EXAM 3: Viruses

Question 1

Virus

Answer 1

An acellular infectious agent

Question 2

Viroid

Answer 2

A subviral particle lacking a capsid and consists soley as RNA

Question 3

Virusoid

Answer 3

A subviral particle lacking a capsid, consists solely as RNA, and requires a virus for replication

Question 4

Prion

Answer 4

A misfolded, infectious protein

Question 5

Capsid

Answer 5

The proteinaceous coat that protects viral genetic information

Question 6

Capsomere

Answer 6

The subunits of a capsid

Question 7

Prophage

Answer 7

A bacteriophage genome that is integrated into the DNA of a host cell bacterial cell

Question 8

Provirus

Answer 8

A virus genome that is integrated into the DNA of a host cell

Question 9

Latency

Answer 9

The ability of a pathogenic virus to lie dormant within a cell

Question 10

Oncogene

Answer 10

A gene that has the potential to cause cancer

Question 11

Acellular infectious agents components

Answer 11

• Viruses – protein and nucleic acid
• Viroids – only RNA (plant pathogen that lacks protein coat)
• Virusoids – only RNA (dependent on plant virus to replicate)
• Prions – proteins only (misfolded protein)

Question 12

Characteristics of viruses:

Answer 12

1. Miniscule, acellular infectious agents having either DNA or RNA
2. Causes many infections of humans, animals, plants, and bacteria
3. Have extracellular and intracellular states
4. NO cytoplasmic membrane, cytosol, or organelles→ contains protein coat or may contain envelope
5. Neither grow nor respond to environment → viral particles assemble into viron, exhibit tropism & use host actin tails
6. Cannot carry out metabolic pathway → can redirect host cell metabolic pathways
7. Cannot respond to environment → attach host cell receptors for entry, assemble exit host cell and affect others
8. Cannot reproduce independently→ Do replicate in host cell, like many parasitic organisms that require a host

Question 13

List the criteria used to classify viruses

Answer 13

Determined by ICTV (international committee on taxonomy of viruses):
* Nucleic acid type
* Presence or absence of envelope
* Capsid symmetry
* Dimension of virion and capsid

David Baltimore Identification
– focuses on viral genome and process used to synthesize viral mRNA
- list them in order

DNA categorized by:
* Their genomic structure
* Presence of an envelope
* Size and shape of their capsid
* The host cells they attack
3 Types: dsDNA, ssDNA, dsDNA-RT

RNA viruses are categorized by:
* Their genomic structure
* Presence of an envelope
* Size and shape of their capsid
4 Types: +ssRNA, –ssRNA, dsRNA, RNA retroviruses

Question 14

Compare and contrast lytic and lysogenic replication cycles

Answer 14

• Both are mechanisms of viral reproduction, they occur within a host cell

Lysogenic
- Infected host cells grow and reproduce normally for generations before they lyse
- virus is considered dormant because it inserts its own genome into the host’s genome and does not harm the host.
- Each time the host genome undergoes replication, so does the viral genome.
* Viral genetic information integrated into the chromosome
* Prophages: inactive phages
* Provirus: inactive virus

Lytic
- Replication cycle usually results in death and lysis of host cell
- virus takes over host to replicate and does cause harm to the host. The viral particles produced can lyse the host cell to find other hosts to infect
- Stages: Attachment, Entry, Synthesis, Assembly, Release (AESAR)

Lysogenic conversion results when phages carry genes that alter phenotype of a bacterium
Two advantages to lysogeny for virus:
* Phage remains viable but may not replicate
* Multiplicity of infection ensures survival of host cell

Question 15

Compare and contrast various mechanisms of viral entry and exit

Answer 15

Entry:
Varies between naked or enveloped virus
3 mechanisms of entry of animal viruses
1. Direct injection of nucleic acid
2. Fusion of the viral envelope with host membrane (nucleocapsid enters)
3. Endocytosis in vesicle (endosome aids in viral uncoating)

Exit:
Enveloped viruses use budding
* Viral proteins are first incorporated into host membrane
* Nucleocapsid may bind to viral proteins
* Envelope derived from host cell membrane, but may be golgi, ER, or other
* Virus may use host actin tails to propel through host membrane

