or DNA).

Most temperate phages integrate into the host chromosome, whereas some replicate as plasmids. Which kind of relationship do you think would be more likely to maintain the phage in the host cell, and why?

Integration, because plasmids are frequently lost during cell division, which could leave a daughter cell without the virus genome.

The concentration of virus that infects or kills 50% of the host cells is referred to as the

LD50 AND ID50

Which is a filamentous phage?

M13

Would ID50 and LD50 necessarily be the same for a given virus? Why or why not?

No, because a virus may be highly infectious (very low ID50 value) but only marginally lethal (very high LD50 value), for example the rhinovirus (common cold virus).

An antibiotic is added to a culture of E. coli, resulting in death of the cells. Bacteriophages are then added. Would the phages replicate in the E. coli cells? Why or why not?

No, because the bacteriophages would depend too much on having the active machinery of a living host cell for replication.

Would you expect the number of virions to be the same if you measured them by the plaque assay or by counting using the electron microscope? Why?

No—the plaque assay only measures viable virus particles, while the electron microscope cannot distinguish between defective and viable virus.

The family to which the Rhinovirus belongs is the

Picornaviridae.

What part of the E. coli T4 phage attaches to the host cell receptors?

Protein fibers at the end of the phage tail.

What part of the attached bacteriophage enters through the host cell wall?

The nucleic acid.

You add an unknown phage to a mixture of F+ and F− cells of E. coli and plate out the bacteria. The bacterial colonies that grow are all F−. How can you explain this phenomenon?

The phage bound to a receptor on the sex pilus, and therefore only infected the F+ cells (leaving the F− cells alone).

Which statement about most phages that contain single-stranded DNA is NOT true?

They contain a negative-sense DNA strand.

Which of the following is not a characteristic of normal cells?

They grow as multiple layers.

Which statement(s) about cells taken from a tumor is/are true?

They may be used to grow viruses

A virion is a(n)

complete, extracellular virus particle.

The changes that occur in virally infected cells are characteristic for a particular virus and are referred to as the

cytopathic effect.

If the infecting phage lacks some critical pieces of DNA necessary for replication, it is called

defective.

The activation of the SOS system in a bacterium infected with a prophage results in

destruction of the viral repressor through host protease activity AND complete lysis of the bacterial culture.

The best known chronic infection involves

hepatitis B.

The best-known examples of viruses that cause latent infections are

herpes AND chickenpox.

Normal tissue taken from animals and prepared immediately as a medium for viral growth is termed a(n)

primary culture.

Phage-encoded enzymes are

produced in a sequential manner AND are used to customize the cell for viral production.

The approximate viral concentration of a sample may be determined by

quantal assay.

During attachment of phage to E. coli, the phage

randomly bumps into the bacteria AND attaches to proteins or carbohydrates on the bacterial surface.

A key feature of all viral infections is the

separation of viral nucleic acid from the capsid.

The terms helical, icosahedral, and complex refer to

shapes of viruses.

The filamentous phages all contain

single-stranded DNA.

Attachment of animal viruses to the host cell may be by means of

spikes.

The genome of retroviruses is made of

ssRNA.

In the replication of phage containing positive-sense DNA,

the host's enzymes are used to make dsDNA.

During penetration of E. coli by the T4 phage

the tail acts as a "hypodermic needle," injecting the phage DNA into the cell.

A filamentous phage is incapable of causing a lytic infection.

true

A lysogenic cell contains viral DNA (a prophage) integrated into the host chromosome

true

If a virus utilizes a lytic life cycle of reproduction, it will not induce tumors.

true

Non-enveloped and enveloped viruses both may enter the host via endocytosis.

true

The host range of a virus depends on the presence of host receptor molecules.

true

The integration of phage DNA into host DNA occurs in much the same fashion as seen in transformation, transduction, or conjugation.

true

Virulent as well as temperate phages can serve as generalized transducing phages.

true

Retroviruses are unique in that they

use RNA as a template to make DNA.

A phage that replicates inside the host cell and then lyses its host during its release is a

virulent or lytic phage

Plant viruses may be transmitted by

worms contaminated seeds. humans. insects. All of the choices are correct.

Plant viruses enter the host plant via

wound sites.

