Viruses

Question 1

Are viruses considered living or non-living?

Answer 1

Living: contain genetic material which can undergo mutations when viruses are in a living host cell, hence viruses can evolve

Non-living:
- acellular and lack cellular organelles
- do not carry out metabolism
- unable to reproduce independently
- unable to respond to stimuli effectively

Question 2

Why are viruses obligate parasites?

Answer 2

depend on host cell to complete reproductive cycle

Question 3

Antigenic drift

Answer 3

• viral RNA-dependent RNA polymerase has poor proofreading mechanism, and fast replication rate of the virus, viral RNA is single stranded and does not have a backup copy to carry out repair mechanism
• occurs when there is an accumulation of random mutations in the viral genome, causing a change in ribonucleotide sequence, hence resulting in modified surface antigens

Question 4

Antigenic shift

Answer 4

• when two different strains of the virus infect a single host cell, there could be random genetic reassortment of viral RNA segments, causing different combinations of haemmaglutinin and neuraminidase

Question 5

T4 phage (lytic phage) – Attachment and penetration

Answer 5

• attachment sites on tail fibres adsorb onto complementary receptor sites of the host cell membrane
• bacteriophage releases lysozyme to digest bacterial cell wall
• this release of molecules triggers a change in conformation of the base plate which causes the contraction of tail sheath, which will drive the hollow core tube through cell wall
• when the tip of the hollow core tube reaches the plasma membrane, phage DNA is injected into the bacterial cell wall, the empty capsid remains outside

Question 6

T4 phage Replication, Maturation, and Release stages

Answer 6

• host cell macromolecular synthesizing is used to synthesize phage proteins
• early phage proteins degrade host DNA, phage DNA is then synthesized using host cell nucleotides and phage proteins.
• phage enzymes and structural components are made
• phage DNA and capsid assemble into a DNA filled head
• head, tail and tail fibres assembled independently and join in its specific sequence
• phage lysozyme synthesised within the cell breaks down the bacterial cell wall, bacterial cell membrane lyses and release newly formed virions

Question 7

Lysogenic cycle replication

Answer 7

• linear phage DNA circularizes and is inserted into host cell genome by enzyme integrase to form prophage
• expression of the phage genome is repressed by repressor proteins, hence new phages are no synthesized
• prophage remains latent and replicates along with bacterial chromosome
• during spontaneous induction, cellular proteases are activated, they destroy the repressor proteins
• the prophage is then excised from the bacterial genome, the replication phase of lytic cycle then occurs

Question 8

Entry of influenza virus

Answer 8

• haemagglutinin recognizes and binds to complementary sialic acid receptors on host cell membrane
• virus enters the cell via endocytosis where the host cell membrane invaginates and buds off to form an endocytotic vesicle
• endocytotic vesicle fuses with lysosome which lowers the pH to allow viral envelope to fuse with lipid bilayer of vesicle, releasing nucleocapsid to cytosol
• capsid degraded by cellular enzymes and the 8 viral RNA segments are enter the nucleus

Question 9

Replication of Influenza

Answer 9

• viral RNA dependent RNA polymerase use viral RNA genome to synthesize mRNA
• mRNA is either used as a template to synthesise new viral RNA genome or is used as a template for translation to form viral structural components

Question 10

Maturation and Assembly of Influenza

Answer 10

• Capsid proteins associate with embedded viral glycoproteins
• RNA genome then interacts with capsid proteins to inititate budding process.
• Newly formed virions then bud off by evagination, acquiring some of the host cell membrane with associated glycoproteins
• neuraminidase facilitates this by cleaving the sialic acid on host cell receptors

Question 11

Attachment and Penetration of HIV

Answer 11

• gp120 recognizes and binds to the complementary CD4 receptors of T helper cells
• with the help of gp41, the viral envelope. fuses with the host cell membrane, releasing nucleocapsid into the cytosol
• cellular enzymes degrade capsid to release viral RNA segments into the cytosol

Question 12

Replication of HIV

Answer 12

• reverse transcriptase makes DNA strand using viral RNA as a template to form a DNA-RNA hybrid
• RNA is then degraded and 2nd DNA strand is made to form double stranded DNA
• DNA enters the nucleus where it is inserted into host cell genome by integrase to form a provirus and remains latent for a long time
• upon activation, viral DNA is transcribed into RNA which can then act as the mRNA template which can be translated to form viral polyproteins or can be part of the genome of the new virions

Question 13

Maturation and assembly of HIV

Answer 13

• viral RNA genome and polyporteins assemble where glycoproteins are embedded
• newly formed virions bud off by evagination, acquiring some of the host cell membrane with embedded glycoproteins
• viral protease cleaves polyproteins to form viral structual proteins and enzymes