Methods of studying viruses
Isolation and cultivation (I) - In vivo
Animals and eggs are the first method used for virus cultivation. Suitable living mammals are chosen for cultivation of viruses. Only healthy animal that are free from any communicable disease are selected. The specific virus then can be introduced into the healthy animals. Examples of animal selected for virus cultivation are chimpanzees, mouse, sheep and rabbits. The animals are then slaughtered at the end of incubation period and are washed thoroughly and viruses are obtained from them. However, this method are inconvenient, it also concerns the safety in handling animals and animal rights.

Just like how we go through different stages in life, viruses too undergo various stages in their life cycle. I am just wondering whether it is as challenging as a human’s life.

Stages of Virus Life Cycle
There are 6 stages in a virus life cycle.

1. Attachment
Attachment is a specific binding between viral surface proteins and their receptors on the host cellular surface. This specificity determines the host range of a virus. This means that the virus particle is only able to attach to the host cell if it has the appropriate attachment protein. If not, attachment would not take place. For instance, HIV attacks only human's immune cells (mainly T cells), because its surface protein can interact with ‘CD4’ and chemokine receptors on the T cell's surface.

2. Penetration / Entry
Following attachment, viruses may enter the host cell through receptor mediated endocytosis or other mechanisms.

Enveloped viruses
· Entry by fusing with the plasma membrane

Some enveloped viruses fuse directly into the plasma membrane. Thus, the internal components of the virion such as the genome are immediately delivered to the cytoplasm of the cell.

· Entry via endosomes at the cell surface

Some enveloped viruses require an acid pH for the fusion to occur as they are unable to fuse directly with the plasma membrane. These viruses are taken up by invagination of the membrane into endosomes. As the endosomes become acidified, the latent fusion activity of the virus proteins becomes activated by the fall in pH and the virion membrane fuses with the endosome membrane. This results in delivery of the internal components of the virus to the cytoplasm of the cell.
Non-enveloped viruses Non-enveloped viruses may cross the plasma membrane directly or may be taken up into endosomes. They then cross (or destroy) the endosomal membrane.

3. Uncoating
Uncoating is a process that viral capsid is degraded by viral enzymes or host enzymes. The viral genome then enters the cell.

Pions is discovered by a man named Stanley Prusiner. Before his discovery it is known as 'slow virus' cause of it long dely between infection and th onset of signs and symptoms. Prions is a proteinaceous infectious agent that was different from any other known infectious agent in that it lacked nucleic acid. Scientists resisted the concept of prions because particles lack any nucleic acids violate the 'universal' rule of protein sythesis. Proteins are translated from a molecule of mRNA, and the information is a copy of a DNA molecules. So how can prions carry the information required to replicate themselves?

Prions are single proteins, called PrP, and all mammals contains a gene that codes for the primary sequence of amino acids in PrP. Normal, functional structure that folds with several apha-helices, called cellular PrP and the disease causing form having beta-pleated sheets, called prion PrP. Prion PrP is able to convert normal cellular PrP into prion PrP by inducing the conformational change in the shape of cellular PrP. The prion PrP will cause the cellular PrP to refolds to form prion PrP, forming more prions. Thus, the rule of proteins synthesize is maintained as prions do code for new prions but instead convert extant cellular PrP into prions. Prion diseases involve fatal neurological degeneration, the deposition of fibrils in the brain, and the loss of brain matter such that eventually large vacuoles form.

Scrapie is a fatal, degenerative disease affecting the central nervous system of sheep and goats.This disease is classified as transmissible spongiform encephalopathies (TSE). It is later pass to cow, through meat and bone meal that contained scrapie-infected sheep product. This disease is now known as bovine spongiform encephalitis (BSE also known as mad cow). Then it passed to man when we eat BSE infected beef or mutton and this diease is know as Creutzfeldt-Jakob disease in human.

Jiani

Condoms can be used for protected sex.

Virus depends on host that they infected to reproduce. Without a host, viruses exist as a protein coat or capsid and sometime, it is enclosed within a membrane. The protein coat encloses genes that are either DNA or RNA which carry genetic information. When virus comes into contact with a host cell, it can insert its genetic material into its host.

Virus classification
· The nature of viruses do not immediately land themselves to the classical taxonomy
· Viruses only exhibit biological activities inside their hosts.

Lwoff’s scheme for classification
-classification based on shared properties instead of properties of their hosts
-based on nucleic acid of the virus
-symmetry of capsid
-presence or absence of envelop
-the dimensions of virion and capsid.

Baltimore’s system for classification (unlike Lwoff’s, it is not based on physical properties)
- Based on viral genome only and its relationship to mRNA.
- Central dogma of molecular biology.
- According to Baltimore classification of viruses, it is divided into the following seven classes:

ss = single strand; ds = double strand.
(+) RNA is the one which can function as mRNA for the synthesis of proteins. (-) RNA cannot function as mRNA.


ICTV Classification
- nature of genome & sequence relatedness
- virus structure
- natural host range
- cell and tissue tropism
- physicochemical properties of virions
- antigenic properties of viral proteins