M genetics of viruses

Question 1

why are viruses regarded as non-living organisms (they are obligate parasites)

Answer 1

1. do not have cellular organisation 2. do not grow and develop 3. are unable to reproduce on their own 4. unable to adapt to the environment without help of a host cell 5. unable to acquire and use energy in order to maintain metabolic processes

Question 2

why is it advantageous for a bacteriophage to have a lysogenic cycle?

Answer 2

Under unfavourable conditions, phage can remain dominant in host cells. Phage genome is replicated each time the host cell replicates and bacteria is not killed, allowing a large number of virions to be produced when induced to transit to lytic cell. Survival of bacteriophage is ensured since host cell survives.

Question 3

describe how infection by HIV causes diseases

Answer 3

1. HIV recognises CD4 on T helper cells and infects them 2. HIV causes gp120 fragments to be exhibited on the CSM of infected helper T cells causing these helper T cells to become targets for destruction by cytotoxic T cells 3. The loss of helper T cells caused the immune system to fail and thus unable to mount a immune response against foreign particles 4. such that individual is susceptible to opportunistic secondary infections 5. AIDS results when infections become unmanageable 6. Cancer may result if HIV integrates into middle of TSG to switch it off

Question 4

lytic cycle vs lysogenic cell cycle

Answer 4

1. In lytic, phage DNA is NOT integrated and remains in the cytoplasm but in lysogenic, phage DNA is integrated into bacterial DNA as a prophage 2. In lytic, phage DNA directs synthesis of phage proteins and replication of phage DNA but in lysogenic, phage DNA is replicated TOGETHER with bacteria DNA during binary fission 3. lytic, host cell lyses to release bacteriophages but in lysogenic, host cell remains intact (unless phage in induced to transit to lytic)

Question 5

how does antigenic shift result in new strains of influenza?

Answer 5

multiple different strains of viruses may CO-INFECT a single host cell -> genetic recombination of viral genomes lead to new combinations of RNA segments during RNA assembly when new viruses are assembled in the host cell + major change in viral glycoproteins

Question 6

how does antigenic drift result in new strains of influenza?

Answer 6

the high mutation rate of viral RNA genomes of existing influenza viruses OR errors in replicating their RNA genomes are not corrected due to lack of proofreading mechanism of viral RNA-dependent RNA polymerase + minor change in viral glycoproteins