Structure of viruses
Viruses are often considered non-living as they exist in an inert state outside of a host cell. They consist of a strand of nucleic acid, either DNA or RNA, surrounded by a protective protein coat (the capsid). Sometimes they have a further membrane of lipid, referred to as an envelope, surrounding the protein. They may also have a tail section.

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Structure of viruses
Transmission of virusesViruses can be transmitted by travelling within a fluid, such as mucus droplets from a sneeze.

Once they have entered a living host they begin the process of replication. This replication process always follows a particular pattern.

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Attachment
Stage : Attachment
Description :
Virus binds to a specific receptor on the host cell surface.

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Penetration
Stage : Penetration
Description : The viral nucleic acid enters the cell.

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Synthesis of new components.
Stage : Synthesis of new components.
Description : Viral nucleic acid takes over control of the cell metabolism stopping the cell's normal nucleic acid and protein synthesis.
Viral nucleic acid is replicated using nucleotides from the host cell.
Protein costs are manufactured using the amino acids of the host cell.


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Assembly
Stage : Assembly
Description : Whole virus particles are made when the nucleic acids are surrounded by the protein coats.

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Release
Stage : Release
Description : Many viral particles are released when the cell bursts open (lysis) or by slow leakage.





Cellular defenceWhen an organism becomes infected with either a bacteria or virus it attempts to limit the damage that can be done to its tissues.

AnimalsThere are two types of white blood cells – phagocytes and lymphocytes.

Phagocytes engulf and breakdown foreign particles. The phagocyte detects the foreign object and moves towards it. The membrane of the phagocyte then projects out and encloses the foreign object in a vacuole.

The lysosomes (powerful enzyme containing vesicles) inside the phagocyte fuse with and release enzymes into the vacuole. The enzymes breakdown the foreign object and the products are either absorbed into the cell or released.

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Phagocytes
Lymphocytes produce antibodies in response to glycoprotein markers (antigens) on the surface of viruses/ bacteria.

Antibodies are specific proteins that attack and destroy substances that enter the body which are carrying foreign antigens. The production and action of antibodies is called the immune response.

Once the body has produced antibodies in response to an antigen (primary response), the memory of the antigen is retained. If the organism invades again the body recognises the antigen and produces antibodies in higher concentrations more quickly (secondary response).

PlantsIsolation - Plants produce resins that isolate areas of infection and prevent the spread of the pathogen to other parts of the plant.

Toxic compounds

Example 1. Hydrogen cyanide is a respiratory inhibitor produced by some plants, such as clover, in response to tissue damage. These cyanogenic plants are less likely to be eaten.

Example 2. Tannin is an acidic chemical, found in tea plants, which most herbivores find distasteful. Whilst nicotine, produced by tobacco plants, is a poison. Both of these compounds reduce the likelihood of grazing.