Chapter 13: Viruses, Viroids, and Prions

Question 1

Define virus

Answer 1

• Minuscule, acellular, infectious agent having either DNA or RNA
• Cause infections of humans, animals, plants, and bacteria (EX common cold, influenza, herpes, AIDS)
• Cannot carry out any metabolic pathway
• Neither grow nor respond to the enviroment
• Cannot reproduce independently; recruit the cell’s metabolic pathways to increase its numbers
• No cytoplasmic membrane, cytosol, organelles
• Has extracellular and intracellular state

Tiny infectious acellular agent w/ nucleic acid surrounded by proteinaceous capsomeres that form a covering called a capsid

Question 2

Define virion

Answer 2

Extracellular state
- Consists of a protein coat (capsid) surrounding nucleic acid; also known as nucleocapsid
- Some have phospholipid envelope
- Outermost layer provides protection and recognition sites that bind to host cells

Question 3

Discuss the viral genomes in terms of dsDNA, ssDNA, ssRNA, dsRNA, and number of segments of nucleic acid.

Answer 3

• Viruses show more variety in nature of their genomes than do cells and are smaller than genomes of cells
• Primary way scientists catgetorize and classify viruses
  **- A viral genome may be DNA or RNA but never both **
  Can be:
• Double stranded DNA (dsDNA)
• Single stranded RNA (ssRNA)
• Single stranded DNA (ssDNA)
• Double stranded RNA (dsRNA)
  May be linear and segmented or single and circular

Question 4

Explain the mechanism by which viruses are specific for their host cells

Answer 4

• Most viruses infect only particular host’s cells due to the affinity of viral surface proteins for complementary proteins on the host cell surface
• Some viruses may be so specific that they infect not only a particular host but also only a particular kind of cell in that host (EX. HIV specifically binds to CD4 receptors on T helper cells
• Some viruses are generalists that they infect many kinds of cells in many different hosts including bacteria, archaea, protozoa, fungi, plants, animals and humans (EX. Influenza virus, West Nile virus)

Question 5

Compare and contrast viruses of fungi, plants, animals, and bacteria.

Answer 5

• A virus that infects bacteria is referred to as a bacteriaphage or phage
• Viruses of plants were first identified from tobacco plants. Viruses of plants are intoduced to plant cells either through abrasions of the cell wall or by plant parasites
• Fungal viruses appear to only exist within cells and have no extracellular state. Because fusion of cells is typically a part of a fungal life cycle, viral infections can easily be propagated by the fusion of an infected fungal cell with an uninfected one

Question 6

Discuss the structure and function of the viral capsid.

Answer 6

Capsids - Protein coats that provide protection for viral nucleic acid and menas of attachment to host’s cells
- Composed of proteinaceous subunits called capsomeres
- Capsomere may be made of single or multiple types of proteins

Question 7

Discuss the origin, structure, and function of the viral envelope.

Answer 7

• A virus with an outer envelope surrounding the capsid is an enveloped virion
• Virion without an envelope is called a non-enveloped or naked virion
• Matrix proteins fill the region between capsid and evelope

Question 8

Describe viral envelope

Answer 8

• A virus with an outer envelope surrounding the capside
  - Envelope is portion of membrane system of the host
• Matrix proteins full the region between capsid and envelope

Question 9

Discuss the origin of viral envelope

Answer 9

• Acquired from host cell during viral replication or release

Question 10

Discuss the structure of viral envelope

Answer 10

• Viral envelope is composed of phospholipid bilayer and proteins
• Some proteins are virally coded glycoproteins (spikes)

Question 11

Discuss the function of viral envelope

Answer 11

• Envelope’s proteins and glycoproteins often play a role in host recognition and binding

Question 12

What fills the region between the capsid and envelope

Answer 12

Matrix proteins

Question 13

List the characteristics by which viruses are classified

Answer 13

• Type of nucleic acid
• Presence of envelope
• Shape
• Size
• Host
• Disease

Question 14

What are the five stages of the lytic replication cycle as it typically occurs in bacteriophages.

Answer 14

• Attachment
• Entry
• Synthesis
• Assembly
• Release

Question 15

Define lytic replication

Answer 15

• Replication cycle usually results in death and lysis of host cell where viral replication is dependent on host’s organelles and enzymes to produce new virions

Question 16

Define lysogeny

Answer 16

• Modified replication cycle
• Infected host cells grow and reproduce normally for generations before they lyse
• The phages are called temperate phages or lysogenic phages
• The inactive bacteriophage is called a prophage