Basic Study of Virus, It's Shape, Structure And Genetic material

 Bacteria are the widely known pathogens. Back in the late nineteenth century, scientists could physically observe many of these infectious agents under a microscope, and even grow some of them inthe laboratory. But in the 1890s, two scientists named D. M. Iwanowsky and Martinus Beijerinck were puzzled by an unusual infectious agent. They observed a disease in tobacco plants called mosaic disease, but the infectious agent was too small to be seen with a light microscope.

  They could only grow this agent in media that contained living cells, and to their surprise, it could pass right through the filters they typically used to catch bacteria. Beijerinck called the mysterious infectious agent a “filterable virus”, where the term virus comes from a Latin word literally meaning “poison.”

  About ten years later, two other scientists named FĂ©lix d’Herelle in France and F.W. Twort in England independently discovered such a “filterable virus” that infects bacteria. A virus that infects bacteria came to be known as a bacteriophage, and with that the field of modern virology was born.

  A virus is just a bundle of genetic information, which is either RNA or DNA but never both, contained within a protective protein coat. Viruses are inert particles, meaning they can’t move around on their own. viruses are also incapable of metabolism or replication on their own. When a virus finds its way to a host cell, however, the virus hijacks the cell’s machinery, using the cell to make more copies of it self.

  Viruses are considered obligate intracellular parasites, and most scientists agree that viruses are not alive, as they don’t fit the criteria we use to define life. Viruses can be grouped into two general categories based on the type of cells they infect, those being prokaryotic and eukaryotic. Although both groups are viruses, as we said the ones that infect bacteria are referred to as bacteriophages, or sometimes just phages.

How big are viruses?
   Most viruses are a hundred to a thousand times smaller than the cells they infect, which explains why early microbiologists could not see them. They can range from about 10 nm in diameter, with as few as 10 genes, to about 800 nm in diameter, which is quite a range.

  A viral particle, also called a virion, is comprised of a nucleic acid surrounded by a protein coat. The protein coat is called a capsid and it protects the nucleic acid from any chemicals or enzymes the virus might encounter.
• Together, the capsid and the nucleic acid it protects is called the nucleocapsid.
• Some viruses have a lipid bilayer outside of the capsid, which is called an envelope.
• Viruses without an envelope are called naked viruses. Phages, in fact, are nearly all naked.

  Since all viruses contain a single type of nucleic acid, they’re first classified as either an RNA virus or DNA virus. Some viruses have a circular genome, while others have a linear genome. The genome itself can be single-stranded or double-stranded, and the type of genome that a virus has informs its replication strategy.

Shapes Of Viruses

Generally, viruses fall into three shapes:
        • Helical,
        • Icosahedral,
        • Complex.

  Helical viruses look cylindrical when viewed with an electron microscope, and can be either short and rigid, or long and filamentous.
Icosahedral viruses look spherical under an electron microscope, but their surface is actually much like that of a soccer ball, with 20 flat triangles arranged around the surface.
  Complex viruses are more intricately constructed and tend to vary more in shape.

  Viruses also have specific protein components that enable the virion to bind to receptors on the surface of host cells. When it comes to viral classification and nomenclature, it can get pretty messy. The good news is that there’s an entire international committee dedicated to classifying viruses, called the International Committee on Taxonomy of Viruses, or ICTV. According to this system, a species of virus is defined as a population of viruses that share a pool of genes distinct from the gene pools of other viruses.

  So even though we don’t consider them to be alive, viruses can still be grouped into a species. Species of viruses can be grouped into a genus. A group of related genera, which is plural for genus, can be grouped into a subfamily. A group of related subfamilies can be grouped into a family. And finally, a group of related families is called an order. For the most part, classification into families is based on characteristics of the virion and its genome.

The criteria for classification of virus
First, there is morphology.
- What is the size and shapeof the 
  virion?
- Does the virion have an  
  envelope?
This is all usually determined by electronmicroscopy.
Next, we have all of the structural proteins.
- What is the molecular weight of  
  the virus?
And lastly, there are genome properties.
- What type of nucleic acid does it  
  have?
- Is it double stranded or single   
  stranded?
- If single stranded, is it positive 
  sense or negative sense?
- What is the sequence of 
  nucleotides?

  These are the relevant criteria that are considered.Unfortunately, the names of viruses have no consistent pattern, other than the names of virus families ending in the italicized suffix viridae. Some are named for the appearance of the virus, some are named for the geographic region they were discovered in, some are named for the disease they cause, and some by their route of transmission. While bacteria are typically called by both their genus and species name, viruses are typically referred to by their species name only. And to make matters worse, many virologists refer to viruses by informal names.