Nonenveloped viruses lyse the host cell
* viral proteins may attack peptidoglycan or membrane

Question 16

Describe the potential effects of viral infection on host cells, including latency and oncogenesis

Answer 16

Latency:
* Viruses remain hidden or dormant inside host cell which cause latent infections
* May cause an acute infection before becoming dormant
* Remain dormant by existing as circular viral genome molecules outside of host chromosome or by becoming proviruses by integrating into host genome

Oncogenesis:
* Process of normal cells turning into cancer cells
*Activation of oncogenes
*Series of genetic and cellular changes
*Cells divide in uncontrolled manner

Question 17

Describe the role of viruses in oncogenesis. Discuss the functioning of oncogenes

Answer 17

• Oncogenesis allows viruses to encode for proteins that break down the host cell genome and prevent cell death which leads to uncontrolled cell growth.
• Oncogenes are a gene that when activated have the potential to cause cancer
  Metastasis: spread of cancerous cells throughout the body
  Benign: tumor remains in place
  Malignant: abnormal growth of neoplastic cells that form lump of tissue
  Neoplasia: uncontrolled division in multicell. animals

Viruses cause ~20-25% of human cancers
- Some viruses can target oncogenes
- Some carry copies of oncogenes as part of their genomes
- Some promote oncogenes already present in host
- Some interfere with tumor repression when inserted into hosts repressor gene

Question 18

Discuss ways to culture and visualize viruses

Answer 18

CULTURING
In mature organisms:
* In bacteria
* In Plants and animals

In embryonic chicken eggs:
* Inexpensive, among the largest of cells, free of contaminating microbes, contain nourishing yolk

In cell tissue culture:
* Consists of cells isolated from an organism & grown on a medium/broth
Two types of cell cultures: Diploid & Continuous cell cultures

VISUALIZING
Plaque assays:
* Dilutions of virus preparation made and plated on lawn of host cells
* Number of plaques counted
* Results expressed as plaque-forming units/ mL

Viral plaques:
* Viral plaques in a lawn of bacterial growth

Question 19

Define viroid, virusoid, and prion. Compare these subviral agents to viruses and other living organisms.

Answer 19

Viroid: A subviral particle lacking a capsid and consist solely as RNA
* extremely small, circular pieces of RNA that are infectious and pathogenic in plants
* Do not encode gene products
* May appear linear due to H bonding

Virusoid: A subviral particle lacking a capsid, consist solely as RNA, and requires a virus for replication
* Infectious agents composed only of closed, circular ssRNAs
* Do not encode gene products
* Requires host cell DNA-dependent RNA polymerase to replicate
*Cause plant diseases → found in nucleolus or chloroplasts; disease caused by RNA silencing

Prion: A misfolded, infectious protein, only affecting the brain
* Proteinaceous infectious agents (domino effect)
- Cellular PrP protein
* Made by all mammals
* Function unknown
* Normal structure with a-helices called cellular PrP

• Prion PrP
• Disease-causing form with B-sheets called prion PrP
• Converts cellular PrP into prion PrP by inducing conformational change

Question 20

Are viruses alive? Discuss why the status of the virus as a living organism is controversial. How does viral infection lead to morbidity and mortality?

Answer 20

• No viruses are not alive because they do not meet all the requirements (cannot carry out metabolic pathways and cannot reproduce independently/require a host)
• Others think it’s a complex pathogenic chemical that lacks characteristics of life (list them) CRRMG

Others consider them to be the least complex living entities because they:
* Use sophisticated methods to invade cells
* Have the ability to take control of their host cells
* Are able to replicate themselves

Viral infection leads to morbidity and mortality:
* Viruses cause 20-25% of human cancers
- Some carry copies of oncogenes as part of their genomes
- Some promote oncogenes already present in host
- Some interfere with tumor repression when inserted into hosts repressor gene
- Specific viruses are known to cause 15% of human cancer

• Once viral infections invade a host cell they can begin to replicate and destroy more and more cells (possibly activating oncogene and uncontrolled cell growth)