Genetics of Viruses

Question 1

outline reproductive cycle of HIV

Answer 1

Attachment/Adsorption:
- gp120 glycoproteins on viral envelope recognise and bind to specific receptor molecules on cell surface membrane of helper T-cell, promoting viral entry into the cell

Entry:
- the virus envelope fuses with the cell surface membrane
- the capsid proteins are degraded by host cell’s enzymes, releasing the viral RNA and reverse transcriptase into the cytoplasm

Integration:
- reverse transcriptase catalyses the synthesis of a single DNA strand complementary to the viral RNA
- the viral RNA is degraded and reverse transcriptase catalyses the synthesis of a second DNA strand complementary to the first
- the newly synthesised double-stranded viral DNA then enters the cell’s nucleus and integrates, as a provirus, into the host cell DNA via the action of integrase
- provirus never leaves the host’s genome, remaining permanently in the host cell

Synthesis of viral components:
- with host cell activation, proviral genes are transcribed into RNA molecules by the host’s RNA polymerase
- RNA molecules functions as viral genomes for the next viral generation
- RNA molecules function as mRNAs, which is translated into both viral and capsid proteins and viral glycoproteins
- vesicles embedded with viral glycoproteins migrate toward and fuse with the csm, as such, the viral glycoproteins become embedded on the csm

Viral assembly/Maturation:
- capsid proteins enclose the viral genome and viral proteins
- capsid then assembles with viral glycoproteins during budding

Release/Budding:
- each new virus buds from the cell, surrounded by the host csm studded with viral glycoproteins

Question 2

outline reproductive cycle of influenza

Answer 2

Attachment/Adsorption:
- haemagglutinin glycoproteins on viral envelope recognise and bind to specific receptor molecules on csm of epithelial cells of the respiratory tract, promoting viral entry into the cell

Entry:
- virus enters the host cell via endocytosis, forming endosome
- viral envelope fuses with endosome’s membrane, exposing the capsid to digestion by cellular enzymes, releasing viral RNA molecules, viral proteins and enzymes into the cytoplasm

Synthesis of viral components:
- the negative sense RNA functions as template for synthesis of positive sense RNA strands by viral RDRP
- positive sense RNA functions as mRNA which is translated into capsid proteins and viral glycoproteins
- vesicles embedded with viral glycoproteins migrate towards and fuse with the csm, as such, viral glycoproteins become embedded on the csm
- positive sense RNA functions as template for replication of new copies of viral RNA genome

Viral assembly/Maturation:
- capsid proteins enclose the viral genome and viral proteins
- capsid then assembles with viral glycoproteins during budding

Release/Budding:
- each new virus buds from the cell, surrounded by the host csm studded with viral glycoproteins

Question 3

antigenic drift

Answer 3

• the process by which virus varies genetically in minor ways from year to year
• spontaneous point mutations in viral genes cause small differences in the structure of the viral surface antigens
• (for influenza) point mutations in the genes encoding the two major viral surface glycoproteins, haemagglutinin and neuraminidase, happen continually over time as the virus replicates, may result in new strains of influenza forming
• (for HIV) error-prone reverse transcriptase produces mutations in the genes encoding the two major viral surface glycoproteins, gp120 and gp41, as the virus replicates

Question 4

antigenic shift

Answer 4

-a major change in the surface antigens of virus, caused by reassortment of their segmented genome with that of another virus
- when more than one strand of influenza virus coinfect a single cell in a single host, genetic reassortment occurs whereby there is random assembly of different RNA segments from different strains, producing a virus with novel combinations of RNA segment