Virology

Question 1

Helical capsids

Answer 1

Long, hollow protein tubes which can be rigid or flexible, with the nucleic acids wound in a spiral and enclosed within the capsid (Willey, et al., 2011, pp.118).
This may be folded within an envelope if it is an enveloped helical capsid (Willey, et al., 2011, pp.118).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 2

Icosahedral capsids

Answer 2

Made up of capsomers (ring or knob-shaped units) consisting of either five or six protomers (pentamers/hexamers) (Willey, et al., 2011, pp.118).
Hexamers form the edges and faces while pentamers are found at vertices (Willey, et al., 2011, pp.118).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 3

Capsids

Answer 3

Composed of protomers which are proteins that form the capsid (Willey, et al., 2011, pp.115-116).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 4

Binal symmetry

Answer 4

Head is icosahedral and tails are helical (Willey, et al., 2011, pp.119).
Tails consist of a central hollow tube surrounded by a sheath with a complex, hexagonal baseplate that has a pin and a jointed tail fibre at each corner (Willey, et al., 2011, pp.119).
Head contains genome and is extended by some hexamer rows in the middle (Willey, et al., 2011, pp.119).
A collar joins the tail to the head (Willey, et al., 2011, pp.119).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 5

Describe the first stage of viral multiplication.

Answer 5

The first stage is attachment to host cell, where viruses randomly collide with host cells and attach by binding to its receptors (Willey, et al., 2011, pp.122).
Viruses have host preferences due to the variation in receptors it uses for attachment (Willey, et al., 2011, pp.122).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 6

Describe the entry stage of viral multiplication.

Answer 6

Bacteriophages inject genome into host cytoplasm, leaving capsid outside (Willey, et al., 2011, pp.122-123).
Eukaryotic viruses entire nucleocapsid enters host and uncoats itself (Willey, et al., 2011, pp.115-127).
Eukaryotic viruses enter cells by endocytosis or fusion (Willey, et al., 2011, pp.115-127).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 7

Describe how viral nucleocapsid enters the cell by endocytosis.

Answer 7

1) Viruses are engulfed by receptor-mediated endocytosis (Willey, et al., 2011, pp.123).
2) Endocytic vesicles containing viruses fuse with endosomes (Willey, et al., 2011, pp.123).
3) Low pH and endosomal enzyme help in uncoating of the virus (Willey, et al., 2011, pp.123).
4) Nucleocapsid is released when viral envelope fuses with endosomal membrane (Willey, et al., 2011, pp.123).
5) After completion of uncoating, viral nucleic acid is released into cytoplasm (Willey, et al., 2011, pp.123).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 8

Describe how the viral nucleocapsid of eukaryotic viruses enters the cell by fusion.

Answer 8

Viral spikes bind to host cell surface receptors causing membrane lipids to rearrange (Willey, et al., 2011, pp.123).
The contacting membranes merge, forming a proteinaceous fusion pore, allowing entry of the nucleocapsid (Willey, et al., 2011, pp.123).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 9

What is the third stage in viral multiplication?

Answer 9

The third stage is synthesis of the virus (Willey, et al., 2011, pp.123).
Synthesis of dsDNA viruses is similar to the normal flow of information in cells (Willey, et al., 2011, pp.123).
RNA viruses however must carry or synthesise the enzymes needed for the synthesis stage because cellular organisms don’t have the enzymes needed for RNA synthesis (Willey, et al., 2011, pp.123).

Viral protein synthesis is tightly regulated (Willey, et al., 2011, pp.123).
Early proteins are involved in taking over the host cell and late proteins are involved in self-assembly and release (Willey, et al., 2011, pp.123).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 10

What is the fourth stage in viral multiplication?

Answer 10

The fourth stage is assembly (Willey, et al., 2011, pp.123).
Virion components are built separately and joined together later on (Willey, et al., 2011, pp.123).

Scaffolding proteins are used to assemble the phage prohead, which are removed after construction (Willey, et al., 2011, pp.123).
The packosome, a complex of proteins, is used to incorporate DNA into the prohead (Willey, et al., 2011, pp.123).
Consists of a portal protein (located at base of prohead) and terminase complex (moves DNA into prohead) (Willey, et al., 2011, pp.123).

Baseplate is constructed first, with the tail tube built on it and the heath assembled around the tube (Willey, et al., 2011, pp.123).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 11

What is the fifth stage of viral multiplication?

Answer 11

The fifth stage is virion release (Willey, et al., 2011, pp.125).

The virus is released by lysis of host cell (Willey, et al., 2011, pp.125).
A protein creates holes in plasma membrane (Willey, et al., 2011, pp.125).
Lysozyme attacks peptidoglycan (Willey, et al., 2011, pp.125).

The virus can also be released by budding (Willey, et al., 2011, pp.125).
The viral proteins attach to plasma membrane and the nucleocapsid is released while simultaneously the envelope is formed by membrane budding (Willey, et al., 2011, pp.125).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 12

Describe what retroviruses are.

Answer 12

Retroviruses are enveloped + sense ssRNA viruses.
They replicate using reverse transcriptase.

Question 13

Describe how a retrovirus infects a cell.

