virus

Question 1

What is the structure of a virus

Answer 1

consists of viral material, protein coat called capsid and sometimes an envelope.

Question 2

What is the structure of a capsid?

What is the function of a capsid?

Answer 2

• consist of protein subunits called capsomeres. The viral genome determines the arrangement of the capsomeres.
• to protect the viral genome from being degraded by nucleases.
• it contains special sites on the surface for virus to attach to the host cell.
• contains proteins that allow virus to penetrate into the host cell.

Question 3

How does T4 phage replicate

Answer 3

Attachment/adsorption: It uses long tail fibres to bind to the specific receptor sites on the host cell via weak chemical bonds. This anchors baseplate pins onto the host cell surface membrane.

Penetration: the bacteriophage tail releases lysozyme to digest the cell wall to release molecules which will trigger chase in the baseplate.
This will cause the contraction of the contractile tail sheath, penetrating the hollow core through the cell. The bacteriophage injects the linear ds DNA into the host cell.

Synthesis of viral genome: the phage DNA is transcribed to form the viral mRNA and is translated to form phage viral proteins using host cell transcription and translation machinery.
Early proteins: viral nuclease is produced when phage DNA enters the cell. It degrades the host cell DNA to provide free deoxyribonucleotides for the replication of phage DNA using host cell DNA polymerase to form new viral genome for new phages.
Late proteins: proteins making up the capsid and the tail.lysozyme.

Assembly: components of phage self assemble to form complete viruses.

Lysozyme digest cell wall of host causing osmotic lysis.

Question 4

How does lamda phage replicate?

Answer 4

Attachment: the short tail fibre bind the specific receptor site on the csm.

Penetration: the phage injects viral genome into the cell
The viral genome circularises to prevent degradation by exonucleases
If conditions not suitable it will undergo lysogenic cycle when the viral genome integrates into the host cell genome at specific recognition site by intergrase to form prophage.

The repress or protein expressed prevents the transcription of all the genes so that when the host cell replicate, the prophage DNA is replicated tgt with the host cell DNA.

Environmental signals cause the degradation of the repressor proteins. Phage DNA is excised from the bacterial chromosome. Lysogenic cycle change to lytic cycle.

Phage DNA is transcribed to viral mRNA and then translated to viral proteins
Phage DNA replicates
Components self assemble to form complete virions.
Lysozymes breaks down the cell wall to cause osmotic lysis.

Question 5

Describe the structure of influenza.

Answer 5

Contains 8 single stranded segments of negative sense RNA that are packaged with nucleoproteins to form helical ribonucleoproteins. 3/8 of the segments code for the segments of RNA-dependent RNA polymerase. The rest code for viral proteins like haemaglutinin and neuraminidase.

Question 6

Describe the structure of HIV virus.

Answer 6

Contains 2 single strands of positive sense RNA (genome). Viral genome is the same as the viral mRNA. The viral genome consist of 3 major genes.
Gag: codes for structural proteins
Pol: codes for viral enzymes ( reverse transcriptase, HIV protease, integrase)
Env: code for viral glycoproteins (gp120 and gp41
RNA is tightly bound by nucleocapsid for stability.

Question 7

How does influenza replicate?

Answer 7

1. viral haemaglutinin attaches to the sialic acid containing residue on glycoprotein receptor on the csm of the host cell.
2. The virus enters the cell via receptor-mediated endocytosis. The csm invaginates and pinched off to enclose the intact virion to form a endosome.
3. Acidification of the endosome occurs which causes the viral envelope to fuse with the endosomal membrane.
4. This releases the ribonucleoproteins into the host cell cytoplasm to complete the uncoating process.
5. Viral ribonucleoproteins enter the nucleus where replication and transcription occurs by the RNA-dependent RNA polymerase attached to the viral ribonucleoproteins.
6. Positive sense RNA is produced. It acts as a template for the production of more negative sense RNA and it acts as viral mRNA to be transcribed and translated to form viral proteins.
7. The viral mRNA is transferred to the cytoplasm to be translated. RNA-dependent RNA polymerase, matrix proteins and nucleoproteins are synthesised by free ribosomes but haemaglutinin and neuraminidase are synthesised by ribosomes on RER.
8. The glycoproteins are modified by the Golgi apparatus and transported by vesicles to be embedded into the host csm.
9. The subunits of RNA -dependent RNA polymerase nucleoproteins are transported back into the nucleus to bind to the viral RNA to form ribonucleoproteins attached with RNA -dependent RNA polymerase.
10. The ribonucleoproteins are transported to where the glycoproteins are embedded in the membrane.
11. Mature virions are synthesised by budding. The csm with embedded glycoproteins from the viral envelope. Neuraminidase cleaves the sialic acid from the host cell receptor to facilitate the release of virions

Question 8

How does HIV replicate?

Answer 8

1. The gp120 protein binds to the CD4 receptor on the cell surface membrane, facilitating coreceptor binding.
2. This causes the conformational change of the gp41 transmembrane protein, exposing the fusion domain. The fusion domain inserts into the host csm
3. Viral envelope fuses into the host csm releasing nucleocapsid into the cytoplasm. The empty capsid degrades releasing viral enzymes and RNA.
4. The RNA genome is reverse transcribed by reverse transcriptase to form double stranded DNA. DNA strand complementary to the RNA is synthetised by reverse transcriptase to form the RNA-DNA hybrid. The RNA strand is degraded and another DNA strand is synthesised to form the double stranded DNA.
5. Viral DNA enters the nucleus is inter grated into the host cell genome by viral integrase. To form a provirus which is in latent stage.
6. Upon activation, proviral DNA is transcribed by RNA polymerase in the host cell to form viral mRNA for the translation of virla poly proteins.
7. Env poly proteins are cleaved by celluer protease to form gp 120 and gp41
8. They are transported by vesicles and embedded into the csm
9. The RNA genome and gag and pol polyproteins are transported to the csm where glycoproteins are embedded
10. Immature virions are released by budding, the csm with glycoproteins embedded form the viral envelope.
11. HIV protease cleaves the polyproteins to became viral enzymes and structural proteins.
12. The capsid encloses the RNA genome and viral enzymes to complete maturation.