Exam 4: Viruses Part 1

Question 1

What key features are shared by all living things?

Answer 1

Cellular organisms
requirement for energy at all times
respond to the environment
possess DNA
growth
reproduction
evolution

Question 2

Which of the features of all living things are shared by viruses?

Answer 2

Possess DNA or RNA, reproduce only in hosts, and evolve very rapidly.

Question 3

Which of the features of all living things are not shared by viruses?

Answer 3

Acellular, require energy only when infecting, no response to environment, no growth.

Question 4

Do you think viruses should be considered alive? Why or why not?

Answer 4

No, I don’t think they possess enough of the key features of life. No consistent energy requirement and no growth. I think all other features can have caveats but theses are the two big ones.

Question 5

Roughly how many genes are possessed by bacteria such as Chlamydia?

Answer 5

Chlamydia is the smallest known cellular human pathogen at about 1,000 genes.

Question 6

What is the smallest number of genes possessed by a virus? How are they able to function with such a small number of genes?

Answer 6

3 genes, they lack many of the critical life functions so dont need genes to code for all of that. Instead they “borrow” this needed machinery from host cells.

Question 7

What does the term obligate intracellular parasite mean?

Answer 7

Have to be inside a host cell and derive nutrition from living hosts.

Question 8

Describe the variation in viral genomes.

Answer 8

Genetic material can be either DNA or RNA. Can be single-stranded or double-stranded. May be divided into one or more pieces. Can be linear or circular.

e.g HIV has two identical single stranded RNA that are linear. Papillomavirus one double stranded DNA that is circular.

Question 9

What structure surrounds the viral genome? Of what substance is this structure comprised? What is the combination of this structure and the genome called?

Answer 9

Capsid. Composed of proteins termed capsomeres, sometimes single type of protein and sometimes multiple types. Together the genome and capsid comprises the nucleocapsid.

Question 10

Describe the three basic viral shapes.

Answer 10

Polyhedral- commonly icosahedral (20 sides).

Helical- capsomeres form a tube.

Complex- many different shapes,
common with bacteriophages.

Simplest virus consists of only a nucleocapsid “naked virus” or “naked nucleocapsid”.

Question 11

Describe the nature of a viral envelope. What is it made of? Where is it in terms of the other parts of the virus? Do all viruses possess an envelope?

Answer 11

Enveloped viruses possess an envelope external to the capsid. Phospholipid bilayer with embedded proteins. Region between the capsid and envelope is filled with matrix proteins.

Acquired from the host cell during viral release, some proteins are virally encoded and some are encoded from by the host. Envelope not involved in active transport.

Question 12

How does an envelope affect the “sturdiness” of a virus?

Answer 12

Envelopes are both beneficial and deleterious. Provides protection from the immune system since it is derived from host membranes. However, they are more sensitive to alcohol, desiccation and detergents than naked viruses. Naked viruses are more stable outside the host.

Question 13

What is a viral spike? Of what substance is a spike comprised? Where is a spike located with respect to other viral components? (Be sure to consider both enveloped and nonenveloped viruses in your answer.)

Answer 13

Viral spikes give specificity to certain hosts. Spikes are viral surface molecules present in the capsid of naked viruses or the surface of enveloped viruses. Composed of glycoproteins. They act complementary to receptors on the cellular surface.

Question 14

Describe the physical interaction between a virus and a host cell.

Answer 14

Viruses display host specificity. The viral “spikes” connect complementary to the “receptors” on the host cell’s surface.

Question 15

What is the host range of a virus? What determines host range?

Answer 15

A given virus only infects cells within a given receptor, this is the “host range’. Determined by the viral “spikes”.

Can be a single species, strain of a single species, multiple species, specific cell types within a multicellular organism, etc.

E.g HIV only infects T-cells of virus primates. Hepatitis virus specifically infects liver cells.

Question 16

What are the key criteria used to classify viruses into evolutionary groups?

Answer 16

Genome structure
Presence/absence of envelope
Shape
Size

Question 17

Are all viruses descended from a common viral ancestor?

Answer 17

No. viruses are not descended from a common viral ancestor. Viruses represent genetic material that has escaped from genomes of cellular life.

Question 18

What classification groups are viruses place into?

Answer 18

All viruses have been classified into families. Only 3 orders have been established.

Question 19

What is the general form for the name of a viral family?

