Viruses

Question 1

define viruses and explain what it means

Answer 1

Viruses are obligated intracellular parasites.
.
Meaning:
- By necessity, they can only survive in a living host cell.
- They rely on hijacking host cell machinery to carry out metabolic processes.
.
Why do they need to hijack host cell?
- They require materials like nucleic acids, amino acids, ribosomes, ATP to help them reproduce.
- They are metabolically inert virions in extracellular condiotions.
- they are metabolically active and replicate in intracellular conditions.

Question 2

Diff types of viruses

Answer 2

1. Bacteriophage
   - Only infect bacteria cells.
   - Includes temperate phages like the lambda phage
   .
2. Animal virus
   - Infects animal cells only
   EG: influenza, HIV

Question 3

Can you see viruses under a light or electron microscope?

Answer 3

Electron microscope

Question 4

Struc of viruses and phages

Answer 4

Phages:
- Head: viral genome + capsid (icosahedral shaped protein coat) = nucleocapsid
- Phages have linear ds DNA, not RNA
- tail
- tail fibres
.
Influenza:
- Viral membrane
- capsid (protein coat), within the viral membrane
- RNA
- Viral proteins

Question 5

Roles of viral genome in survival

Answer 5

• Viral genome can be single-stranded/ ds, linear/circular, DNA/ RNA, segmented/ unsegmented
  .
• Encode both structural and regulatory proteins that aid in survival and replication of virus.
• Structural protein form viral components like the capsid proteins.
• Regulatory proteins control gene expression and protein synthesis.

Question 6

Struc of capsid (present in all viruses)

Answer 6

A capsid is:
- a protein coat surrounding the viral genome
- Protein subunits used to form the capsid are called capsomere
- Capsids are usually helical (bananas) or icosahedral (diamond) shaped

Question 7

Role of envelope in virus survival
(only in envelope viruses, which usually only infect animal cells)

Answer 7

• Envelope is made of phospholipids from the CSM of the host cell,
  .
• Virus buds off from host cell, taking the phospholipid bilayer with them.
  .
• Envelope surface contains viral glycoproteins that aid in attachment and adsorption of virus when infecting the next host cell.

Question 8

State diff enzymes and their roles in viruses

Answer 8

• Viruses contains enzymes that aid in entry and exit to the host cell, and polymerases that aid in gene expression & transcription.
  .
  EGS:
• Lysozyme is an enzyme in bacteriophages that break down the peptidoglycan bacteria cell wall, resulting in lysis of the host cell.
  .
• Neuraminidase is an enzyme on the envelope of influenza (animal virus) that aids with its release from the host cell.
  .
• RNA-dependent RNA polymerase (RNA template, synthesises RNA) like viral replicase influenza.
• It uses -ve sense RNA as a template to synthesise +ve sense RNA.
• the +ve sense RNA is used as a template to replicate viral RNA or acts as mRNA to synthesise viral proteins using host cell machinery.
  .
• RNA dependent DNA polymerase (RNA template, synthesises DNA) like reverse transcriptase in HIV
• reverse transcribes viral RNA into a complementary ss DNA strand via CBP.
• the DNA strand is then used as a template to synthesise a complementary DNA strand via CBP.
  .
• Integrase
• Catalyses the integration of viral DNA into host chromosome to form a provirus

Question 9

Different between lysosome and lysozyme

Answer 9

1. Lysosome
   - an organelle that contains hydrolytic enzymes
   - fuses with vesicle to digest food particles taken in via endocytosis
   - Autophagy (digestion of worn out organelles.
   - Autolysis (diff from apoptosis bcuz its uncontrolled and leads to tissue inflammation)
   .
2. Lysozyme
   - an enzyme found in bacteriophages which breaks down the peptidoglycan bacterial cell wall, resulting in lysis of the host cell.

Question 10

Reproductive cycle of virus summary

Answer 10

APUSAR
.
1. Attachment/ adsorption
2. Penetration and Uncoating
3. Synthesis / Replication
4. Assembly
5. release

APU – How viruses enter
SAR – How viruses reproduce

Question 11

Describe the Reproductive cycle of a virus

Answer 11

1. Attachment
   - Viral glycoproteins are comp in shape to receptors of the host cell
   - Virus recognises and binds to the host cell receptors to attach to the cell surface of the host cell.
   .
2. Penetration & Uncoating
   - virus is engulfed into a vesicle
   - Uncoating of capsid to release viral genome into host cell cytoplasm.
   .
3. Synthesis
   - Virus takes over host cell machinery & raw materials carry out replication and transcription of viral genome, and then viral protein synthesis using host ribosomes.
   - Proteins like capsomere, glycoprotein and enzymes will be synthesised.
   .
4. Assembly
   - Nucleocapsids are assembled from viral nucleic acids and capsomeres
   - Glycoproteins are embedded into host cell membranes (FOR ENVELOPED VIRUSES ONLY).
   .
5. Release
   - New viruses lyse the host cell or bud off the membrane to free themselves.
   - New released virions can now infect next host cell.

Question 12

How do viruses challenge cell theory?

