Propagation - Life cycle

Question 1

Step 1: attachment and penetration

Answer 1

1. The virus gains entry by endocytosis and is carried into the cytoplasm in a vacuole via the cell membrane
   § Attachment: Interaction of the virion with specific ‘receptor sites’ on the surface of the host cell
   § Penetration: Viral particle is taken into the cell, this sometimes involves endocytosis and the subsequent appearance of endosomes in the cell

Question 2

Step 2: Uncoating

Answer 2

2. Leaving its protein capsule redundant on the cell’s surface
   § Uncoating: The separation of the viral nucleic acid from the capsid

Question 3

Step 3: Eclipse/Replication

Answer 3

3. Viral nucleic acid is then released to take over the genetic machinery of the host cell
   § Eclipse/replication: Viral particles present as small non-infective subunits within the host cell and is not visible. Intense synthesis of viral components
   □ The synthetic and replicative phase of animal viruses are highly regulated and extremely complex and the molecular level
   □ DNA viruses (except poxviruses): enter the host cell’s nucleus and are replicated and transcribed in the nucleus
   □ RNA viruses (except retroviruses): are replicated and transcribed in the cytoplasm
   Almost immediately upon entry, the viral nucleic acid alters the genetic expression of the host and instructs it to synthesize the building blocks for new viruses
   The RNA of the virus becomes a message for synthesizing viral proteins (translation)
   ◊ Viruses with positive sense RNA molecules already contain the correct message for translation into proteins
   ◊ Viruses with negative sense RNA molecules must first be converted into a positive sense message

Some viruses come equipped with the necessary enzymes for synthesis of viral components- others utilize the ones in the host

In the next phase, new RNA is synthesize using host nucleotides (proteins for the capsid, spikes, and viral enzymes are synthesized on the host’s ribosomes using its amino acids)

Question 4

Step 4: Assembly

Answer 4

4. Viral DNA becomes incorporated into the DNA of the host, assuming command of genetic control
   § Assembly: Newly synthesized viral genomes and capsid proteins come together to form new viruses

Question 5

Step 5: Release

Answer 5

5. The host synthesizes viral protein rather than its own so that the new virus particles are generated and eventually released completing the life cycle
   § Release: non-enveloped viruses generally lyse or burst the cell. Enveloped viruses leave the host cell by a budding process resembling exocytosis