- How does it fit into the living scheme of organisms?
The prime and most important component of a virus (or viroid) is its nucleic acid (either DNA or RNA) which codes for the making of a virus just like itself. (HIV virus which causes AIDS is two copies of single stranded RNA with protein core and lipid outer coat; Smallpox virus is double stranded DNA, Polio is m-RNA that replicates itself as m-RNA).
A virus can not reproduce on its own, it must infect a living cell in order to reproduce. The nucleic acid directs and utilizes the host cell machinery to reproduce more of itself. It is important to note that the viral component is only a
small part of the large reproductive machinery needed to make a new virus or a new cell. The bulk of that reproductive machinery (most enzymes, energy supply, ribosomes, basic building block components, right environment) comes from the living cell that was invaded. The virus simply replaces the coding part of the cell and redirects that coding. (A poor analogy would be thinking of a virus as a replacement steering wheel on a car, whereby you certainly would not call a steering wheel a car, but it can dramatically affect the direction of the car.)
Viruses are very small (0.03 to 0.3 µm), and thus they are very hard to detect. A given virus is designed to infect a given type of cell. There are even viruses that infect bacteria (bacteriophage, or simply phage). These are the best studied virus. (Why?)
The smallest virus codes for 3-10 proteins
The largest for 100-200 proteins
The nucleic acid is generally surrounded with a protein that provides a definite structure, plus possibly having some enzymatic activity (Sometimes a lipid membrane type of coat is also possible - derived from the host.)
Is a virus alive?
Yes - They reproduce which is a definite form of organization. True, they can not do it on their own, yet all living systems are open systems and depend upon their environment for survival. It is just that the environment a virus requires is another living cell. This is actually true for many symbiotic and parasitic cells.
No - A virus is not a living machine . It is only part of a machine, namely the blueprints. The virus only provides the plans, the cell has done the real living work. Outside of its host the virus shows no living qualities.
Of this major scheme, the virus only provides the DNA or the m-RNA (the blue prints) while the living cell has to provide the living factory (energy, enzymes, ribosomes, amino acids, nucleic acids, etc.)
Viruses have been found in a wide variety of forms.
Single stranded DNA, double stranded DNA, single stranded RNA.
Just nucleic acid with no coating (viroid), protein coated (e.g. phage), and membrane and protein coated.
Some viruses have been found such that they are directly incorporated into the DNA of the host organism and the viral DNA is dormant until stressful situations when it becomes activated.
As viruses are reproduced in a cell, imperfect replication of the DNA or RNA code can occur just as can occur in normal cells DNA replication. Such changes are mutations, just like in normal cells. And, just like in normal cells, usually such mutations are a disadvantage to the virus BUT occasionally the change is an advantage. Hence viruses DO mutate and evolve. Some viruses mutate and evolve more rapidly than others. Human disease viruses that have a high mutation rate are hard to control - in this high mutation rate category are the flu viruses and the HIV virus. The smallpox virus and polio virus have a low mutation rate and we have been able to almost totally eliminate these!