Answer 12

1. Cells are the basic unit of life
   - Viruses lack cell machinery that living cells have.
   - Viruses cannot carry out metabolic processes on their own without a living host cell
   .
2. All cells arises from pre-existing cells
   - Virions, when in an extracellular environment, are not able to replicate without a host cell.
   .
3. All living organisms are made of 1 or more cells
   - Viruses are acellular, they do not have protoplasm and organelles.
   - so they cannot carry out metabolic processes w/o host cell

Question 13

3 Principles of cell theory

Answer 13

1. Cells are the basic unit of life
2. All cells arises from pre-existing cells
3. All living organisms are made of 1 or more cells

Question 14

How do viruses support cell theory?

Answer 14

1. Cells are the basic unit of life
   - have impt genetic info
   - Have the potential to evolve over time & adapt to environment.
   .
2. All cells arises from pre-existing cells
   - All new viruses come from original viruses.
   - Viruses can replicate when they enter a host cell.
   .
3. All living organisms are made of 1 or more cells
   - nil

Question 15

If viruses both support and challenge cell theory, then what are they?

Answer 15

Viruses are bets described as infectious particles that are active in intracellularly and inactive in extracellularly

Question 16

3 Reasons viruses are living things

Answer 16

1. They can regulate metabolic processes
   - such as genome replication, gene expression and protein synthesis.
   .
2. They can reproduce and synthesise new virions
   .
3. _they have great genetic diversity _
   - they can rapidly alter their genome as they evolve to better survive in the host cell.

Question 17

2 Reasons viruses are NOT living things

Answer 17

1. They cannot carry out metabolic processes on their own
   .
   They cannot:
   - synthesise proteins
   - grow
   - response to stimuli
   - evolve
   On their own
   ____________________________________
2. They are accellular
   they lack:
   - a phospholipid membrane
   - membrane organelles

Question 18

What reproductive cycles can temperate phages undergo?

Answer 18

1. lysogenic cycle
   then
2. lytic cycle

Question 19

Lambda phage reproductive cycle

Answer 19

1. Attachment
   - Tail fibre recognises and binds to receptors on the cell surface of a bacteria cell that has a complemetary 3D conformation.
   .
2. Penetration
   - Lysozyme from phage hydrolyses bacteria peptidoglycan cell wall.
   - Phage DNA is injected through tail tube and into the cell
   .
3. Prophage formation (lysogenic)
   - Phage DNA circularises and is integrated at the Prophage insertion site (PIS)
   - Prophage is formed and remains latent
   - Host cell undergoes binary fission
   - subsequent generations of bacteria will have the prophage
   .
4. Prophage induction
   - occurs upon env trigger (UV, heat, DNA damage)
   - induces the phage to switch to lyric cycle.
   - Lytic genes of phage are now switched on
   - Phage genome will be excised from the host chromosome to stat viral replication and synthesis
   .
5. Rep and synthesis (lytic)
   - phage hijacks host cell machinery to synthesise phage nuclei acids and proteins
   - Host bacterial chromosome is degraded
   - host DNA polymerases and free deoxyribonucleotides used to replicate phage DNA
   - host RNA polymerases transcribe template phage DNA into mRNA.
   - Host ribosomes, tRNAs and translation factors are used to translate the phage mRNA
   - _Viral proteins and enzymes needed for replication and inhibition of host cell gene expression_ are synthesised.
   .
6. New phages are assembled
   .
7. Release
   - Lysozyme hydrolyses the bacteria cell wall
   - cell lyses to release new lambda phages
   -

Question 20

Influenza reproductive cycle (no lysogenic cycle)

Question 21

HIV reproductive cycle

Answer 21

1. Adsorption
   - gp 120 glycoproteins on HIV recognise and bind to CD4 receptors, w a specific 3D conformation that is complementary in shape, on the CSM of T-helper cells.
   - This causes a conformations change in HIV, to expose the gp 41 glycoprotein
   .
2. Penetration
   - gp 41 bring HIV closer to tbe host cell
   - gp 120 binds to the co-receptor, facilitating fusion of viral envelope with host CSM.
   - Nucleocapsid is released into host cell
   .
3. Provirus formation
   - Viral SS RNA is reverse transcribed by reverse transcriptase into complementary ss DNA via CBP
   - Viral DNA is used as template by host polymerases to synthesis complementary DNA strand via semi-conservative DNA rep
   - Double stranded viral DNA formed
   - Viral DNA is integrated into host chromosome by integrase
   - Provirus is formed
   - HIV remains in latent phase
   .
4. Replication and synthesis
   - When host cell is involved in an immune response, it activates the virus,
   - latent phase ends.
   - (Viral genome replication and protein synthesis occurs)
   - Viral DNA is transcribed by host RNA polymerases into RNA
   - Synthesised RNA can act as new viral genome
   - or mRNA to be translated by host ribosomes for synthesis of viral proteins
   .
5. Assembly
   - viral components like enzymes, capsid, RNA genome etc⁉️⁉️
   - assembled near host cell membrane.
   - gp 120 and 41 glycoproteins embedded into host CSM
   .
6. Release
   - New HIV virus budsoff from host cell, taking a part of the host’s studded membrane to form its new viral envelope

Question 22

Genome of lambda phage, influenza and HIV viruses