book 7 (reproductive cycle of bacteriophages via lytic & lysogenic cycle)

Question 1

describe the reproductive cycles of bacteriophage that reproduce via lytic cycle (T4 bacteriophage)

Answer 1

1. the attachment sites on the phage tail fibres recognises and bind to complementary receptor sites on the bacterial cell wall
2. lysozyme at the tip of base plate degrades the bacterial cell wall and plasma membrane, allowing tail sheath to contract and drive a hollow tube through the cell wall and membrane. this injects the DNA present in the capsid head into the bacterial cytoplasm
3. the viral DNA is used as a template to make new viral DNA genome using host DNA polymerase. the viral DNA is transcribed to form the viral mRNA using host RNA polymerase, viral mRNA is then tranlsated to form viral proteins and enzymes using host ribosomes . the viral enzymes and proteins are used to:
   - shut down the host’s macromolecular (protein, RNA, DNA) synthesis
   - hydrolyse the host DNA to recycle the nucleotides to be used for the synthesis of new copies of phage DNA
   - to direct the host’s transcription and translation machinery
4. viral proteins then self-assemble to form the capsid head, tail and tail fibres of the new bacteriophages. the phage genome is packaged inside the capsid head. lysozyme is packaged into base plate.
5. phage-encoded lysozyme breaks down the bacterial peptidoglycan cell wall, causing the lysis of host cell and release of viral particles which spread to nearby cells and infect them

Question 2

describe the reproductive cycles of bacteriophages that reproduce via the lysogenic cycle (Lambda phage)

Answer 2

1. injection of the genome through the cell wall and plasma membrane into the bacterial cytoplasm
2. the phage inserts its genome into the bacterium chromosomal DNA at specific attachment sites to become a prophage which will remain in the latent period whereby the prophage gets replicated together with the host DNA so every daughter cell contains a prophage. the expression of phage genes is blocked by a viral repressor protein until when the host cell exhibits signs of stress, induction occurs where the repressor protein is destroyed by an activated bacterial enzyme. the prophage is then excised from the host genome and enters the lytic cycle.
3. host DNA polymerase is used to replicate viral prophage to form new viral DNA genome. prophage is transcribed to form viral mRNA using host RNA polymerase. viral mRNA is then translated to form viral proteins and enzymes using host’s ribosomes. the viral proteins and enzymes are used to:
   - shut down the host’s macromolecular synthesis-
   - hydrolyse the host DNA to recycle the nucleotides for the synthesis of new copies of phage DNA
   - direct the host’s transcription and translation machinery
4. viral proteins self-assemble to form the capsid head, tail and tail fiber of the new bacteriophages. the phage genome is packaged inside the capsid head.
5. the lysis of host cell and release of viral particles which spread to nearby cells to infect them.

Question 3

outline the characteristics of the Influenza virus genome

Answer 3

1. single-straded
2. has negative sense RNA
3. segmented – contains different nucleotide sequences
4. 8 molecules

Question 4

outline the characteristics of the HIV virus genome

Answer 4

1. single-stranded
2. positive sense RNA
3. non-segmented – identical
4. 2 molecules

Question 5

describe how influenza virus enters the cell

Answer 5

1. haemagglutinin on the viral envelope recognises and binds to sialic acid receptors on epithelial cell’s plasma membrane
2. the virus enters the host cell via receptor-mediated endocytosis where host plasma membrane invaginates and pinches off, placing the virus in an endocytic vesicle/endosome
3. the acidic conditions of endosome activates the M2 proton channel to allow protons into the virus to make the inside of virus acidic. this causes the viral envelope to fuse with the membrane of the endocytic vesicle, releasing the viral nucleocapsid into the host cell’s cytoplasm. the viral capsid is then enzymatically removed, releasing the viral genome which is then transported to the nucleus of the cell

Question 6

describe the replication process of influenza virus

Answer 6

1. the viral RNA-dependent RNA polymerase uses negative sense RNA as a template to synthesise a complementary positive-sense RNA strand. positive RNA strand acts as a template to replicate more negative-sense RNA viral genome
2. the viral RNA-dependent RNA polymerase uses a negative sense RNA strand as a template to synthesise a complementary mRNA strand which migrates to the cytoplasm where it serves as a template for host cell’s ribosomes to synthesise viral proteins and viral enzymes during translation

Question 7

describe how newly synthesised influenza virus are formed and released

Answer 7

1. the newly synthesised viral envelope proteins are transported through the rER and golgi body, and then inserted in the host plasma membrane
2. newly synthesised viral enzymes and viral nucleocapsid are assembled
3. the newly formed viral nucleocapsid and viral enzymes migrate through the rER and golgi body, then inserted in the host plasma membrane
4. the host plasma membrane pinches off to form the viral envelope via budding, enclosing the viral nucleocapsid and enzymes
5. the sialic acid is cleaved and removed from the envelope of the new viral particle by neuraminidase. this aids in the release of the new viral particles from each other, thus preventing agglutination of viruses.
6. the new viral particles released are able to travel and infect other cells