Ch 19: Genetics of Viruses

What is special about viruses that contain RNA?

-These are the only example where RNA carries genes from one generation to next -RNA viruses may be double or single stranded

Stages: Attachment Penetration Viral DNA attaches to host DNA and remains latent

-Viral DNA only replicates when host DNA replicates -Viral DNA attaches to specific site of host DNA -Eventually might become lytic

What is reverse transcriptase and where is it found?

An enzyme carried by retroviruses that make DNA from an RNA template

Why are viruses referred to as obligate parasites? A. They invariably kill any cell they infect. B. They must use enzymes encoded by the virus itself. C. They can incorporate nucleic acids from other viruses. D. Viral DNA always inserts itself into host DNA. E. They cannot reproduce outside of a host cell.

E

What is the function of reverse transcriptase in retroviruses?

It uses viral RNA as a template for DNA synthesis.

Where does the envelope of enveloped virsuses come from, and what's it made from?

It's made from lipids, proteins and carbs. The envelope comes from the modified host membrane (after infection of the host)

Which of these binds to receptor molecules on the host cell membrane?

Glycoproteins on the viral envelope recognize and bind to receptors on the host cell.

What are prions?

Misfolded versions of normal brain protein

Capsid

Protein coat on outside of virus

What are the 2 names given to viruses attached to host DNA?

Provirus or Prophage

What type of genetic material do viruses contain?

Nucleic acid, either DNA or RNA; never both

Shape is determined by the pattern of the capsomeres

Shape is typically either helical or polyhedral or combination of both

Which cell undergoes a lysogenic cycle?

Temperate viruses

A researcher lyses a cell that contains nucleic acid molecules and capsomeres of tobacco mosaic virus (TMV). The cell contents are left in a covered test tube overnight. The next day this mixture is sprayed on tobacco plants. Which of the following would be expected to occur?

The plants would develop the typical symptoms of TMV infection.

Which of the following represents a difference between viruses and viroids?

Viruses have capsids composed of protein, whereas viroids have no capsids.

What is a lytic cycle?

When a host cell is taken over, used to replicate viruses, and (typically) destroyed

How are neuron infections different?

When nerve cells are infected with poliovirus they can not be replaced because neurons do not divide (so the damage is permanent)

What is Penetration?

Whole virus may enter host or only nucleic acid may enter

Which of the following describes plant virus infections?

They are spread via the plasmodesmata.

Viruses that infect bacteria are called __________.

bacteriophages

In many ways, the regulation of the genes of a particular group of viruses will be similar to the regulation of the host genes. Therefore, which of the following would you expect of the genes of the bacteriophage?

control of more than one gene in an operon

Which of the following accounts for someone who has had a herpesvirus-mediated cold sore or genital sore getting flare-ups for the rest of his or her life?

copies of the herpesvirus genome permanently maintained in host nuclei

What is self-assembly?

many copies of viral nucleic acid and capsomere - they combine spontaneously to form the mature virus particle

Which of the following characteristics, structures, or processes is common to both bacteria and viruses?

genetic material composed of nucleic acid

Which of the following can be effective in preventing the onset of viral infection in humans?

getting vaccinated

What is Release?

host cell may be lysed to release virus particles, in animal cells virus may be released by exocytosis

Most human-infecting viruses are maintained in the human population only. However, a zoonosis is a disease that is transmitted from other vertebrates to humans, at least sporadically, without requiring viral mutation. Which of the following is the best example of a zoonosis?

rabies

Obligate parasites

require a host cell

The lytic cycle of bacteriophage infection ends with the _____.

rupture of the bacterium

The host range of a virus is determined by

the proteins on its surface and that of the host.

Some viruses can be crystallized and their structures analyzed. One such virus is Desmodium, or yellow mottle virus, which infects beans. This is a member of the tymovirus group and has a single-stranded RNA genome of ~6,300 nucleotides. Its virion is 25—30 nm in diameter, and is made up of 180 copies of a single capsid protein that self-associate to form each capsomere, which has icosahedral symmetry with 20 facets. 30) If this virus has a positive RNA strand as its genome, it begins the infection by using this strand as mRNA. Therefore, which of the following do you expect to be able to measure?

translation rate

To which cycle does a temperate virus go when they infect the host?

