Viruses

Viruses are replicative bits of DNA or RNA within a protein coat; sometimes they are also enveloped in a lipid bilayer that comes from the host cell. Viral nucleic acids code for proteins that assemble viruses within host cells. Although often called organisms, viruses are not true organisms but rather obligate molecular parasites that in principle lack the full complement of cellular machinery necessary to reproduce on their own. The biggest taxo-nomic division in living beings is that between prokaryotes, bacteria with no nuclei in their cells, and eukaryotes, all other nonbacterial cells, which as a rule possess nuclei in their cells (there are a few exceptions, such as the blood cells of mammals, which lack nuclei).

The five-kingdom classification scheme divides all life into bacteria (prokaryotes) and their evolutionary descendants—protoctists, plants, animals, and fungi. Notice that viruses do not appear within these kingdoms. Viruses do not grow bigger and split in two as microorganisms do. They can reproduce prodigiously, but not on their own. Viruses are so small that thousands of them can fit in the nucleus of a eukaryotic cell; whereas an animal cell consists of 10,000 genes coding for specific proteins, some viruses have as few as four genes. Lacking the chemistry of self-maintenance called metabolism, they are much smaller than cells. Outside of cells viruses are inert—they cannot reproduce, feed, or grow. Little more than pieces of DNA or RNA, viruses are best grouped with plasmids, repli-cons, naked DNA, and other movable bits of nucleic acid that are sometimes integrated into the cyclical functioning of cells, thereby reproducing themselves. An extracellular virus particle, called a virion, may be nothing other than a package of nucleic acid surrounded by many repeat copies of an encasing protein.

Viruses are usually shaped as helical rods or icosahedral spheres. Some are differentiated into rocket ship and hypodermic shapes, with a head full of nucleic acid and a tail to attach to the host cell and inject the viral genes. Some symmetrical viruses have been crystallized using X-ray diffraction crystallography. Advances in electron microscopy and computer imaging improve our understanding of these quasi-beings.

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Worm Farming

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