Spongiform encephalopathy occurs in all of the following EXCEPT

plants.

A limiting factor for viral infection of animals cells is

presence of specific receptor molecules on the host cell.

An infection in which the virus is continually present in the body is referred to as

persistent.

Animal viruses are divided into a number of families whose names end in

-viridae.

In the case of T-even phages, the burst size is about

200 per host cell.

The time from absorption to release for T-even phage is about

30 minutes.

There are ______ major families of DNA-containing viruses that infect vertebrates.

7

Which statement regarding phage replication is TRUE?

A lysogen is a bacterial cell with an integrated prophage.

Why do animal viruses have envelopes and phages rarely do?

Animal viruses will often use the envelope in order to fuse with the plasma membrane of a new target cell, gaining entry into the cytoplasm.

The nucleocapsid is composed of

DNA or RNA, and protein.

What are two ways that phage can replicate without directly lysing their host cell?

Extrusion and lysogeny

Completed filamentous phages are often found in the cytoplasm of infected bacteria

FALSE

The receptors to which animal virus attachment proteins usually bind are

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

Why is it virtually impossible to eradicate a disease caused by a zoonotic virus?

You would 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 answer choices are correct.

The site at which a virus has infected and subsequently lysed the infected cell, releasing its progeny to infect and lyse surrounding cells, thereby forming a "clear zone," is

a plaque.

If you had three tubes of flu viruses and you exposed tube 1 to protease, tube 2 to protease and RNase, and tube 3 to protease and DNase, and the contents of tube 2 were be degraded, you would be able to conclude that

an RNA virus.

Prions are

an infectious protein

Genetic exchange in segmented viruses that allows a zoonotic virus to infect humans is an example

antigenic shift.

Prions

are made of protein only

Transducing virulent phages do not lyse the cells they invade because

bacterial DNA has replaced critical viral DNA in the phage.

Viruses that infect bacteria are referred to as

bacteriophages.

In addition to lysis, animal viruses may exit the host cell by

budding

The shape of the virus is determined by its

capsid

Entry of bacteriophages and animal viruses into host cells

differs because bacteriophages leave the capsid outside the cell, while animal virus entry involves the entry of the whole nucleocapsid.

Viruses are commonly referred to by their ______ name.

disease AND species

If reasonably pure preparations of virus are available, the number of virus present may be determined by

electron microscopy.

When a non-enveloped animal virus adsorbs to the host cell with its protein spikes, the virions are taken into the cell by the process of

endocytosis.

The viral envelope closely resembles the

eukaryotic cytoplasmic membrane.

An exit method used by viruses that does not immediately destroy the host bacterium is

extrusion.

Bacteriophages, 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.

false

Capsids are made of a number of capsomeres that are covalently bonded to one another.

false

Capsids are made of a number of capsomeres that are covalently bonded to one another. True or False?

false

Electron microscopy is useful for counting viruses and distinguishing between infective and non-infective virions.

false

Infective and non-infective viruses may be distinguished by growth on MacConkey's agar.

false

The structure of plant, animal, and bacterial viruses are each, fundamentally, very different from one another.

false

Cells infected with animal viruses lyse because

functions necessary for cell survival are not carried out and the cell dies.

Which of the following BEST describes the protein coat of a virus? It

is called a capsid, protects the nucleic acid, AND is involved in the recognition of host cell receptors by non-enveloped viruses.

All of the statements about specialized transduction are correct EXCEPT

it involves the random transmission of any gene.

Resistance of some animals to certain viral diseases is based on

lack of specific receptors on the host cell.

The phenomenon responsible for the ability of Corynebacterium diphtherium to produce the virulent toxin responsible for the devastating effects of diphtheria is called

lysogenic conversion.

In the region of budding, the inside of the cytoplasmic membrane becomes coated with

matrix proteins.

Carriers

may have a persistent infection AND may be a source of infection.

Assembly of the T4 phage

may involve some self-assembly AND may involve the use of scaffolds.

Using phages to treat a bacterial infection is an interesting idea because

of the increasing problem of antibiotic resistance in pathogens.

Filamentous phages

only infect E. coli that have pili.

In latent infections, the virions are

only produced during reactivation.

One group of animal viruses that are able to agglutinate red blood cells are the

orthomyxoviruses