Usually, when viruses enter a living plant or animal cell, they grow, multiply and cause disease. Some can live in a cell without harming the victim (host) cell. But if the same viruses enter another kind of host, they produce disease. In addition to causing plant diseases, some viruses cause animal illness, as distemper in dogs, and colds, mumps and polio in man.
Viruses have various shapes. They may be spherical, rod-like or bottle-shaped. None of them can move by themselves.
Chemically, a virus consists of a core of NUCLEIC ACID, either RNA, DNA or both. This core is surrounded by a sheath of non-acid PROTEIN. Some viruses leave the sheath behind when they enter a cell. Others are believed to separate the sheath and acid core right after invading the host cell.
Once inside a living cell, viruses organize the cytoplasmic activities (enzyme systems) of the host and start to reproduce. Using raw materials supplied by the host cell, virus particles of RNA or DNA make copies of themselves.
Scientists have been unable to decide whether viruses are living or non-living. Because of their chemical composition, and their behavior within a cell, they appear to be living particles. It has been said that viruses are "cell nuclei in search of some cytoplasm." On the other hand, viruses can be crystallized. In this form, they are inert like non-living substances.
|Kissing the Blarney Stone|
No one believes now that the Blarney Stone has any real power. But every year many tourists go to see the castle. Some of them kiss the Blarney Stone just for the fun of saying that they have done so.
Kissing the Blarney Stone is not easy. First a person must climb to the very top of the castle. Then he has to be held so that he can hang upside down over the stone railing there. Many people decide to be satisfied with kissing some other stone in the wall.
All woodpeckers nest in holes of trees and poles. All lay white eggs. Short, stiff tails, and four-toed feet support their bodies as they cling to tree trunks. The northern black-backed and boreal woodpeckers are the only types with three toes and without some red coloring. The yellow-bellied sapsucker girdles trunks with borings and thus is one of the few woodpeckers that could prove harmful to trees.
What is a depressant?
A depressant is a drug that reduces the activity of various body functions. Some depressants such as anesthetics, sedatives, antiepileptics, narcotics analgesics, and some muscle relaxants, slow nervous and muscular activity by acting on the central nervous system. Tranquilizers are depressants that affect only part of the nervous system. They induce relaxation without causing total depression.