1. What was some early evidence of the existence of viruses? Why were they difficult to study?

2. What was Wendell Stanley's contribution to our knowledge of viruses?

3. What are the four forms of viral genomes?

4. What is a capsid? What are capsomeres? What different shapes may capsids have?

5. As you see, all viruses consist of a nucleic acid enclosed in a protein coat. Some viruses also have a membranous envelope. What are the components of a viral envelope? Which component is derived from the host cell, and which is of viral origin?

Answer No 1:

The earliest evidence for the occurrence of viruses was the discovery of an infectious agent in the sap of a tobacco plant. Experiments were set with different filters from which bacteria could pass but viruses being even smaller than bacteria could not pass.

Viruses were difficult to study because they are very small and couldn't be seen even under a microscope.

Answer No 2:

Wendell Stanley made his studies on the tobacco mosaic virus (TMV). Wendell Stanley developed techniques and crystallized this virus. At that time, it was known that even a very simple cell does not undergo crytallization. Hence, Wendell Stanley got to known that this structure is some infectious agent which has the ability of crystallization.

Answer No 3:

The four forms of viral genomes are:

double-stranded DNA single-stranded DNA double-stranded RNA single-stranded RNA.

Answer No 4:

The capsid can be described as proteins which make up a shell to enclose the genome of a virus. Capsomeres can be described as the protein sub-units from which a capsid is made. The shapes of a capsid can be characterized into

rod-shaped: helical viruses, or inside spherical viruses. icosohedral: icosohedral viruses or the heads of bacteriophages.

Answer No 5:

The components of a viral envelope involve phospholipids and membrane proteins and proteins and glycoproteins. The phospholipids and membrane proteins are derived from the host cell. The proteins and glycoproteins are of the viral origin.