Viral properties

Question 1

What technique is used to prove that a virus causing a particular disease?

Answer 1

Koch’s Postulates

Question 2

Define virus.

Answer 2

Infectious obligate intracellular parasites

Question 3

What is the average size of a virus?

Answer 3

100nm

10-500 nm

Question 4

What are the two broad types of virus morphology and give examples?

Answer 4

Non-enveloped: protein capsid, tends to be more symmetrical
e.g. adenovirus, picornavirus, calicivirus

Enveloped membrane:
lipid envelope around that which is derived from the host membrane
-are pleomorphic- lots of different shapes e.g. ebola virus

Question 5

What is the main classification of viruses and what is it based on?

Answer 5

Baltimore Classification -

- which places viruses into groups depending on the pathway they use to make their genomes into mRNAs.

Question 6

What are the different groups under this classification?

Answer 6

Some viruses have DNA or RNA
-DNA Viruses (double stranded, single stranded) RNA Viruses (positive sense, negative sense, double stranded) DNA and RNA Viruses (retroviruses, Carry RNA genome but go through DNA intermediate to produce more RNA)

Question 7

What is the difference between positive sense RNA and negative sense RNA?

Answer 7

Positive sense RNA can be translated straight away Negative sense (antisense) RNA must be transcribed into a positive sense copy

Question 8

What are some common features among RNA viruses and retroviruses?

Answer 8

RNA genome limited by instability of RNA
Use their own polymerase to replicate: error prone
No proofreading capacity – high mutation rate

Question 9

What are some common features among DNA viruses?

Answer 9

Larger because DNA is more stable so there is space for accessory genes: these give them advantage by e.g. supressing the immune system
-some have their genome as one long starnd while others have it in segmented

Question 10

What are the good and bad aspects of having a segmented genome?

Answer 10

Good – allows an opportunity for exchanges of genetic material and fast evolution Bad – all the segments need to be assembled when the virus leaves the cell

Question 11

Describe the replication cycle of HIV-1.

Answer 11

GP120 glycoprotein receptors on the HIV bind to CD4 receptors on T cells and bind to co-receptors (CCR5 or CXCR4) allowing the membranes to fuse and the viral contents to enter the cell. Some copies of the virus genome gets replicated Some gets reverse transcribed to viral DNA using reverse transcriptase, which is integrated into the host genome. It is then transcribed and translated into proteins The proteins and copies of the genome then assemble to form new virus particles, which exit the cell.

Question 12

Describe a generic virus replication cycle

Answer 12

1. Attachment to the host cell by specific interaction between the virus attachment protein and a host cell receptor (a molecule on the surface of a host cell that has a completely different role but that the virus has evolved to use as its key for entry to the interior of the cell).
2. Entry of the virus genome to the host cell and synthesis of viral mRNA (primary transcription).
3. Translation of viral mRNAs into viral proteins by host cell ribosomes in the cytoplasm.
4. Replication of the viral genome, usually by making a small number of complementary copies that are then themselves copied at high numbers into new genomes (asymmetric replication).
5. Assembly of the virus proteins into new virions, association and encapsidation of the new genomes.
6. Exit from the infected cells and dissemination to new hosts.

Question 13

What is the cytopathic effect?

Answer 13

Death of a cell as a result of being infected by a virus

Question 14

How can viral plaques be used to quantify the amount of virus in a sample?

Answer 14

Plaque Assay – - take a sample from the patient that contains the virus

• the virus undergoes serial 10-fold dilutions and a known volume of it is spread on a monolayer of susceptible cells. A plaque will appear where an individual virus has killed some cells. The number of plaques can be counted and scaled up to quantify the amount of virus in a sample
• if you count the plaques you can measure how many viruses there were in the sample, as 1 virus causes 1 plaque

Question 15

What are 2 other ways of detecting the presence of virus in a sample?

Answer 15

Syncytia formation (bundle of cells stuck together)and Immunostaining

Question 16

What are the three phases of growth of a virus?

Answer 16

Eclipse
Logarithmic
Cell Death

Question 17

What are the five techniques used to diagnose a viral infection?

Answer 17

Detecting Genome (PCR): have primers which are complementary to a specific part of the viral genome
Detecting Antigen (Indirect Fluorescence Antibody, ELISA)
Detecting Viral Particles (Electron Microscopy, Haemagglutination Assay)
Detecing Cytopathic Effect in cultured cells
(virus isolation)
Detecting antibodies to virus (serology)

Question 18

Why are plaques formed?

Answer 18

Each plaque is the result of one single virus that was originally put onto the cell mono-layer

Question 19

what are the 2 key features of viruses?

Answer 19

1. Structural simplicity: Virus particles are structurally simple. They have highly repetitive units that are so regular they can be crystallized. TMV was crystaliized in 1932. This achievement fed the debate as to whether viruses were alive or not. Their capacity to form crystals made them seem more like an inorganic substance.

2 .Intracellular parasitism.