Viruses

Contents Definition Shapes Basic structure of a virus Viruses are obligate parasires How viruses infect a cell Viral Reproduction / Effect of viruses on cell Medical importance of viruses Economic importance of viruses Beneficial viruses Conditions caused by viruses

Definition virus: (plural = viruses) are non-cellular micro-organisms, made up of a protein coat and one type of nucleic acid (DNA or RNA) - obligate parasites = can only multiply inside a living cell - cause disease. Are they living or non-living? Examples of diseases caused by viruses are influenza (flu), HIV (causes AIDS), chicken pox, measles, mumps, rubella.

Shapes Viruses are found in a number of different shapes e.g. bacteriophage, etc. Note: names of various shapes not needed for L.C.

Basic Structure of a Virus Consists of a nucleic acid (DNA or RNA) and a protein coat. Some may have a membrane coat outside of this, made from the membrane of the cell that was infected; with virus proteins inserted into it e.g. influenza.

Viruses are obligate parasites Can only replicate within other living cells. Can only replicate within a limited no. of organisms. Some only replicate within a specific type of cell of the host.

How viruses infect a cell To infect a cell, the surface proteins of the virus only bind with certain proteins on the surface of the cell. Viruses are specific.

Viral Reproduction / Effect of Viruses on Cells (1/2) When a virus enters a cell one of two things may happen: - 1. Viral DNA takes over the host cell – makes the host cell produce copies of the viral protein and DNA (or RNA) from materials within the cell. These are then assembled into new viruses and released by the bursting (lysing) of the host cell.

Viral Reproduction / Effect of Viruses on Cells (2/2) Viral DNA may be inserted into the host DNA. Becomes integrated with host DNA and is passed on from generation to generation when cell replication takes place. These are lysogenic cells – and the new DNA has no apparent effect. Generations lateer – environmental conditions may change (e.g. radiation, chemicals, etc.) and activate the virus to make more copies of itself and lysing the host cell.

The possible effects of a virus on a cell

Medical importance of viruses They cause many diseases. Antibodies cannot cure viral diseases. Vaccinations available for some viral diseases e.g. chickenpox, measles, etc. No protection against other viral diseases e.g. AIDS. Some new medicines interfere with viral replication e.g. medication against cold sores.

Economic importance of viruses They damage crops e.g. tobacco mosaic virus. Sick people effect the economy – out of work – need to be cared for. In parts of Africa 10% of the population is infected with HIV – devastates their societies.

Beneficial viruses Very hard to find. Bacteriophages used to destroy certain harmful bacteria. Some kill disease-causing bacteria / fungi e.g. Lambda bacteriophage attack E. coli. Used in medical research e.g. in genetic engineering and the production of DNA profiles e.g. retroviruses produce the enzyme reverse transcriptase which is needed for cutting the DNA molecules in these processes. Some viruses live in rhubarb and reduce size of plant.

Conditions caused by viruses not examinable for information only

Influenza

Other conditions caused by viruses Foot & mouth disease Distemper Myxomatosis Rabies (hydrophobia) Common cold Measles Mumps Rubella = German measles Smallpox Chickenpox Poliomyelitis = Polio Viral pneumonia Genital herpes AIDS

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