THE FRAGRANCE

dna viruses: replicATION :

three objectives :

1. to make their copy of dna genome

2. to make their proteins ( for this , they have to make m rna from dna and then , proteins from this m rna)

3. to do all this, they need enzymes present in the host...now these enzymes are present in nucleus of the host ..so, viruses have to enter the nucleus of the host ..

so, we have two types of viruses :

nuclear dna viruses :DNA replication occurs in the nucleus.

cytoplasmic dna viruses : POX VIRUS

NUCLEAR DNA VIRUSES :

1. PARVO :

Human parvovirus B-19 replicates in dividing cells – primarily in erythrocyte progenitors in the bone marrow - and causes fifth disease (erythema infectiosum). This is usually a mild disease but the decreased production of red blood cells can be a problem in people with various types of severe hemolytic anemia

2. papova viruses :

Papillomaviruses are difficult to grow in culture. and are a part of teh group DNA tumor viruses).

polyomaviruses :

Depending on the host cell, they can either transform the cell or replicate the virus and lyze the cell.

* read copy....

*other important points are : -

*Attachment is via ionic interactions which are temperature-independent. The viral attachment protein recognizes specific receptors, which may be protein, carbohydrate or lipid, on the outside of the cell. Cells without the appropriate receptors are not susceptible to the virus.

*Enveloped viruses

(A) Entry by fusing with the plasma membrane. Some enveloped viruses fuse directly with the plasma membrane. Thus, the internal components of the virion are immediately delivered to the cytoplasm of the cell .
(B) Entry via endosomes at the cell surface (figure 2)

Some enveloped viruses require an acid pH for fusion to occur and 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

*Nucleic acid has to be sufficiently uncoated that virus replication can begin at this stage. When the nucleic acid is uncoated, infectious virus particles cannot be recovered from the cell - this is the start of the ECLIPSE phase - which lasts until new infectious virions are made.

*Not all released viral particles are infectious. The ratio of non-infectious to infectious particles varies with the virus and the growth conditions.

*Cytopathic effect (CPE)The presence of the virus often gives rise to morphological changes in the host cell. Any detectable changes in the host cell due to infection are known as a cytopathic effect. Cytopathic effects (CPE) may consist of cell rounding, disorientation, swelling or shrinking, death, detachment from the surface, etc.
Many viruses induce apoptosis (programmed cell death) in infected cells. This can be an important part of the host cell defense against a virus - cell death before the completion of the viral replication cycle may limit the number of progeny and the spread of infection. (Some viruses delay or prevent apoptosis - thus giving themselves a chance to replicate more virions.)

*
Assays for plaque-forming units
The CPE effect can be used to quantitate infectious virus particles by the plaque-forming unit assay (figure 5).
Cells are grown on a flat surface until they form a monolayer of cells covering a plastic bottle or dish. They are then infected with the virus. The liquid growth medium is replaced with a semi-solid one so that any virus particles produced as the result of an infection cannot move far from the site of their production. A plaque is produced when a virus particle infects a cell, replicates, and then kills that cell. Surrounding cells are infected by the newly replicated virus and they too are killed. This process may repeat several times. The cells are then stained with a dye which stains only living cells. The dead cells in the plaque do not stain and appear as unstained areas on a colored background. Each plaque is the result of infection of one cell by one virus followed by replication and spreading of that virus. However, viruses that do not kill cells may not produce plaques .