viruses

Question 1

why may viruses be considered living

Answer 1

• contain genetic material
• their genetic material may undergo mutations
• able to evolve to adapt to new environments

Question 2

why may viruses not be considered non living

Answer 2

• acellular and lack cellular organisms
• do not carry out metabolism
• unable to reproduce independently
• unable to respond to stimuli

Question 3

why are viruses obligate parasites

Answer 3

depend on host cells to complete their reproductive cycle

Question 4

negative vs positive sense RNA

Answer 4

negative: viral genome is complementary to viral mRNA
positive: viral genome has same sequence to viral mRNA

Question 5

structure of capsid

Answer 5

protein coat that surrounds genome
composed of protei subunits called capsomeres
together with nucleic acid, forms nucleocapsid

Question 6

function of capsid

Answer 6

surrounds nucleic acid and serves to protect, attach and introduce the genome into host cells

Question 7

structure of envelope

Answer 7

a phospholipid bilayer, derived from host cell membrane, surrounding the nucleocapsid
embedded with viral glycoproteins

Question 8

genome of bacteriophages

Answer 8

double stranded DNA

Question 9

structure of genome of influenza

Answer 9

8 different segments of single stranded negative sense RNA, each associated with proteins
3 of the RNA segments are packed with 3 polymerase proteins which together form the enzyme RNA dependent RNA polymerase (RdRp)

Question 10

what do 5/8 of the RNA strands in influenza’s genome code for

Answer 10

haemaglutinin
neuraminidase
nucleoprotein
matrix protein
non structural proteins

Question 11

structure of genome of HIV

Answer 11

2 identical copies of single stranded positive sense RNA are bound to nucleocapsid proteins

Question 12

what does the HIV genome code for

Answer 12

contains 3 major genes: 5’ gag-pol-env-3’
gag codes for structural proteins
pol codes for HIV’s viral enzymes
env codes for the glycoproreins gp 120 and gp41

Question 13

structure of HIV’s capsid

Answer 13

conical shaped and contains reverse transcriptase, integrase and protease

Question 14

structure of influenza’s envelope

Answer 14

has the glycoprotein haemagglutinin and the enzyme neuraminidase embedded

Question 15

structure of HIV’s envelope

Answer 15

has the glycoproteins gp 120 and gp 41 embedded where gp 120 is attached to gp 41

Question 16

describe attachment stage of lytic/lysogenic life cycle

Answer 16

attachment site on tail fibres adsorbs to complementary receptor sites on bacterial surface

Question 17

describe penetration stage of the lytic/lysogenic life cycle

Answer 17

1. bacteriophage releases lysozyme which digests bacterial cell wall
2. molecules from bacterium are released, triggering a change in shape of proteins of base plate
3. tail sheath contracts, driving the hollow core tube through the cell wall
4. when the tip of the hollow core tube reaches the plasma membrane, phage DNA is injected into the bacterial cell
5. empty capsid remains outside bacterial cell wall

Question 18

describe replication of the lytic life/lysogenic life cycle after spontaneous induction

Answer 18

1. Host cell macromolecular
   synthesizing machinery is used
   to synthesise phage proteins
2. phage proteins synthesied earlier will degrade host DNA
3. Phage DNA is then synthesized using host cell nucleotides and phage proteins
4. phage proteins synthesied later are phage enzymes and
   structural components

Question 19

describe replication of lysogenic life cycle

Answer 19

1. Linear phage DNA circularizes
   and inserted into host cell
   genome by enzyme integrase
2. The integrated phage DNA is
   known as a prophage
3. expression of phage genes is
   repressed by phage repressor
   proteins so new phages
   are not synthesized
4. Prophage remains latent and replicates along with bacterial chromosome
5. prophage will be found in all progeny cells, remaining latent or enters spontaneous induction

Question 20

describe spontaneous induction (SI) of lysogenic life cycle

Answer 20

During spontaneous induction,
cellular proteases are activated, destroying the repressor
proteins
The prophage is then excised
from the bacterial genome
The replication phase of lytic
cycle then occurs

Question 21

describe the maturation stage of the lytic/lysogenic after SI life cycle

Answer 21

Phage DNA and capsid assemble into a DNA-filled head
head, tail and tail fibers assemble independently & join in a
specific sequence.

