Virology

Question 1

What is an example of Unexplained illnesses caused by viruses?

Answer 1

‘Orphan’ viruses – for example : Echo (Enteric Cytopathogenic Human Orphan) viruses

Question 2

What are the restrictions on the areas where you can find viruses?

Answer 2

Need a host to reproduce - need a host and host cells
Most of the time they need a specific host cell type as well - biomolecular interaction
Some are very selective - have a particular target

Question 3

If you look at virus particle microscopically you will see it as one of four possible shapes:

Answer 3

Head + tail
Helix
Icosahedron
Variable (pleomorphic)

Question 4

What are the parts of a bacteriophage?

Answer 4

Head containing nucleic acid (ss DNA)

Tail which penetrates host cell wall to allow injection of phage DNA

Question 5

What is an example of a Helical virus?

Answer 5

Tobacco Mosaic Virus
Nucleic acid
Glycoprotein capsid

Question 6

What is an example of a icosahedral virus?

Answer 6

Polio virus
Capsid proteins arranged in icosahedral form ( 20 faces)
Polio capsid compromises four different glycoproteins – VP 1, VP2, VP3 and VP4

Question 7

What is an example of a pleomorphic virus?

Answer 7

Influenza virus

Question 8

Enveloped particle of Herpes virus

Answer 8

Lipid envelope, giving characteristic ‘fried egg’

Core virus particle, containing ds DNA; 120-200mn

Question 9

Non- enveloped Herpes virus particles

Answer 9

~ 110 nm particles without envelopes

Question 10

Size of virus particles

Answer 10

Escherichia coli 1300 x 4000 nm
Poxviridae (e.g. Smallpox virus ): 300 x 240 x 100nm
Herpesviridae (e.g. Herpes Simplex virus): 120-200nm with envelope
Bacteriophage - T4 : 200nm diameter
Tobamovirus (e.g.Tobacco Mosaic Virus) 18 X 300 nm
Picornaviridae (e.g. Polio virus : 22nm diameter)

Question 11

What is the structure of a virus particle?

Answer 11

Capsid proteins form into regular structures call capsomers, which give the virus particle its symmetry ( helical, icosahedral etc)
Nucleic acid + protein = ‘nucleocapsid’, or ‘virion’
The reason that each type of virus particles form their characteristic shape relates to the size of the particle and biochemistry of the capsomers

Question 12

The aim of replication in viruses is to?

Answer 12

produce more copies of viral genome

produce viral proteins – structural capsid proteins and enzymes to help with replication of the genome

Question 13

How do we classify viruses?

Answer 13

their genome and taxonomy
Baltimore classification (7 groups)
International Committee on Taxonomy of Viruses system
- In general, when referring to the virus genus or species, italics NOT used

Question 14

Replication of viruses- what are the 3 examples?

Answer 14

Three examples:
Polio virus ( ss +ve sense RNA)
Herpes virus ( ds DNA)
Influenza virus (ss –ve sense RNA,  segmented)

Question 15

Polio virus

Answer 15

Picorna (‘pico’ + ‘rna’ virus)
Virus particles infection human host gut epithelial cells and ( rarely) neurones in central nervous system
Once virus has entered host cell, replicates very quickly and produces progeny within about 6 hours

Question 16

Replication of poliovirus

Answer 16

Virus particle uses its surface capsid glycoproteins (VPs 1-3) to attach to receptive host cell
Bind to host cells with receptor CD155 (only) – e.g. gut epithelium
VP 4 is lost from virus particle during the process of entry into host cell
Virus ‘uncoats’ in cytoplasm and genome is released
Replication occurs inside cytoplasm of host cell
Since polio virus genome is positive sense RNA, it acts as mRNA inside cell
Migrates to ribosomes where translation takes place
The virus genome contains codes for all the virus proteins
Translated into single polyprotein strand, which is cleaved into functional proteins by host cell enzyme
Host cell RNA and protein production are closed down
Each virus protein strand is broken up into:
RNA polymerase
Proteases
Structural proteins
Whole process is repeated until host cell resources exhausted
Progeny virion assembled and cell lyses, releasing new particles to infect other cells

Question 17

Herpes simplex virus

Answer 17

Lipid envelope, giving characteristic ‘fried egg’ appearance
Core virus particle, containing ds DNA
Virus infects skin cells ( lips, genital mucosa) and neurones
Replication = ‘lytic’ cycle
Also ‘latent’ stage where virus ‘hides’ from host immune system

Question 18

Replication of Herpes viruses

Answer 18

Virus particle has 11 capsid glycoproteins , gA – gK.
It uses its gB, gC, gD and gH to attach to receptive host cell
Herpes virus envelope fuses with host cell membrane , with the involvement of gB
The virion enters the cytoplasm of the host cell
Transported to the nucleus where particle uncoats
Linear ds viral DNA closes into a circular form
uses host cell enzymes to transcribe viral mRNA in three stages:
- First (immediate early) – regulatory proteins for the next phases
- Second (delayed early) – DNA polymerase and other enzymes ( e.g. thymidine kinase)
- Third (late) – structural proteins for new virus particles
In the ‘delayed early’ and ‘late’ stages, the genomes and proteins for the new (progeny) virus particles are being made
By the ‘late’ stage of replication, the viral DNA polymerase is going around the circular genome repeatedly, making many copies
-‘Rolling circle replication’
Whole process takes place in host cell nucleus
Once the host cell’s resources have been exhausted, the process of assembling new virus particles begins
Nucleocapsids bud out of the host cell membrane and thus acquire lipid material for the envelope

Question 19

Influenza virus

Answer 19

Genome comprises 8 segments of RNA of varying lengths
Each one contains codes for specific proteins
e.g Haemagglutinin glycoprotein coded for on segment 4
e.g. Neuraminidase coded for on segment 6

Question 20

Replication of influenza virus

Answer 20

Virus targets suitable host cell and uses HA and NA to enter
HA attaches to sialic acid component of host cell receptors ( respiratory epithelial cells)
NA acts as an enzyme to bind to and then cleave the sialic acid molecule from the host cell glycoprotein
As virus particles are released from host cell surface, viral membrane components absorbed into host membrane , aided by HA
Nucleocapsid is released into host cell cytoplasm
Migrates to nucleus and uses its own RNA polymerase to transcribe +ve sense RNA molecules
The viral mRNAs move back to cytoplasm
Assembly of progeny virus particles
Each particle collects 8 segments
Fairly error prone, but only particles with 8 different segments viable and able to infect new cells / new host

Question 21

Errors in replication of influenza virus

Answer 21

Nucleic acid replication not always accurate
Mammalian cells have complex processes to correct errors in DNA replication, but RNA replication does not need to be so strictly controlled
Thus, errors in replication of RNA viruses inside host cells are not always corrected

Question 22

Antigenic drift in Influenza virus

Answer 22

Depending on the nature of the error:
Could mean progeny virus not viable
Could give rise to new ‘strain’ of virus
New strains of influenza A virus regularly arise, through this ‘antigenic drift’

Question 23

Outline antigenic drift

Answer 23

2 types of influenza A virus infect single host cell
RNA segments from both viruses in host cell cytoplasm
New virus with Haemagglutinin from X and Neuraminidase from Y: ‘Antigenic shift’

Question 24

Influenza A

Answer 24

In nature, these ‘re-assortment’ events happen quite commonly
The new version of Influenza A may not be a viable virus
The ‘Swine flu’ strain of H1N1 appears to have sequences from both avian and swine viruses
Currently low influenza activity – some cases of Influenza A H3 strains and someone Influenza B reported in recent weeks