Answer 13

1) First it enters the host cell.
2) Capsid is enzymatically removed.
3) Virus carries a protein called reverse transcriptase, which uses the viral RNA to synthesise DNA.
4) Viral DNA and integrase enter the hosts nucleus.
5) Integrase integrates the viral DNA into the hosts chromosome.
6) When the hosts DNA is transcribed, viral RNA is produced.
7) Ribosomes use the viral RNA to produce viral proteins.
8) Viral proteins self-assemble to form the capsid.
9) The immature virus is released by budding.
10) Immature virus undergoes enzymatic changes to become mature, after which it infects other cells.

Question 14

Define lysogeny.

Answer 14

Lysogeny is the relationship of a temperate phage and its host cell (Willey, et al., 2011, pp.115-127).
Instead of producing new virions immediately, it reproduces along with it (Willey, et al., 2011, pp.115-127).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 15

Define temperate phage.

Answer 15

A bacteriophage which can either begin multiplying and lyse the host cell or remain within the host cell without destroying it (Willey, et al., 2011, pp.126-127).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 16

Define prophage.

Answer 16

The form of the temperate phage that remains within its host cell, integrated into the host chromosome (Willey, et al., 2011, pp.126-127).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 17

Define lysogen.

Answer 17

A lysogen is a bacterial cell which has been infected by a temperate phage (Willey, et al., 2011, pp.126-127).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 18

Define induction.

Answer 18

Induction is when a lysogen enters the lytic cycle and the prophage begins synthesising virions (Willey, et al., 2011, pp.126-127).

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 19

Describe the life cycle of HIV.

Answer 19

First virus gp120 binds to CD4 of T helper cells, macrophages and dendritic cells, with the help of coreceptors CXCR4 and CCR5.
Next, fusion of the viral envelope with the plasma membrane releases viral contents into the host cell.
Reverse transcriptase then converts viral RNA into DNA.
Integrase inserts viral DNA into the host chromosome, producing a provirus.
When the hosts DNA is transcribed, the viral DNA is also transcribed to produce viral RNA.
The viral RNA is translated to produce the proteins and enzymes it needs.
When assembly of the virus is complete, it is released from the cell and enters the blood.

Question 20

Describe the lysogenic cycle of bacteriophages.

Answer 20

The first step is adsorption, where the bacteriophage attaches to the cell wall.
The second step is penetration, where the bacteriophage injects its viral genome.
The third stage is prophage formation, where the viral genome is integrated into the host cell genome.
The fourth step is maintenance, where the bacteriophage remains dormant for a while, and divides and replicates with the viral genome.
The fifth step is spontaneous induction, where changes in the environment cause viral genome to be spontaneously excised.
The bacteriophage now undergoes the lytic cycle.
It synthesises viral proteins and nucleic aid, which are assembled into new virions that are released.

Question 21

How does a bacteriophage switch between the lytic and lysogenic pathway?

Answer 21

CI and CRO are transcription factors which influence whether the phage will enter the lytic cycle or the lysogenic cycle.
CI binds to OR1 (Operator Region 1) and OR2, and prevents lytic growth.
CRO binds to OR2 and OR3 and prevents lysogeny.

When bacteria are growing actively, they produce HFLA, which breaks down CII.
Thus, CII cannot transcribe CI.
As there is more CRO than CI, the phage enters the lytic cycle.

When bacteria are growing slowly, they don’t produce HFLA.
Thus, CII will be present and can transcribe CI.
Therefore, as there is more CI than CRO, the phage enters the lysogenic cycle.

Question 22

Describe lysogenic conversion.

Answer 22

Lysogenic conversion is the transformation of non-virulent bacteria into highly virulent pathogens.
This occurs when they are infected by lysogenic bacteriophages .

Question 23

Give some examples of lysogenic conversions.

Answer 23

When C. diphtheriae is infected with phage B, it produces diphtheria toxin (Willey, et al., 2011, pp.126-127).
V. cholerae becomes toxic when infected with phage CTX and produces the cholera toxin.
When lambdoid prophages infect S. dysenteriae, it produces dysentery toxins Stx1 and Stx2.

References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.

Question 24

Describe phage therapy.

Answer 24

Phage therapy is the therapeutic use of lytic bacteriophages to treat pathogenic bacterial infections.
There are several concepts for phage therapy: the phage must be lytic; a single dose should treat infection; the phage should be non-toxic and highly specific for targeted bacteria; and antibiotic-resistant bacteria should remain sensitive to phage-mediated lysis.

Question 25

Compare the use of bacteriophages and the use of antibiotics for treatment of bacterial pathogens.

Answer 25

Bacteriophages have a very narrow host range and have fewer side effects.
Also, new phages can be selected quickly in a few days.
However, they have a ten-fold lower rate.

Antibiotics target both pathogenic microorganisms and the normal microflora, and have much more side effects.
Furthermore, resistance to antibiotics can be spread through many bacteria, and is not limited to the targeted bacteria.
Also, developing new antibiotics can take up to several years.

Question 26

Describe several drugs which can be used against HIV.

Answer 26

Several drugs against HIV inhibit reverse transcription.
For example, Nucleoside RT inhibitors and Nucleotide RT inhibitors act as competitive substrate inhibitors as they lack the 3’ hydroxy group, thus chain synthesis is stopped and the next group won’t be able to bind.
Another reverse transcription inhibitor is Non-Nucleoside RT inhibitor.
Protease inhibitors and Integrase inhibitors also are drugs against HIV.