viruses lecture 4

Question 1

why are viruses considered non-living 2

Answer 1

1. cannot replicate independently of host cell

2. cannot grow or metabolise

Question 2

3 factors that affect the host range of viruses

Answer 2

1. correct attachment: viral protein needs to bind to receptor on cell surface
2. after entry appropriate cellular machinery must be available for virus replication
3. needs mechanisms for release of virus particles to ensure infection can spread

Question 3

2 components make up a virus

Answer 3

1. genetic materila
   - single/double stranded
   - DNA/RNA
   - linear/circular
   - one/ more molecules
2. enclosed by a protective coat= capsid

Question 4

5 basic structural forms of viruses

Answer 4

1. naked icosahedral
2. naked helical
3. enveloped icosahedral
4. enveloped helical
5. complex

Question 5

example of naked icosahedral virus 3

Answer 5

poliovirus, adenovirus, hepatitis A

Question 6

example of naked helical virus 1

Answer 6

tobacco mosaic virus

Question 7

example of enveloped icosahedral virus 3

Answer 7

herpes virus, yellow fever virus, rubella virus

Question 8

example of enveloped helical virus 4

Answer 8

rabies virus, influenza virus, mumps virus, measles

Question 9

example of complex virus 1

Answer 9

pox virus

Question 10

virion

Answer 10

the complete, infective form of a virus outside of a host cell

Question 11

viruses exist in 2 phases

Answer 11

1. extracellular

2. intracellular

Question 12

virus replication process 6 steps

Answer 12

1. infection of host (attachment, entry and uncoating)
2. transcription of viral genes
3. replication- new nucleic acid generated
4. translation-protein synthesis
5. assembly of new virus particles
6. release of mature virus

Question 13

bacteriophages exhibit what 2 types of life cycles

Answer 13

1. lytic

2. lysogenic

Question 14

difference between lytic and lysogenic life cycles of bacteriophages

Answer 14

lytic cycle
-viral replication is followed by host cell lysis to release the virus particles

lysogenic

• viral genetic material is integrated into host’s genome for replication some time in the future
• infected bacteria (phage) is latent and called a prophage

Question 15

lytic cycle (virulent phages) 5 stages

Answer 15

1. attachment: capsid combine with bacterial cell wall receptors (‘lock and key’)
2. penetration: viral enzyme digests part of cell wall & viral DNA is injected into cell
3. biosynthesis: biosynthetic apparatus of host is taken over for viral DNA and coat protein production
4. maturation: assembly of DNA & capsid protein to produce new phage particles
5. release: lysozyme, encoded for by a viral gene, is produced and disrupts the cell wall, thereby releasing the phage particle resulting in host cell death

Question 16

lysogenic life cycle of bacteriophages 5 stages

Answer 16

1. attachment: capsid combine with bacterial cell wall receptors (‘lock and key’)
2. penetration: viral enzyme digests part of cell wall & viral DNA is injected into cell
3. viral DNA is integrated into the genome of the bacterial cell; the virus is called a prophage
4. Prophage is replicated within host DNA and all bacterial daughter cells (lysogenic cells) carry a copy of viral DNA
5. environmental factors such as UV radiation may induce prophages to leave host and induces a lytic cycle thus producing new virus particles

Question 17

transduction

Answer 17

• host’s DNA may be packaged in the heads of prophages

- bacterial genes may then be transferred to next host via the bacteriophage

Question 18

in lysogenic life cycle viral genes are expressed as if they are……

Answer 18

host’s genes

Question 19

example of phage conversion

Answer 19

Vibrio cholera

• normally exists in aquatic environment as harmless form
• deadly, disease-causing bacteria due infection of the V.cholerae cell by a bacteriophage that introduces a gene coding for the cholera toxin

Question 20

HIV 3 characteristics

Answer 20

human immunodeficiency virus

• retrovirus
• genome of 2 identical, positive, single stranded RNA molecules
• infects white blood cells presenting CD4+ receptors on their host cell membranes

Question 21

5 stages in replication of HIV

Answer 21

1. reverse transcriptase converts single-stranded RNA genome of virus to double-stranded cDNA (copy)
2. integrase integrates cDNA into host’s genome
3. transcription , HIV derived mRNA is produced
4. translation of viral mRNA results in new virus production
5. release does not involve destruction of host cell, occurs via budding