Exam 3- Viruses

Question 1

What is a virus?

Answer 1

a non-cellular particle that must infect a host cell to reproduce

Question 2

What are the general characteristics of viruses?

Answer 2

1. most viruses are very small
2. contain nucleic acid (DNA or RNA)
3. nucleic acid is packaged in a protein coat
4. some are enclosed in an envelope
5. some have spikes
6. each species infects a particular group of host species called the host range

Question 3

What is the size of a typical virus?

Answer 3

50-100 nm

Question 4

What is a capsid?

Answer 4

made of repeating protein subunits called capsomeres, packages & protects the viral genome

Question 5

What is an envelope?

Answer 5

a lipid bilayer that surrounds the capsid & is usually derived from the host cell membrane

Question 6

What are the spikes on a virus?

Answer 6

glycoproteins in the envelope

Question 7

What are spikes involved in?

Answer 7

attachment to a host cell & escape from a host cell

Question 8

Explain tissue tropism.

Answer 8

tissue &/or cell type that virus can infect

Question 9

All viral replication cycles must achieve the following:

Answer 9

1. host recognition & attachment
2. genome entry
3. biosynthesis
4. assembly of virions
5. exit & transmission

Question 10

What do all viruses require for reproduction?

Answer 10

a host cell

Question 11

Explain bacteriophage.

Answer 11

viruses that infect bacteria, most mesophyilic & mildly thermophilic

Question 12

Lytic cycle of a bacteriophage

Answer 12

1. attachment- phage attaches to host cell
2. penetration- phage inserts DNA into host cell
3. biosynthesis- production of phage DNA & phage proteins by the infected cell
4. maturation- self-assembly of phage particles; virus genome is packaged into capsid
5. release- phage degrades infected host cell wall; host cell lyses (bursts); new phage particles are released

Question 13

Describe a virulent phage.

Answer 13

reproduce only through the lytic cycle, resulting in lysis of the host bacterial cell

Question 14

Describe a temperate phage.

Answer 14

can reproduce through the lytic cycle; can also undergo lysogeny, where the phage genome inserts into the host genome

Question 15

Lysogenic cycle of a bacteriophage.

Answer 15

1. attachment- phage attaches to host cell
2. penetration- phage inserts DNA into host cell
3. integration- phage DNA integrates into bacterial chromosome by recombination; the integrated phage DNA is called a prophage
4. excision- prophage excises from chromosome (phage induction); excision can be stimulated by UV or chemicals
5. lytic cycle- excised phage enters lytic cycle

Question 16

General life cycle of animal viruses

Answer 16

1. attachment- virus attaches to host cell membrane
2. penetration- virus enters host cell by endocytosis or fusion
3. uncoating- capsid removed by viral or host enzymes
4. biosynthesis- infected host cell produces viral nucleic acid & viral proteins –> may require viral enzymes
5. maturation- viral nucleic acid & viral capsid proteins assemble into virus particle
6. release- virus escapes host cell by budding (enveloped viruses), exocytosis, or rupture

Question 17

What is the primary factor to determine the life cycle of an animal virus?

Answer 17

the form of its genome

Question 18

What do DNA viruses use during the life cycle?

Answer 18

the host replication machinery

Question 19

What do RNS viruses use during the life cycle?

Answer 19

an RNA-dependent RNA-polymerase to copy their RNA

Question 20

What do retroviruses use during the life cycle?

Answer 20

a reverse transcriptase to copy the RNA genome into DNA & an integrase to insert the DNA into the host chromosome

Question 21

Describe Human Papillomavirus.

Answer 21

-HPV- small icosahedral virus; no envelope
- 8 kb genome
- most infections go away without treatment
-infects epithelial cells that cause genital warts
-also integrate into genome of host basal cells that can cause cancer
- direct contact
-prevention is Gardasil HPV vaccine
-80% adults in US are infected

Question 22

What DNA viruses can cause cancer?

Answer 22

-Epstein-Barr virus
-HPV
-Hepatitis B virus
-Human herpes virus-8

Question 23

What RNA viruses can cause cancer?

Answer 23

-human T-cell leukemia virus
-hepatitis C virus

Question 24

Describe Influenza A virus.

Answer 24

-(-) stranded RNA virus, segmented genome, enveloped with spikes
-upper respiratory infection- fever, sore throat, headache
-high risk- very old, very young, immunocompromised
-transmission through droplets
-seasonal flu vaccine
-Tamiflu (Oseltamivir) & Zanamivir

Question 25

What are spikes on a virus?

Answer 25

glycoproteins in the envelope of the virus

Question 26

What are antibodies?

Answer 26

proteins produced by the host that bind to specific antigens

Question 27

What is antigenic drift?

Answer 27

gradual change in surface molecules of virus (host antibodies may not recognize altered antigens)

Question 28

What is reassortment of genome segments from?

Answer 28

different viral strains infecting the same host at the same time can generate new viral strains

Question 29

Describe Human immunodeficiency virus.

Answer 29

-HIV: retrovirus
-infects CD4+ T cells; progressive loss of the immune system
-transmission from contact with blood, semen, vaginal fluid, breast milk
-no cure, no vaccine

Question 30

What genes are found in all retroviruses?

Answer 30

gag, pol, env