Question 22

describe the release stage of lytic/lysogenic after SI life cycle

Answer 22

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

Question 23

how can bacteria defend themselves against phages

Answer 23

1. develop lysogenic relationship with phage
2. develop receptor sites that are no longer complementary to phage attachment sites
3. develop restriction enzymes which recognise foreign phage DNA and cleave the DNA off

Question 24

describe attachment stage of influenza’s life cycle

Answer 24

the glycoprotein haemagglutinin binds to complementary sialic acid receptor on host cell
membrane

Question 25

describe the penetration stage og influenza's life cycle

Answer 25

1. host plasma membrane invaginates and pinches off, placing the virus in a endocytic vesicle, so the virus enters host cell by endocytosis 2. Endocytic vesicle fuses with lysosome, which lowers the pH 3. the low pH environment stimulates viral envelope to fuse with lipid bilayer of vesicle 4. nucleocapsid is released into cytosol

Question 26

describe the uncoating stage of influenza's life cycle

Answer 26

Capsid degraded by cellular enzymes, releasing the 8 viral RNA segments into the cytosol the RNA segments will then enter the nucleus

Question 27

describe what happens in the replication stage of influenza's life cycle

Answer 27

Viral RNA-dependent RNA polymerase uses viral genome as a template to synthesise mRNA fate of syntheised mRNA 1. enters cytosol where it will be translated into viral structural components Capsid proteins are made in the cytosol and Envelope glycoproteins are made in the RER & eventually are embedded in host cell membrane 2. acts as template for synthesis of new viral RNA genome in the nucleus after synthesis, it will exit nucleus

Question 28

describe what happens in the maturation stage of influenza's life cycle

Answer 28

1. viral glycoproteins are transported by vesicles from the ER and get incorporated into the plasma membrane 2. capsid proteins associate with these glycoproteins at the plasma membrane 3. Nucleoproteins associate with the RNA genome to form the helical nucleoprotein and then interact with capsid proteins that have associated with the glycoproteins embedded on the plasma membrane. 4. the interaction of the capsid with the nucleoprotein initiates the budding process.

Question 29

describe what happens in the release stage of influenza's life cycle

Answer 29

the new virions are released by budding 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 30

describe what happens in the attachment stage of HIV's life cycle

Answer 30

gp120 binds to complementary CD4 receptors on T helper cells or (macrophages) with the help of a co- receptor.

Question 31

describe what happens in the penetration stage of HIV's life cycle

Answer 31

With the help of gp41, the viral envelope fuses with host cell membrane, so nucleocapsid is released into cytosol, leaving envelope behind

Question 32

describe what happens in the uncoating stage of HIV's life cycle

Answer 32

Capsid gets degraded by cellular enzymes and the 2 viral RNA strands and enzymes are released into the cytosol

Question 33

describe what happens in the replication stage of HIV's life cycle (before activation version)

Answer 33

1. Reverse transcriptase makes DNA strand using viral RNA as template to form a DNA-RNA hybrid. 2. the RNA is then degraded and the remaining DNA strand is used as a template to synthesise a 2nd DNA strand, producing a double stranded DNA molecule 3.Viral DNA enters nucleus  inserted into host cell genome by integrase 4. the integrated Viral DNA is known as provirus 5. the provirus can remain latent for a long time

Question 34

describe what happens in the replication stage of HIV's life cycle (after activation version)

Answer 34

1. Upon provirus being activated, viral DNA transcribed to viral RNA which enters cytosol 2. Viral RNA can either act as mRNA and be translated into proteins or become part of the genome of the new virions fate of mRNA 1. is translated in the ribosome in the cytosol to form viral polyproteins 2. is translated in the RER. the proteins are glycosylated in the RER into envelope glycoproteins gp120 and gp 41 and eventually are embedded in the host cell surface membrane.

Question 35

describe what happens in the maturation stage of HIV's life cycle

Answer 35

For HIV, maturation is completed only after release of virus. The viral RNA genome and polyprotein assembles at the cell surface membrane where viral glycoproteins gp41 and gp120 have been inserted.

Question 36

describe what happens during the release stage of HIV's life cycle

Answer 36

Newly formed viruses bud off by evagination, acquiring host cell membrane with embedded viral glycoproteins Viral protease cleaves polyproteins, forming viral enzymes and proteins. The viral RNA genome and enzymes are then encapsulated by a protein coat to form a capsid The HIV virus (virion) is now mature and able to infect neighbouring cells.

Question 37

describe antigenic drift

Answer 37

When the influenza virus replicates in its host cell, mutations frequently occur due to the poor proofreading mechanism of the viral RNA-dependent RNA polymerase and the fast replication rate of the virus. Over time, there is an accumutation of mutations in the viral genome. Sometimes, these mutations produce viruses with MODIFIED surface antigens with different conformation. If these viruses infect a host that does not have the antibodies that recognise these modified surface antigens, thehost becomes susceptible to the virus.

Question 38

describe antigenic shift

Answer 38

When a bird strain of influenza A and human strain of influenza A infect a single cell of an intermediate host (e.g.a pig), genetic reassortment can occur. Genetic reassortment is when new viruses are being assembled in the host cell, new combinations of RNA segments come together. Sometimes, genetic reassortment produces viruses with NEW surface antigens such as new glycoproteins If these viruses infect a human host the host becomes susceptiblet to the virus, as the host will not have the antibodies that recognise these new surface antigens,

Question 39

condition for antigenic shift to occur

Answer 39

as long as species barrier is crossed genetic assortment may or may not be involved