Answer 19

Family name typically ends in suffix -viridae. Generally derived from defining characteristics of the group. e.gPicornaviridae contains very small RNA viruses. Sometimes they are named for an important member, e.g. Herpesviridae ( named for a virus that causes genital herpes).

Question 20

What is the general form for the name of a viral genus?

Answer 20

Genus name typically ends in suffix -virus. Named for a feature of the group. e.g Lentiviruses are slow, lentus is Latin for slow.

Question 21

What is the general form for the name of a viral species and type?

Answer 21

Species names are generally named for diseases caused. e.g. Polioviruses cause polio.

Types, akin to subspecies, strains, etc. eg., HIV-1 group M subtype B.

Question 22

Convenient non-evolutionary grouping of viruses:

Answer 22

zoonose
sexually-transmitted viruses
respiratory viruses
enteric viruses

Question 23

Why are viruses grouped together in ways that do not represent evolutionary groupings?

Answer 23

Unlike cellular organisms, viruses do not have genes that are common to all species and so a single viral phylogenetic tree cannot be created.

Question 24

What is an enteric virus? What types of diseases are caused by enteric viruses?

Answer 24

Ingested in fecal-contaminated material, fecal-oral transmission. Rotaviruses, hepatitis E (enteric hepatitis), hepatitis A. Often causes gastroenteritis. Some systemic (body wide) disease e.g. Poliovirus.

Question 25

What is a respiratory virus? What types of diseases are caused by respiratory viruses?”

Answer 25

Inhaled and replicate in the respiratory tract. Remain localized in the respiratory tract. E.g., Rhinoviruses, coronaviruses. Inhaled viruses causing systemic diseases are not included.

Question 26

Why is the measles and mumps virus not considered a respiratory virus even though it is inhaled?

Answer 26

Because they do not stay in the respiratory system, they cause body wide disease.

Question 27

What is a zoonotic virus?Give Examples.

Answer 27

Transmitted from one animal species to another (including humans).

E.g., rabies (not transmitted to other humans, humans don’t bite humans).

E.g., Arboviruses. Arthropod-borne viruses (arthropods are “bugs”), Infect arthropods, replicate, infect vertebrates.

E.g. West Nile virus, zika virus, dengue virus.

Question 28

What are arthropods? What is the relevance of arthropods to viruses?

Answer 28

Bugs like mosquitoes, spiders, and ticks. The virus is able to replicate inside the arthropods. The arthropod has the ability to bite vertebrates transferring the virus.

Question 29

What are some examples of viruses that are transmitted through sexual activity? Is sexual activity the only means by which these viruses are transmitted?

Answer 29

Transmitted during sexual activity. May cause lesions in genital tract. e.g. Herpesviruses, papillomaviruses. Some cause systemic infections e.g., HIV and hepatitis B and C.

Question 30

Describe the structure of a viroid. Can they cause disease? Explain.

Answer 30

Viroids are non living infectious agents. Small, circular RNA molecules, no capsid. Pathogenic in plants. Viroid-like agents affect some fungi.

Question 31

What is a prion? What types of diseases are caused by prions?

Answer 31

Prions are non-living infectious agents. Consist exclusively of proteins, no genetic material. Reproduce via conversion.

Animals cause BSE “bovine spongiform encephalopathy” or “mad cow disease, chronic wasting disease (deer, elk), and scrapies (sheep).

In humans it causes vCJD (variant Creutzfeldt-jakob disease) and Kuru (shaking death, caused by cannibalism of the human spine and brain, consumption of the prions).

Question 32

How do prions “reproduce?”

Answer 32

Via conversion. Causes other proteins to alter their shapes to become prions. Reproduction does not involve the production of new proteins.

All mammals make PrP membrane proteins, which plays a role in normal brain function. This PrP can form two stable tertiary structures cellular PrP which has numerous a-helices (normal form) and prion PrP which has numerous B-pleated sheets (abnormal form). The Prion PrP bumps the cellular PrP and converts it to the Prion form.

Question 33

What is the effect of prions in the brain?

Answer 33

The prion shape clumps together, neurons fail and die, and the brain gains a spongy appearance “spongiform encephalopathies”.

Question 34

What is the difference between the PrP proteins in your brain and prion proteins?

Answer 34

PrP proteins in the brain have a-helices while the prion proteins have b-pleated sheets.

Question 35

Can all humans develop prion diseases? Why or why not?