Lysogenic cycle

What are the 2 causes of viral diseases?

1. Some viruses damage or kill cells by causing the release of hydrolytic enzymes from host lysosomes 2. Other viruses cause the infected cells to produce toxins that lead to disease symptoms

Which of the following is characteristic of the lytic cycle?

A large number of phages are released at a time.

Which of the following series best reflects what we know about how the flu virus moves between species?

An animal such as a pig is infected with more than one virus, genetic recombination occurs, the new virus mutates and is passed to a new species such as a bird, the virus mutates and can be transmitted to humans.

Viral envelopes can best be analyzed with which of the following techniques?

Antibodies against specific proteins not found in the host membranes

What are the 5 stages of the lytic cycle?

Attachment to host cell, Penetration, Synthesis of genome and proteins, Self assembly, Release

RNA viruses require their own supply of certain enzymes because A) host cells rapidly destroy the viruses. B) host cells lack enzymes that can replicate the viral genome. C) these enzymes translate viral mRNA into proteins. D) these enzymes penetrate host cell membranes. E) these enzymes cannot be made in host cells.

B

Double-stranded viral DNA is incorporated into a host cell as a _____. promoter provirus transposon lac homeoboxes

B. "Provirus" is the name given to double-stranded viral DNA that has been incorporated into a host cell's genome.

As a result of the lytic cycle, _____. the host cell is not destroyed the host cell's DNA is destroyed viral ribosomes are produced viral DNA is incorporated into host cell DNA a prophage is created

B. The host cell's DNA is destroyed, and ultimately, the host cell itself is destroyed in the lytic cycle.

Which of the following is characteristic of the lytic cycle? The viral genome replicates without destroying the host. The virus-host relationship usually lasts for generations. A large number of phages is released at a time. Viral DNA is incorporated into the host genome. Many bacterial cells containing viral DNA are produced.

C

To cause a human pandemic, the H5N1 avian flu virus would have to A) spread to primates such as chimpanzees. B) develop into a virus with a different host range. C) become capable of human-to-human transmission. D) arise independently in chickens in North and South America. E) become much more pathogenic.

C) become capable of human-to-human transmission.

Intracellular State

Can take over host cell and replicate themselves

Individual proteins in capsid

Capsomeres

Retrovirus

Complex single-stranded RNA virus

44) Emerging viruses arise by A) mutation of existing viruses. B) the spread of existing viruses to new host species. C) the spread of existing viruses more widely within their host species. D) mutation of existing viruses, the spread of existing viruses to new host species, and the spread of existing viruses more widely within their host species. E) none of these.

D) mutation of existing viruses, the spread of existing viruses to new host species, and the spread of existing viruses more widely within their host species.

A bacterium is infected with an experimentally constructed bacteriophage composed of the T2 phage protein coat and T4 phage DNA. The new phages produced would have A) T2 protein and T4 DNA. B) T2 protein and T2 DNA. C) a mixture of the DNA and proteins of both phages. D) T4 protein and T4 DNA. E) T4 protein and T2 DNA.

D. T4 protein and T4 DNA.

How are the genes coded?

Either by DNA or RNA, but RNA is never the genetic material in a cellular organism

How are animal viruses taken up?

Endocytosis

What are vaccines and how do they work?

Harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen. They sensitize the immune system to react vigorously if exposed to real virus.

What is the source of a viral envelope?

Host cell membrane

You isolate an infectious substance that is capable of causing disease in plants, but you do not know whether the infectious agent is a bacterium, virus, viroid, or prion. You have four methods at your disposal that you can use to analyze the substance in order to determine the nature of the infectious agent. I. treating the substance with nucleases that destroy all nucleic acids and then determining whether it is still infectious II. filtering the substance to remove all elements smaller than what can be easily seen under a light microscope III. culturing the substance by itself on nutritive medium, away from any plant cells IV. treating the sample with proteases that digest all proteins and then determining whether it is still infectious 37) Which treatment could definitively determine whether or not the component is a viroid?

