Origin of Viruses
A virus is simply an infectious agent of very small biological composition and hence easily multiply in living cells of humans, animals, or even bacteria. A virus contains encoded genetic material enclosed in a protein shell, or envelope. Like other viral particles, the virus also has a protective envelope which acts as a boundary keeping the viral particles from infecting other cells. In fact, a virus behaves like a dangerous intruder when it invades a human cell. The invader turns into a destructive substance which causes destruction of the infected part and also leads to death of the invading organism.
Since, a virus spreads itself over great distances without any physical contact, it is usually termed as airborne virus. The viruses that spread through contact are known as infective viruses whereas those that replicate themselves by means of a chemical reaction are termed as virucidal viruses. The major categories of airborne viruses are: herpes, SARS, Epstein-Barr, Mumps, Rubella, and Swine flu.
Viruses can be categorized further as either single-celled or multicellular. Single-celled viruses are those that affect single cells while multicellular viruses are those that affect multicellular organisms or tissues. For example, the SARS virus, which is a transmittable virus, affects mostly the lungs and the gastrointestinal tract to the rubella virus, which is a nontransmittable virus can affect both the respiratory tract and the reproductive organs. Tobacco plants are the main reason behind the increased rate of infections.
There are certain aspects that separate the various species of plant viruses. First, they all belong to the hosts’ family in which all members have similar characteristics, except for the ability to produce disease. Second, the host range is highly diverse and consists of several orders of living organisms. Third, the host range of each virus varies significantly depending on the type and location of infection. These factors are important in determining the ability of the virus to multiply and spread.
Virus particles are usually protein molecules. The basic genetic code, however, is encoded within the DNA and is therefore not capable of being read by any type of protein scanning methods. Therefore, viruses may attach to the target organ where it inserts its genetic code. It also takes the form of a polypeptide if it has been created by a living cell. The viral envelope, capsid, and other outer membrane proteins enable the virus to latch onto the target cell or organism.
The viral envelope of the virus contains instructions for the initiation of the replication of the virus. In comparison with bacterial viruses, whose envelopes are complex due to the genetic material that codes for their formation, animal viruses have simpler envelopes. However, to produce their viral envelopes, these animal viruses require only the genetic material encoded in their DNA, whereas the bacterial viruses need an external DNA resource that is passed on from one cell to another. In addition, although bacterial viruses may be single stranded agents, there are multiple combinations of single stranded and tandem replications that are essential for their progression through the bacterial cell cycle.
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