Virus

Question 1

Define obligate parasite [2]

Answer 1

An organism that
(i) cannot live independently without its host,
(ii) requires host to complete lift cycle

Question 2

How can you be sure that something is a virus? [3]

Answer 2

It must be:
1. Obligate parasite
2. Genome only made up
of 1 type of nucleic acid- DNA OR RNA, NEVER both*
3. Viral components must assemble into complete
viruses to infect other cells (self-assembly)

Question 3

What is the difference between the lytic and lysogenic cycles in bacteriophages?

Answer 3

Lytic cycle: The virus causes host cell lysis, releasing new phages.

Lysogenic cycle: Viral DNA is incorporated into the host genome, forming a prophage that can later enter the lytic cycle.

Question 3

What are the general steps in the replication cycle of a virus? [5]

Answer 3

Mnemonic: April Mr
1.Attachment: Virus binds to specific host cell antigens.
2. Penetration: Viral genetic material is injected into the host cell.
3. Replication: Virus uses host’s metabolism and machinery to synthesize its own nucleic acid.
4.Maturation: Viral components self-assemble to form new viruses.
5. Release: New viruses exit the host cell via budding, exocytosis, or lysis.

Question 3

How do viruses differ from living organisms based on the cell theory? [give at least 3]

Answer 3

Living characteristics: Possess genetic material, capable of propagating genetic information, undergo mutations, and react to environmental stimuli.

Non-living characteristics: Acellular, lack cell organelles, do not undergo metabolism, cannot reproduce independently, do not grow, and do not respond to stimuli outside of a host cell.

Question 4

How do you confirm a virus is a RETROVIRUS? [2]

Answer 4

1. Presence of reverse transcriptase (MOST IMPT)
2. and a (+) RNA genome.

Question 4

What is antigenic drift and antigenic shift in the context of viral genomes?

Answer 4

Antigenic drift: Minor changes in the structure of surface antigens due to the accumulation of point mutations.

Antigenic shift: Major changes in surface antigens due to changes in host species or genome reassortment between different subtypes.

Question 4

Describe the general structure of HIV

Answer 4

1. (+) RNA genome [What does this mean w.r.t mRNA?]
2. 2* IDENTICAL SS RNA associated w Nucleoproteins
3. Has Reverse Transcriptase*
4. Has gp41 & gp120 embedded in envelope

Question 4

Describe the general structure of Influenza [4]

Answer 4

1. (-) RNA genome [What does this mean w.r.t mRNA?]
2. 8* DIFFERENT SS RNA associated w Nucleoproteins
3. Has RNA-dependent RNA Polymerase*
4. Has Haemagglutinin (80%) & Neuraminidase (20%) embedded in envelope

Question 4

Describe the process of viral integration and the latent phase in HIV. [6]

Answer 4

1. Viral Reverse Transcriptase
   makes DNA strand using
   viral RNA as template to
   form a DNA-RNA hybrid.
2. The RNA is then degraded
   and the 2nd DNA strand is
   made → forming double-stranded DNA molecule
   Integration & Latent Phase
3. Viral DNA enters nucleus
   → inserted into host cell
   genome by integrase →
   forms provirus → that can
   remain latent for a long time
4. Upon activation, viral DNA transcribed to viral RNA which enters cytosol
5. Viral RNA EITHER act as
   mRNA → translated into
   proteins OR become part of the genome of new virions
6. mRNA translated to viral
   polyprotein → modified to gp120 & 41 in RER & eventually embedded
   in host plasma membrane

Question 5

How do enveloped animal viruses like Influenza and HIV enter their host cells?

Answer 5

Influenza: Enters host cells by endocytosis*, followed by fusion of the viral envelope with the endocytic vesicle.

HIV: Viral envelope fuses* with the host cell membrane with the help of gp41, releasing the nucleocapsid into the cytosol.

Question 6

What role do neuraminidase and haemagglutinin play in the life cycle of viruses?

Answer 6

Neuraminidase: Facilitates the release of new virions from the host cell membrane by cleaving sialic acid from the host cell receptor (influenza virus).

Hemagglutinin (HA): binds to complementary
sialic acid receptor on host cell membrane (e.g.
epithelial cells in respiratory tract)

Question 7

What is the significance of viral self-assembly?

Answer 7

It ensures that viral components assemble into complete viruses capable of infecting other cells, a process critical for viral replication and propagation.

Question 8

What bacteriophage undergoes Lytic & Lysogenic cycle respectively?

Answer 8

Lytic: T4

Lysogenic: Lambda

Question 9

In bacteriophages, what capsid head do they have?