I

You isolate an infectious substance that is capable of causing disease in plants, but you do not know whether the infectious agent is a bacterium, virus, viroid, or prion. You have four methods at your disposal that you can use to analyze the substance in order to determine the nature of the infectious agent. I. treating the substance with nucleases that destroy all nucleic acids and then determining whether it is still infectious II. filtering the substance to remove all elements smaller than what can be easily seen under a light microscope III. culturing the substance by itself on nutritive medium, away from any plant cells IV. treating the sample with proteases that digest all proteins and then determining whether it is still infectious 38) If you already knew that the infectious agent was either bacterial or viral, which treatment would allow you to distinguish between these two possibilities?

III

You isolate an infectious substance that is capable of causing disease in plants, but you do not know whether the infectious agent is a bacterium, virus, viroid, or prion. You have four methods at your disposal that you can use to analyze the substance in order to determine the nature of the infectious agent. I. treating the substance with nucleases that destroy all nucleic acids and then determining whether it is still infectious II. filtering the substance to remove all elements smaller than what can be easily seen under a light microscope III. culturing the substance by itself on nutritive medium, away from any plant cells IV. treating the sample with proteases that digest all proteins and then determining whether it is still infectious 39) Which treatment would you use to determine if the agent is a prion?

IV only

What is the function of reverse transcriptase? It catalyzes the formation of a polypeptide from an RNA template. It catalyzes the formation of DNA from a polypeptide template. It catalyzes the formation of RNA from a polypeptide template. It catalyzes the formation of RNA from a DNA template. It catalyzes the formation of DNA from an RNA template.

It catalyzes the formation of DNA from an RNA template.

Why do RNA viruses appear to have higher rates of mutation?

Replication of their genomes does not involve proofreading.

Viral genomes vary greatly in size and may include from four genes to several hundred genes. Which of the following viral features is most apt to correlate with the size of the genome?

Size and shape of the capsid

What is Attachment?

Specificity between proteins on virus surface and receptor molecules on host surface

Poliovirus is a positive-sense RNA virus of the picornavirus group. At its 5' end, the RNA genome has a viral protein (VPg) instead of a 5' cap. This is followed by a nontranslated leader sequence, and then a single long protein coding region (~7,000 nucleotides), followed by a poly-A tail. Observations were made that used radioactive amino acid analogues. Short period use of the radioactive amino acids result in labeling of only very long proteins, while longer periods of labeling result in several different short polypeptides. 33) What conclusion is most consistent with the results of the radioactive labeling experiment?

The RNA is only translated into a single long polypeptide, which is then cleaved into shorter ones.

What determines the damage done by a virus?

The damage caused by a virus depends partly on the ability of the cells in the infected tissue to regenerate by cell division

Which of the following is the most probable fate of a newly emerging virus that causes high mortality in its host?

The newly emerging virus will die out rather quickly or will mutate to be far less lethal.

Which of the following statements describes the lysogenic cycle of lambda (λ) phage?

The phage genome replicates along with the host genome.

The herpes viruses are very important enveloped DNA viruses that cause disease in all vertebrate species and in some invertebrates such as oysters. Some of the human ones are herpes simplex (HSV) I and II, causing facial and genital lesions, and the varicella-zoster (VSV), causing chicken pox and shingles. Each of these three actively infect nervous tissue. Primary infections are fairly mild, but the virus is not then cleared from the host; rather, viral genomes are maintained in cells in a latent phase. The virus can then reactivate, replicate again, and be infectious to others. 41) In electron micrographs of HSV infection, it can be seen that the intact virus initially reacts with cell surface proteoglycans, then with specific receptors. This is later followed by viral capsids docking with nuclear pores. Afterward, the capsids go from being full to being "empty." Which of the following best fits these observations?

The viral envelope mediates entry into the cell, the capsid entry into the nuclear membrane, and the genome is all that enters the nucleus.

True or false: Antibiotics are not effective against viruses

True

What is the replication of viral nucleic acid?

