Structure and Reproductive Cycle of Viruses

Question 1

Why are viruses not considered to be alive

Answer 1

1. Viruses do not grow, feed or respire
2. They are acellular and do not contain cytoplasm or cellular organelles
3. They do not feed
4. They do not replicate independently

Question 2

Why are viruses considered obligate parasites

Answer 2

1. Contains only 1 type of nucleic acid, hence, does not replicate independently of a host cell
2. Lacks resources like amino acids and ATP

Question 3

Why is HIV a retrovirus

Answer 3

They replicate by means of reverse transcription of its single stranded RNA into double stranded DNA, followed by integration of ds DNA into host cell genome by integrase

Question 4

Lytic Cycle of a T4 phage

Answer 4

1. T4 phage uses attachment sites on its tail fibres to recognise and adsorb to specific receptor sites on host bacterial cell wall.
2. T4 phage tail fibres anchors its base plate pins onto the cell surface
3. Conformational change occurs in the tail sheath and base plate whereby tail sheath contracts to drive a hollow tube through the host bacterial cell wall, injecting phage DNA into the bacterial cytoplasm
4. On entry, phage genes are expressed using the host cell’s metabolic machinery and resources
5. These phage genes code for enzymes which disrupt normal cellular functions in the hosr by shutting down host bacterium’s macromolecular synthesis and nucleases which hydrolyses the bacterial chromosome
6. DNA nucleotides released from degraded DNA are then used in the synthesis of new copies of phage DNA
7. Phage uses host cell metabolic machinery to synthesise more phage enzymes and phage structural components
8. When sufficient phage enzymes, DNA and structural components are synthesised, the phage components assemble and enclose newly replicated DNA, forming new phages
9. The phage directs the production of lysozyme, coded for by the phage DNA, which breaks down peptidoglycan cell wall of the host bacterium, resulting in osmotic lysis of the bacterium.

Question 5

Lysogenic cycle of lambda phage

Answer 5

1. Lambda phage uses attachment sites on its tail fibres to recognise and adsorb to a specific receptor site on host bacterial cell wall
2. Tail sheath contracts to drive a hollow tube through the bacterial cell wall, injecting phage DNA into the bacterial cytoplasm
3. Lambda phage circularises inside the bacterium, preventing exonucleases from degrading it.
4. Lambda phage integrates into host bacterium’s chromosome, forming a prophage
5. One of the prophage genes coding for a repressor protein is expressed. Repressor protein represses the expression of most of the phage genes controlling phage replication.
6. Phage replication is blocked, and the phage is said to be dormant
7. Every time the bacterium divides, phage replicates as part of the host bacterium’s DNA and is passed on to each daughter cell
8. Environmental factors could induce lambda phage to enter the lytic cycle.
9. Repressor protein is no longer expressed, and lambda phage DNA is excised from the host bacterial chromosome
10. Phage switches from lysogenic cycle to lytic cycle, resulting in lysis of the host cell.

Question 6

Reproductive cycle of HIV

Answer 6

1. gp120 recognises and binds to CD4 molecule on host cell plasma membrane, inducing conformational change on gp120
2. gp120 binds to chemokine receptor, CCR5 on plasma membrane, inducing a conformational change on gp41 on HIV virion
3. Viral envelope fuses with host cell plasma membrane, allowing viral RNA and capsid to enter the host cell
4. Capsid is degraded by cellular enzymes, releasing viral RNA, integrase and reverse transcriptase into the cytoplasm, via uncoating
5. Reverse transcriptase catalyses the synthesis of a DNA strand, complementary to viral RNA (cDNA)
6. Viral RNA is degraded, and synthesis of a second DNA strand complementary to cDNA occurs, forming a double stranded DNA
7. Double stranded DNA is transported to host cell nucleus, and integrated into host cell chromosome as a provirus, by integrase.
8. During T/S, host’s RNA polymerase transcribes the provirus genes into viral RNA molecules, which can function as mRNA for translation of new viral proteins, or genetic material for new virus particles
9. New virus particles buds off the host cell membrane at ‘exit points’
10. HIV protease cleaves the immature HIV polyprotein into individual functional proteins, giving rise to a mature HIV virus.

Question 7

Reproductive cycle of Influenza virus

Answer 7

1. Haemagglutinin recognises and binds to specific sialic acid containing receptors on host cell plasma membrane
2. Virus enters the host cell by receptor mediated endocytosis, whereby plasma membrane of host cell invaginates and pinches off, engulfing the virus in an endocytic vesicle which enters the cytoplasm
3. Acidification causes fusion of viral envelope with endocytic vesicle membrane, releasing the nucleocapsid
4. Capsid is removed by cellular enzymes, releasing viral RNA and RNA-dependent RNA polymerase, via uncoating
5. Viral RNA serves as template for synthesis of complementary RNA strand (cRNA), catalysed by RNA-dependent RNA polymerase.
6. cRNA acts as a template for synthesis of new copies of viral RNA or functions as mRNA for translation of new viral proteins
7. Glycoproteins synthesised on rER is transported to host cell surface membrane via vesicles and incorporated into the plasma membrane, clustered in patches which serves as exit points for the new viral progeny
8. Newly replicates influenza virus buds off the host cell plasma membrane at ‘exit points’
9. At this point, influenza virus still adheres to the host cell receptors through haemagglutinin
10. Neuraminidase catalyses cleavage of sialic acid residues from HA, facilitating in the release of newly replicated influenza virus

Question 8

Antigenic Shift

Answer 8

1. 2 different strains of influenza virus infects the same host cell simultaneously.
2. Random assembly of RNA segments results in novel combinations of haemagglutinin and neuraminidase genes, resulting in antigenic shift

3.

Question 9

Antigenic Drift

Answer 9

1. Point mutations in genes coding for glycoproteins may occur during replication of viral RNA
2. Accumulation of mutations results in slight changes to the shape of viral glycoproteins, resulting in antigenic drift.
3. The change in shape to the new viral glycoprotein may enable it to be complementary in shape to other membrane receptors found on new types of host cells

Question 10

Capsid

Answer 10

1. Outer protein coat surrounding genetic material of the virus
2. Made up of capsomeres

Question 11

Difference between influenza virus and HIV

Answer 11

1. Influenza virus has 8 segments of single- stranded RNA genome while HIV has 2 copies of single-stranded RNA
2. Influenza virus: RNA-dependent RNA polymerase vs HIV: integrase, HIV protease, reverse transcriptase
3. Influenza virus targets epithelial cells lining the respiratory tract while HIV targets specific immune cells such as CD4+ T cells