Answer 9

Icosahedral capsid head

Question 10

What’s the difference between Animal viruses & Bacteriophages [

Answer 10

1. Genome: Bacteriophages ONLY have DS DNA vs Animal viruses have RNA (usually SS)
2. Bacteriophages are NAKED, without viral envelope while Animal viruses have viral envelopes.
3. Any specific detail that animal viruses have that bacteriophages don’t
   e.g. Animal viruses like influenza have HA & NA while bacteriophages don’t

Question 11

How does HIV infect a cell? [3]

Answer 11

Attachment –> Penetration –> Uncoating

1. gp120 binds to complementary CD4 receptors on T helper
   cells/macrophages with the help of a co-receptor
2. With the help of gp41, the viral envelope fuses with host cell membrane → nucleocapsid is released into cytosol
3. Capsid degraded by cellular enzymes → the 2 viral RNA strands and enzymes (RIP) are released into the cytosol

Question 12

How does Influenza infect a cell? [4]

Answer 12

Attachment –> Penetration & Uncoating

1. Hemagglutinin (HA) binds to complementary
   sialic acid receptor on host cell membrane (e.g.
   epithelial cells in respiratory tract)
2. Enters host cell by endocytosis (invaginates,
   pinches off, placing virus in endocytic vesicle)
3. Endocytic vesicle fuses with Lysosome → lowering its pH → causes viral envelope to fuse with lipid bilayer of e. vesicle →
   nucleocapsid is released into cytosol
4. Capsid degraded by cellular enzymes & 8
   viral RNA segments released into cytosol
   enters nucleus

Question 13

Describe how does the influenza virus replicate and mature

Answer 13

1. Viral RNA-dependent RNA polymerase uses
   viral genome as a template to synthesise mRNA [ (-) → (+) ]
2. mRNA synthesized enters cytosol →
   translated into viral structural components
3. mRNA can also act as template for synthesis
   of new viral RNA genome in the nucleus [ (+) → (-) ]. Viral RNA genome then exits nucleus.

Maturation
4. Viral glycoprotein transported by vesicles from ER to plasma membrane, where it is
incorporated
5. Nucleoproteins interact with RNA genome &
capsid proteins to initiate budding process

Question 14

How does the influenza viruses get released from an infected cell? [2]

Answer 14

Release
* Newly formed viruses bud off by evagination,
acquiring host cell membrane with
embedded viral glycoproteins
* Neuraminidase facilitates the release of the
new virions from the host cell membrane by cleaving sialic acid from the host cell receptor

Question 15

How does bacteriophages generally infects a bacteria cells? [4]

Answer 15

Attachment –> Penetration
1. Attachment sites on tail fibres adsorbs to complementary receptor on bacterial surface
2. Releases Lysozyme which digest bacterial cell wall → allowing release of molecules
from bacterium
3. Released molecules trigger conformation ∆ in shape of base plate → cause contraction of bacteriophage tail sheath → thrusting hollow core tube through
cell wall
4. When the tip of the hollow core tube reaches the plasma membrane, phage DNA is injected into the bacterial cell

Question 16

How does a phage in Lytic Cell cycle replicate? [3]

Answer 16

Lytic Cell cycle
1. Inside host cell, BP DNA
immediately transcribed
to synthesize mRNA
using host RNA
polymerase
2. Phage enzymes coded
by the phage genome
takes over the
bacterium’s macromolecular (protein,
RNA, DNA) synthesising
machinery for its own use
3. Phage uses the host
cell’s nucleotides and
DNA polymerase to synthesise many copies
of phage DNA

Question 17

How does a page in Lysogenic Cell cycle replicate? [4]

Answer 17

1. Linear phage DNA circularizes and
   inserted into host cell genome by enzyme
   integrase, forming prophage
2. Expression of phage genes is repressed
   by phage repressor proteins coded by
   phage genes → no new phage is not synthesized
3. Every time host cell’s machinery e.g DNA
   polymerase replicates bacterial chromosome → prophage DNA replicated
   along with it → allows continuous replication of the lambda phage DNA
   without killing the host bacteria → results
   in large numbers of daughter cells with the
   prophage (advantage!)

Spontaneous Induction*
4. During spontaneous induction, cellular proteases activated → they destroy the repressor proteins → prophage is then excised from the bacterial genome → BP enters lytic cell cycle

Question 18

How does bacteriophages get released from an infected cell?

Answer 18

Phage lysozyme synthesised within the cell breaks down the bacterial cell wall → cell
membrane lyses and release the newly formed virions

Question 19

Describe how and when the HIV virus matures and get released.

Answer 19

*Maturation for HIV only complete with release of virus 1. Viral RNA genome & polyprotein assembles at the plasma membrane where viral glycoproteins have been inserted 2. Newly formed viruses bud off by evagination, acquiring host cell membrane with embedded viral glycoproteins → Viral protease cleaves polyproteins, forming functional viral enzymes and proteins. 3. Viral RNA genome & enzymes then ncapsulated by a protein coat to form a capsid → mature HIV virus (virion) is now able to infect neighbouring cells

Question 20

What are some general reasons why animal viruses are prone to mutations?

Answer 20

1. the fast replication rate of the virus (fast accumulation of random mutations) 2. SS viral RNA → no backup copy for DNA repair 3. Respective viral enzymes like RNA-Dependent RNA pol & Reverse Transcriptase do not have proof reading mechanism