Stages in virus life cycle
1. Attachment with the host receptor protein.
2. Entry of the viral nucleic acid into the host cytoplasm.
3. Replication and expression of the viral components.
4. Assembly (maturation) of the viral components into complete viral units.
5. Release (exit) of the completed virus from the host cell.

Introduction
The basic principle of viral infection of a given cell is that the cell’s outer surface contains the host cell receptor in which the virus attachment protein (VAP) can bind, the receptor on the host cell will then mediates viral entry into cell and the virus will then grow in the cell. However, this is only applicable within related species or to different types of cells within a host. For example, the common cold virus (Picornaviridae) are specific for the human upper respiratory track as they contain the appropriate VAP for binding to the receptors on the cell membrane of the specialized cells in the upper respiratory tract. There are other viruses that cause intestinal diseases only.

Attachment and entry: the virus blinding to the host cell receptor sites (small black spots) through its attaching proteins at the end of the tail fibres. Once firmly attached to the cell, a viral enzyme in the tail punches a hole in the host‘s cell and the core region of the tail is thrust through and the genome is injected into the host’s cytoplasm.

Replication and expression: once inside the host, the viral genome takes over the metabolism of the host and converting it to its own ends. The viral nucleic acid is then transcribed and translated and the various viral proteins proceed to create everything required to make new viruses.

Maturation and release: the various components accumulate inside the cell and start to SPOTANEOUSLY ASSEMBLY into new virus. However, this process is an orderly one so that each component is added in the proper sequence. Enzymes are produced causing the cell to burst or die thus a crop of new viruses are release into the environment.

Routes of host entry
->Endocytosis:
-non-specific endocytosis
-phagocytosis
-receptor-mediated endocytosis
-> Fusion
-> Direct penetration

1. Naked virus enters the host cell through translocation whereby particle crosses cell membrane intact.
2. Naked virus also enter the host cell through genome injection in which virus attaches to cell surface and release its genome that penetrates the cytoplasm by pore that has been created in the plasma membrane. E.g. bacteriophages.
3. Naked virus-Endocytosis: the virus attaches to the host cell receptor molecules and sinks into a clathrin coated pit. The pit invaginates and lastly closes off producing a clathrin coated vesicle and the contained virus is absorbed into the cytoplasm. E.g. adenovirus
4. Enveloped virus- virus enters cell by receptor mediated endocytosis whereby it depends on the interaction of the virus with a specific binding protein in the cell membrane called a receptor. E.g. influenza virus.
5. Enveloped virus-membrane fusion whereby the virus enters the cell by fusing its outer membrane with the plasma membrane of the host cell. The viral contents are then spilled into the cytoplasm of the cell. E.g. HIV virus.

Naked viruses have an exposed protein capsid while enveloped viruses are cloaked in cell membrane, added during budding from the host cell.

By man hua.

Tissue culture methods for detection
Virus, by subverting its host to make more copies of itself and next, replicating itself into large numbers. In doing that, virus usually cause some changes to its infected host such as morphology of the cell changes, cells swelling or shrinking, etc. These changes are known as cytopathic effects (CPE).

Plaque assay is a technique based on one virus infects one cell and spreads to surrounding cells; the virus then killed the cells and thus forming plaques. This method requires that the virus infects a cell line that grows as a monolayer. There is higher accuracy when the concentration of virus sample is low therefore serial dilution of the virus sample is done.

By man hua.