Bacteriophage can inject its genes into a bacterium
Some viruses inject DNA into a cell which is taken into the cell's chromosome. Others inject RNA which is either used by the cell's ribosomes to create new protein or converted first into DNA. Viruses which cause the latter process are called retroviruses.
Once inside the chromosome, the new DNA begins to give out for making new viruses. The host cell is thereby turned into a factory for making new copies of the virus.
When it is full of viruses the cell bursts open, sending out millions of viruses to infect more cells.
Size of Viruses
If a soccer ball were blown up to the size of the Earth () then a typical would be from 1 meter across (not much bigger than a ribosome) to 20 meters across. On average it would be about the size of a small car.
Origin of Viruses
Nobody is sure how viruses began. The existence of very large viruses (megaviruses) which contains enough genes to encode about one thousand proteins, more than most bacteria, suggests that they were originally cells which lost the ability to reproduce on their own, adopting instead a parasitic way of life.
However the existence of bacterial spores suggests that perhaps they evolved into viruses.
cause many diseases in plants and animals. For example the human retrovirus HIV causes acquired immune deficiency syndrome (AIDS). Other viruses cause Burkitt's lymphoma, chickenpox, the common cold, conjunctivitis, Creutzfeldt-Jakob disease (CJD), encephalitis, enterovirus disease, gastrointestinal infections, glandular fever, hepatitis, herpes simplex, infectious mononucleosis, influenza, kuru, Lassa fever, measles, mumps, polio, rabies, respiratory infections, rubella (German measles), shingles, smallpox, some cancers, warts and yellow fever. However, viruses also bring some benefits.
A seems like a totally , killing cells without doing any good. Usually this is true, but viruses can carry useful genes from one cell to another. engineers use viruses for this reason.
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