Viral genes are transcribed, translated to form viral proteins (capsid) Virus takes over host cell - uses energy, subunits, ribosomes, tRNAs of the host cell to make more virus Great deal of variation among viruses , (some have 3 genes, some have 20)

Most molecular biologists think that viruses originated from fragments of cellular nucleic acid. Which of the following observations supports this theory?

Viral genomes are usually similar to the genome of the host cell.

Which cell undergoes a lytic cycle?

Virulent viruses

What is a lysogenic cycle?

Virus genes remain dormant. Viruses can then undergo a lytic cycle where they destroy the cell

Why are viral diseases so difficult to treat?

Virus has no distinctive metabolism of its own - after penetration, virus takes over host cell and uses host metabolism

Extracellular State

Virus particle. Cannot replicate itself

How are plant viruses taken up?

Viruses enter through injuries or insects

Which of the following is the best predictor of how much damage a virus causes?

ability of the infected cell to undergo normal cell division

The herpes viruses are very important enveloped DNA viruses that cause disease in all vertebrate species and in some invertebrates such as oysters. Some of the human ones are herpes simplex (HSV) I and II, causing facial and genital lesions, and the varicella-zoster (VSV), causing chicken pox and shingles. Each of these three actively infect nervous tissue. Primary infections are fairly mild, but the virus is not then cleared from the host; rather, viral genomes are maintained in cells in a latent phase. The virus can then reactivate, replicate again, and be infectious to others. 42) In order to be able to remain latent in an infected live cell, HSV must be able to shut down what process?

apoptosis of a virally infected cell

Why do we recover completely from colds?

because the epithelium of the respiratory tract can repair itself efficiently

The herpes viruses are very important enveloped DNA viruses that cause disease in all vertebrate species and in some invertebrates such as oysters. Some of the human ones are herpes simplex (HSV) I and II, causing facial and genital lesions, and the varicella-zoster (VSV), causing chicken pox and shingles. Each of these three actively infect nervous tissue. Primary infections are fairly mild, but the virus is not then cleared from the host; rather, viral genomes are maintained in cells in a latent phase. The virus can then reactivate, replicate again, and be infectious to others. 40) If scientists are trying to use what they know about HSV to devise a means of protecting other people from being infected, which of the following would have the best chance of lowering the number of new cases of infection?

interference with new viral replication in preexisting cases

Antiviral drugs that have become useful are usually associated with which of the following properties?

interference with viral replication

Which viruses have single-stranded RNA that acts as a template for DNA synthesis?

retroviruses

Some viruses can be crystallized and their structures analyzed. One such virus is Desmodium, or yellow mottle virus, which infects beans. This is a member of the tymovirus group and has a single-stranded RNA genome of ~6,300 nucleotides. Its virion is 25—30 nm in diameter, and is made up of 180 copies of a single capsid protein that self-associate to form each capsomere, which has icosahedral symmetry with 20 facets. 31) In a cell-free system, what other components would you have to provide for this virus to express its genes?

ribosomes, tRNAs, amino acids, and GTP

The difference between vertical and horizontal transmission of plant viruses is that

vertical transmission is transmission of a virus from a parent plant to its progeny, and horizontal transmission is one plant spreading the virus to another plant.

Some viruses can be crystallized and their structures analyzed. One such virus is Desmodium, or yellow mottle virus, which infects beans. This is a member of the tymovirus group and has a single-stranded RNA genome of ~6,300 nucleotides. Its virion is 25—30 nm in diameter, and is made up of 180 copies of a single capsid protein that self-associate to form each capsomere, which has icosahedral symmetry with 20 facets. 28) If this virus has capsomeres with 20 facets, how many proteins form each one?

~6

Some viruses can be crystallized and their structures analyzed. One such virus is Desmodium, or yellow mottle virus, which infects beans. This is a member of the tymovirus group and has a single-stranded RNA genome of ~6,300 nucleotides. Its virion is 25—30 nm in diameter, and is made up of 180 copies of a single capsid protein that self-associate to form each capsomere, which has icosahedral symmetry with 20 facets. 29) How many nucleotides of the genome would you expect to find in one capsid?

~6,300