Answer 35

Yes, human PrP proteins with methionine as the 129th amino acid can fold into the prion form. Valine is normally present at this location. About 40% of humans possess 129-met. Those with 129-met are at risk of contracting prion diseases but those with normal valine are not. Contracted via eating the infected animals.

Question 36

What feature of a virus and of host cells determines the host range of the virus.

Answer 36

The cell poses receptors complementary to spike proteins on the surface of the virus.

Question 37

What processes alter host range?

Answer 37

Phenotypic mixing and genetic reassortment.

Question 38

Describe the process of phenotypic mixing. Is this altered host range permanent? Explain.

Answer 38

Animal cells are sometimes simultaneously infected by two different viruses, host range overlaps but differ. Some mismatched virions can be produced, resulting in genetic material surrounded by the wrong capsid. Host range is temporarily altered, external spikes do not match the viral DNA. So for a single generation one of these mismatched virus spikes will connect it to a cell that it normally does not infect. It will insert its genetic DNA since the DNA is all correct it will produce its normal virus with its normal capsid.

Question 39

Describe the process of genetic reassortment. Is this altered host range permanent? Explain.

Answer 39

Some viruses possess segmented genomes. Multiple strains of these viruses exist, host ranges overlap but differ. Two strains can coinfect a single cell, e.g. avian influenza and swine influenza both infecting a human cell. Two strains with segmented genomes can infect a single cell. Segments of the multipartite genome can be mixed. New virions can have novel combinations of segments. Akin to variation production in meiosis. Depending on which strains spike coding genes are used will determine the host, but now genes that normally cannot enter that host can. Causes “antigenic shift”, new strains can avoid immunity already in place.

Question 40

What is an antigenic shift? How is it beneficial to viruses?

Answer 40

Shift in genetic material due to genetic reassortment. It can be beneficial viruses because the new strains can avoid immunity already in palace. It generates genetic variation speeding up evolution.

Question 41

Compare genetic reassortment of viruses with meiosis. How are they similar?

Answer 41

Similar to meiosis and the assortment of chromosomes. The new strain has all the needed genes but they are a chopped u combination of the two viral strains.

Question 42

What is a latent virus?

Answer 42

Some viruses can remain dormant within host cells. e.g. herpes viruses and HIV. latency can last for years. Similar to bacteriophage’s lysogenic cycle, some latent viruses are integrated into the host chromosome. Integration is permanent, induction does not occur, all descendants carry the provirus. Some latent viruses are not integrated.

Question 43

Which of the following is a sequence at which a restriction enzyme would cut?

• 5’AGCCGT3’
• 5AAGCTT’3’
• 5’AAATTT3’
• 5’AGGCCT3’
• 5’ACCGGA3’
• 5’ACGGCT3’

Answer 43

Not 5’ACGGCT3’

Question 44

Which of the following statements regarding retroviruses is FALSE?

• A cellular enzyme converts the RNA genome into DNA
• Double-stranded DNA is transcribed into mRNA by cellular enzymes
• Retroviral genomes consist of positive-sense RNA
• The virus commonly integrates into the host chromosome as a provirus
• Viral RNA enters the nucleus
• All these statements are TRUE

Answer 44

the virus commonly integrates into the host chromosome as a provirus

Question 45

During the lytic replication cycle of the bacteriophage T4, the phage ______________ enters the cell but the phage ______________ does not.

capsid protein… tail fibers

DNA… protein

envelope… DNA

enzymes… ribosome

flagellum… mitochondrion

RNA… capsid protein

Answer 45

DNA… protein

Question 46

Which of the following does NOT accurately describe the replication cycle of herpes virus?

apsomeres are produced in the cytoplasm and transported into the nucleus

DNA is replicated by cellular enzymes

RNA is transcribed from viral DNA using viral enzymes

the viral DNA enters the nucleus

viral mRNAs are translated by the cell’s ribosomes

all these statements are TRUE

Answer 46

RNA is transcribed from viral DNA using viral enzymes. Only mRNA involved

Question 47

Which of the following statements regarding viral classification is FALSE?

all viruses are not descended from a common viral ancestor

not all known viruses have been classified into genera and families

some viral families have been classified into orders

the presence or absence of an envelope is one criterion upon which viral classification is based

viral classification is based, in part, on the structure of the viral genome

all these statements are TRUE

Answer 47

not all known viruses have been classified into genera and families.All viruses have been classififed into families.