Question

19. During virus growth, it is possible to obtain nucleic acid-free capsids. Under the electron microscope, these capsids look similar to complete virions. Would you expect such empty virions to be infectious? Why or why not? 20. Many DNA viruses encode proteins that stimulate the host to enter the S phase of the cell cycle. Why is this important for DNA virus replication? and are DNA are viruses only depend on active in sphase 21. (a) What viruses encode for reverse transcriptase? (b) What does reverse transcriptase do?

Answer 1

19. Virus needs its genetic material (DNA or RNA) to be infectious. When virus infects a host cell it injects its genetic material inside the host cell which integrates with the host's genetic material. Then the virus will take over the host replication, transcription and translation machinery to make more copies of its genetic material and capsid proteins. The virus then assembles the capsid and genetic material inside to form a function virion and breaks out from the host cell to infect more cells. So the information to replicate and make coat proteins and other required proteins are in the virus genetic material, with out that the virus cannot survive. Hence empty virions are not infectious.

20. DNA viruses depend on host cell's replication machinery to replicate its DNA. Since DNA replication occurs at S or synthetic phase of the cell cycle, the virus will stimulate the cell to enter S phase.

21. Retro viruses ( Virus whose genetic material is RNA) encode for reverse transcriptase. This enzyme will convert the RNA of the virus to its complementary DNA which can then integrate into the host cell's genome and replicate along with it.