Chapter 9

Question 1

Introduction to viruses

Answer 1

Genetic elements in extracellular form.
Viruses give their host cell new properties.
Effect of new properties will vary:
-some viruses are helpful in spreading genetic information (transduction).
-Most viruses harmful to their host

Question 2

Are viruses alive?

Answer 2

1. Have DNA or RNA, but never both.
2. Replication takes place only in a host cell.
3. Does not divide by binary fission or mitosis
4. Lack genes for energy production
5. Depends on host for ribosomes and nutrients.

Question 3

Virus background

Answer 3

Viruses replicate independently of cells chromosome, not independently of cell.
Extracellular form enables existence outside of host.
Extracellular form facilitates transmission.

Question 4

Two states of being

Answer 4

Extracellular 
-inert and cannot replicate
-called virus particles or virions
Intracellular
-Viral replication occurs 
-New viruses released from cell

Question 5

What is a virus?

Answer 5

Virus: genetic element that cannot replicate independently
Virology: study of viruses
Virus particle (virion): extracellular form
-exists outside host and facilitates transmission
-nucleic acid genome surrounded by protein coat and sometimes other things

Question 6

Viral genomes

Answer 6

Either DNA or RNA genomes.
Some circular, but most linear.
Double stranded and single stranded.

Question 7

Viral taxonomy

Answer 7

Viruses can be classified by hosts they infect. 
-bacterial viruses
-archaeal viruses
-animal viruses 
-plant viruses
Kind of genome. 
Sequence of genome.

Question 8

Viral size

Answer 8

Most viruses are smaller than prokaryotes; .02 to .3 um.

Most viral genomes are smaller than those of cells - 5000 bp to 250000 bp.

Question 9

Viral structure

Answer 9

Capsid: protein shell
-composed of individual protein subunits.
Nucleocapsid core: capsid + genome.
Capsomere: subunit of the capsid.

Question 10

Tobacco mosaic virus

Answer 10

Infectivity was reduced 12-fold when virus was exposed to solar radiation for several minutes.

Question 11

Viral shape

Answer 11

Constructed in highly symmetric ways
-Helical symmetry: rod shaped viruses.
–length determined by nucleic acid length
–width determined by protein subunits
-Icosahedral symmetry: spherical viruses.
–Most efficient arrangement of subunits in a closed shell.
Others: Enveloped, complex,…

Question 12

Enveloped virus

Answer 12

Virus that contains additional layers around nucleocapsid.

• membrane surrounding nucleocapsid
• lipid bilayer with embedded proteins
• envelope makes initial contact with host cell.

Question 13

Enveloped virus: protein membrane change

Answer 13

Envelope proteins continually changing producing new antigens.
Antigenic drift: year to year minor variations (mutations) in viral membrane proteins.
Antigenic shift: major changes in these proteins.

Question 14

Complex viruses

Answer 14

Virions composed of several parts, each with separate shapes and symmetries.
Bacterial viruses contain complicated structures.
Icosahedral heads and helical trails.

Question 15

Prepackaged enzymes

Answer 15

Some visions contain enzymes critical to infection.

• Lysozyme
• -makes hole in cel wall
• -lyses bacterial cell
• Nucleic acid polymerases
• Neuraminidases
• -enzymes that cleave glycosidic bonds
• -allows liberation of viruses from cell

Question 16

The Host

Answer 16

Viruses replicate only in certain cells.
Bacterial viruses easiest to grow; model systems.
Animal/plant viruses can be cultivated in tissue or cell cultures.
Plant viruses typically most difficult because they require growth of entire plant.

Question 17

Host range and specificity

Answer 17

Host range: spectrum of hosts that a virus can infect.
Viral specificity: the specific kinds of cells that a virus can infect.
-Ex. papillomaviruses can only infect skin cells.

Question 18

Viral Hosts

Answer 18

Bacteriophage infect bacteria.
Plant viruses only infect plants.
Animal viruses infect only animals.

Genetic information in virus has to match “machinery” of host.