Explain Viruses and their classification?
Viruses are living organisms. Viruses are not living organisms.
No, the above is not a misprint! In fact, viruses defy the normal classification schemes that are applied to living things. First of all, they clearly lack the cellular organization that all living organisms possess. Viruses do not possess the metabolic machinery that would enable them to make proteins or to carry out metabolic processes such as respiration and photosynthesis, convert energy, acquire food, build structures, and reproduce. So right off the bat, viruses do not conform to the defining features that characterize life.
Viruses are considered to be the simplest living organisms. In fact, there are those who would not classify them among the living because they lack the organization of a "true" cell, and must rely upon true cells to reproduce. They basically consist of nucleic acids wrapped in a protein coat.
While viruses do not conform to the traditional definition of life - that is, they are not cells, and they do not reproduce by themselves - they do represent the most fundamental mechanisms of living systems. Some think of viruses as the extreme end of the evolutionary process, where they have evolved to the point of not needing the metabolic machinery that sustains the functions of cellular life. In other words, if viruses can get other cells to perform the processes of reproduction, energy trapping and conversion, then there is no need to build and maintain these organelles themselves!
Other scientists think that viruses are left over from the very first life forms to evolve, the prototypes of cells. Yet other scientists are fragments, or parts of genetic material that broke off from living cells.
Viruses are therefore difficult to classify. They do not fall under any of the traditional groupings of organisms, and so some have suggested that they represent their own kingdom. But because the viruses do not have a common ancestry, they do not lend themselves to such a grouping. About the only traits that viruses have in common are their tiny size, their simple structure, and their parasitic life style.
Viruses are very small - measuring on average between 20 and 300 nanometers across, which is about the size of the smallest bacteria. Also unlike cells, viruses are particles that can be crystalized. Some scientists refer to these particles as "active particles" because they interact with living cells. There are different types of viruses. Some contain DNA (single or double stranded), others RNA (single or double stranded). The RNA and DNA come as either linear or circular molecules, containing anywhere from 4 to a few hundred genes.
The "head" of a virus is made of a protein container called a capsid. The capsid comes in a variety of shapes and sizes - helical, polyhedral, cuboidal, or rectangular. The capsid itself is composed of building block protein subunits called capsomeres. Some types of viruses have an envelope that surrounds the capsid, which is similar to a cellular membrane. The capsid encloses the viral particle, sometimes referred to as the virion, and in some cases, also an enzyme.
LYTIC VIRUSES
Click on the Multimedia button on the left to view the life cycle of a lytic virus.