Viruses can attack plant, animal, even bacterial cells. Amongst the viruses that attack animal cells the most notorious are those that attack human cells causing diseases such as small pox, rabies, measles, influenza and the common cold. Even certain types of leukaemia and other cancers are of viral origin.
Structure of Viruses
Although all viruses are extremely small, they vary in size and organisation. Generally, viruses vary in diameter or length from 20 – 200 nm, with the largest viruses actually being bigger than the smallest cells (bacteria and mycoplasmas).
Most viruses are either rod shaped or roughly spherical. They contain a nucleic acid core surrounded by a specific arrangement of protein molecules, which form a coat known as the capsid. The proteins of the capsid are positioned either to form a helical pattern (when the virus is rod-like) or an isometric pattern making the virus appear like a polyhedron. Viruses causing diseases like chicken pox or the common cold have polyhedral capsids. Helical capsids tend to be found amongst viruses that affect plant cells and bacteria, for example the tobacco mosaic virus that infects the leaves of the tobacco plant. In many animal viruses and in some plant viruses, a lipoprotein envelope protects the capsid (for example with the herpes virus).
Life cycle of a virus
Isolated from other living tissue viruses show no metabolism and cannot reproduce. Viral reproduction requires a host cell; in its simplest form, it follows the following pattern. One or more viruses attach to specific sites on the host cell. Following attachment, the viral nucleic acid is inserted through the plasma membrane and into the host’s cytoplasm. Once inside the host cell, the viral nucleic acid changes the metabolism of the host so that new parts of the virus are formed. These viral components assemble in the cell to form new viruses, which either burst from the host cell causing cell lysis or bud from the host cell, which does not lyse the host cell. The viral life cycle then repeats itself.
On some occasions and only with certain viruses, the injected nucleic acid does not trigger the production of new viruses. Instead, the viral nucleic acid is incorporated into the host’s DNA. The host cell continues to behave in its normal way. However every time the host cell duplicates, so does the incorporated viral nucleic acid. But eventually the incorporated viral nucleic acid will begin to direct the replication of new viruses.
Different types of viruses
In some viruses their genetic material consists of RNA, the RNA can be single or double stranded. Examples of RNA viruses include those that cause polio, mumps and measles. Whereas other viruses, their genetic material consists of DNA, again the genetic material can be single or double stranded. Examples of DNA viruses include those that cause chicken pox and herpes.
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Resistance to antiviral agents
Source:
Cell and Molecular Biology by P. Sheeler and D.E. Bianchi, Wiley
