Viruses

Question 1

Suggest how these anti-viral drugs (enzyme inhibiting) would work in the treatment of patients
infected with HIV

Answer 1

1. idea that drugs would prevent viral replication ;
2. idea that T (helper) {cells / lymphocytes} will not be
   { killed / burst / destroyed}(by virus particles leaving
   cell) ;
3. idea of {inhibition / eq} of reverse transcriptase ;
4. idea that (viral) DNA could not be made;
5. from the (viral) RNA ;
6. idea of {inhibition / eq} of integrase ;
7. idea that (viral) DNA cannot integrate into (host)
   {DNA / genome} / eq ;

Question 2

Suggest why common cold viruses cannot infect cells if they land on
unbroken skin.

3

Answer 2

1. {skin / epidermis} is a barrier / eq ;
2. reference to keratin ;
3. reference to lack of receptors (for the virus) ;

Question 3

3 enzymes are formed in the lytic cycle of norovirus, why are two of them used at the stage where new viruses are made?

3 points

Answer 3

1. to synthesise (common cold) RNA / eq ;
2. for amino acids to bind to tRNA / eq ;
3. to synthesise (common cold) protein
   (capsid) / eq ;

Question 4

Suggest how new viral particles are formed inside the host cells.
(lytic)

5 points

Answer 4

1. reference to synthesis of RNA ;
2. using host cell {enzymes / named enzyme
   / (RNA) nucleotides} ;
3. reference to synthesis of (viral) proteins ;
4. using host cell {enzymes / named enzyme
   / amino acids / ribosomes / tRNA / ATP} ;
5. reference to assembly of {viruses /
   particles} (inside cells) ;

Question 5

Why does not all contaminated food cause food poisoning?

Answer 5

• enzymes kill microorangisms
• stomach acid kills microorganisms
• lack of oxygen for microorganisms
• competition with gut flora
• insufficient numbers of microorganisms to cause food poisoning
• immediate vomitting removes microorganisms

Question 6

Name two types of cell that HIV enters in the immune system.

Answer 6

1. T helper cell
2. phagocytic cells like macrophages

Question 7

How does HIV enter cells?

Answer 7

• HIV has glycoproteins on it’s surface which recognise and bind to the CD4 receptors of the cell (T helper/ phagocytic cell)
• virus envelope fuses with the cell surface membrane

Question 8

Describe the sequence of events following infection of these cells by HIV, that
may lead to the death of the patient. [6]

Answer 8

1. reference to viral RNA ;
2. refernce to production of (viral) DNA using viral RNA as a copy
3. reference to reverse transcriptase
4. incorportaion of viral DNA into host cell’s DNA
5. using intergrase
6. reference to production of more viral RNA and proteins
7. these are assembled
8. infect more cells (T helper cells)
9. lowers immunity to other diseases
10. becuase T helper cells help in immune response
11. death is caused by eg opportunistic disease like pneumonia or TB

Question 9

Some anti-viral drugs prevent HIV entering the host cells.
Suggest how these anti-viral drugs could prevent HIV entering the host cells.

Answer 9

1. idea that the drugs could {bind to / alter shape of}
   {glycoproteins / gp120} ;
2. idea that drugs bind to {receptors / antigens} on
   membrane / eq ;
3. called CD4 (antigen / molecules) ;
4. preventing virus attaching to T (helper / CD4+) cells / eq

Question 10

Describe how enzymes are involved in HIV infection.

Answer 10

1. reverse transcriptase
2. formation of viral DNA
3. from viral RNA
4. ref to intergrase
5. intergrates the viral DNA into host DNA
6. host (T helper cell) destroyed when virus particles leave cell
7. more cells become infected

Question 11

Why are viruses non-living?

Answer 11

• do not perform metabolic reactions
• do not have membrane bound organelles
• cannot replicate independantly of a host cell

Question 12

Why do antibiotics not work on viruses

Answer 12

• viruses are non-living
• viruses do not have a cell wall (which antibiotics usually target)

Question 13

lamda phage virus
- type of nucleaic acid
- presence of absence of an envelope
- arrangement of proteins in their capsid

Answer 13

• nucleaic acid: double stranded DNA
• envelope: absent
• proteins arrangement in capsid: complex protein capsid (non-helical)

Question 14

tobacco mosiac virus
- type of nucleaic acid
- presence of absence of an envelope
- arrangement of proteins in their capsid

Answer 14

• nucleais acid: single stranded RNA
• envelope: absent
• proteins arrangment in capsid: helical

Question 15

Ebola virus
- type of nucleaic acid
- presence of absence of an envelope
- arrangement of proteins in their capsid

Answer 15

• nucleaic acid: RNA single stranded
• envelope: present
• protein arrangement in capsid: helical

Question 16

HIV
- type of nucleaic acid
- presence of absence of an envelope
- arrangement of proteins in their capsid

Answer 16

• nucleaic acid: RNA single stranded
• envelope: present
• arrangment of proteins in capside: complex

Question 17

Explain why there was a delay before the number of lysed cells started to increase. (3)

Answer 17

• first virus has to attatch to host cells and insert its genetic material into the host
• then new viral DNA/RNA needs to be synthesised
• new proteins need to be syntheised (transcription and translation)
• new viruses must be assembled

Question 18

Explain why electron micrscopes may be used to view viruses instead of light microscopes

Answer 18

Electron microscopes have a shorter wavelength of electrons so resolution is higher so they are better able to view viruses which are very small.

Question 19

Briefly describe the lytic cycle of a virus

6 points could be 2-4 marker

Answer 19

• viruses attatches to host cell
• DNA/RNA synthesised
• viral proteins/capsids made
• new viruses/virions assembled
• viruses multiply
• cells lyse/burst

Question 20

What is meant by ‘latency’

Answer 20

• virus nucleaic acid is incorporated into host cells
• virus is dormant/inactive/non virulent
• virus nucleic acid replicates when host cell divides.

Question 21

Ethical implications of using untested new drug

Answer 21

positives;
- (ebola) has high mortality rates
- new drug may help develop it for others
negatives
- unknown side affects
- patient may not be able to provide informed consent
- it is difficult to decided who can be treated if drug is limited

Question 22

What type of cells do EBV attatch to

Answer 22

B lymphoctes/epithelial cells

Question 23

Why are viruses dependent on living cells?

2 MARKS

1st mark

Answer 23

• they do not have appropriate organelles/enzymes
• so unable to replicate independently

Question 24

Describe the lysogenic virus cycle

Answer 24

• virus injects host cell DNA as provirus.
• viral DNA replicates when host cell divides
• virus produces repressor proetins to inhibit transcription
• latent virus enters lytic pathway when host cell is damaged or immune system is weakened