Ch 6 Intro to Viruses

Question 1

What are Viruses?

Answer 1

• Acellular infectious agents smaller than cells
• Obligate intracellular parasites: can only multiple when it invades a host cell for genetic and metabolic material

Question 2

What makes a virus so complicated?

Answer 2

• It causes disease in plants, animals, bacteria, protozoa, fungi, & algae
• There is no agreement as to how or when a virus originated
• Plays a role in the evolution of Bacteria, Archaea, and Eukarya
• Debate as to whether it is living or nonliving
• Does NOT classify as prokaryote or eukaryote

Question 3

Viruses contain only the parts needed to ____________. Viruses bear no resemblance to cells because they _______________.

Answer 3

a. invade and control a host cell
b. Lack protein-synthesizing machinery

Question 4

What structures make up a virus?

Answer 4

1. Virus particle
2. Covering -> Capsid & Envelope (some)
3. Central Core -> DNA or RNA & Matrix protein enzymes (some)

Question 5

Envelope

Answer 5

Def: Lipid Bilayer

Function:
- Exposed proteins on the outside of the envelope, called spikes used to attach themselves to the host

• Infect cells by fusion of the envelope with the host’s plasma membrane

Question 6

Capsid

Answer 6

Def: Protein coat made up of capsomers

Function:
- Attach to host cells
- Protects Nucleic Acids
- Determines shape

Question 7

The capsid together with the nucleic acid is the _________. Some viruses have an external covering called an ________; those lacking an envelope are __________.

Answer 7

a. nucleocapsid
b. envelope
c. naked

Question 8

Two structural capsid types:

Answer 8

1. Helical - continuous helix of capsomers forming a cylindrical nucleocapsid
2. Icosahedral - 20-sided with 12 corners

Question 9

Nucleic acids & their characteristics

Answer 9

1. DNA: usually double-stranded, can be single, linear, or circular
2. RNA: usually single-stranded, can be double-stranded, may be segmented into separate RNA pieces

Question 10

Complex viruses

Answer 10

Def: asymmetrical structures

• lacks a typical capsid, may not be helical or icosahedral
• may have helical tail and attachment fibers

Question 11

Give an example of an enveloped, naked, and complex virus

Answer 11

Envelope: mumps virus
Naked: plum poxvirus
Complex: poxviruse

Question 12

Viral genome

Answer 12

• DNA or RNA
• Carries genes necessary to invade host cell and redirect cell’s activity to make new viruses

Question 13

ssRNA genomes ready for immediate translation are ______________.
ssRNA genomes that must be converted into proper form are _________________.

Answer 13

a. positive-sense RNA
b. negative-sense RNA

Question 14

How are viruses classified?

Answer 14

Structure, Chemical composition, & Genetic make-up

Question 15

How do animal viruses replicate?

Answer 15

Adsorption – virus attaches itself to the receptor of the host cell

Penetration – genome enters the host cell

Uncoating – the viral nucleic acid is released from the capsid

Synthesis – viral components are produced

Assembly – new viral particles are constructed

Release – assembled viruses are released to infect other cells

Question 16

What makes the replication process of RNA and DNA viruses different?

Answer 16

RNA needs DNA to replicate, so it undergoes reverse transcription to create the DNA to replicate.

Traditional transcription: DNA = RNA = Protein
Reverse transcription: Protein = RNA = DNA

Question 17

Penetration: Endocytosis

Answer 17

the entire virus is engulfed and enclosed in a vacuole or vesicle

Question 18

Penetration: Fusion

Answer 18

envelope merges directly with membrane resulting in nucleocapsid’s entry into cytoplasm

Question 19

DNA viruses generally are replicated and assembled in the __________.

Answer 19

nucleus

Question 20

RNA viruses generally are replicated and assembled in the _________.

Answer 20

cytoplasm

Question 21

Negative-sense RNA must be converted into ________________ because a positive-sense RNA contains the message for ____________.

Answer 21

a. positive-sense message
b. translation

Question 22

What is the difference between translation and transcription?

Question 23

How do bacteriophage viruses replicate?

Answer 23

Adsorption – virus sticks onto specific molecules on host cell

Penetration – genome enters host cell

Replication – viral components are produced

Assembly – viral components are assembled

Maturation – completion of viral formation

Lysis & Release – viruses leave the cell to infect other cells