The following points highlight the two main parts of virus particles according to Bawden and Pirie. The parts are: 1. Nucleoid 2. Capsid.
Virus Particle: Part # 1. Nucleoid:
It is present within the protein coat (Fig. 1). Each virus particle has only one type of nucleic acid either RNA or DNA. The viruses containing DNA are called Deoxyviruses and those having RNA are known as Riboviruses.
All the plant viruses had been reported to contain single stranded RNA while animal viruses either single or double stranded DNA. Bacterial viruses contain mostly double stranded DNA. Most of the insect viruses contain RNA and only a few have DNA.
The DNA of several animal and bacterial viruses is circular whereas in others it is like RNA. Shephered (1968) reported that cauliflower mosaic plant virus contains double stranded DNA. Double stranded RNA is also reported in Rice dwarf viruses and wound tumor viruses.
The type of nucleic acid and the number of strands in different viruses are given below (Table 1):
Virus Particle: Part # 2. Capsid:
It is the protein coat surrounding the internal nucleoid. It is made up of repeating protein subunits called capsomeres. The capsomeres are composed of either one or several types of protein. Host specification of viruses is due to protein of capsid.
The number of capsomeres in the capsid of a given virus is constant (32 in the poliomyelitis virus, 252 in the Adenovirus, 2130 in the Tobacco Mosaic Virus etc.) and they are arranged in a very symmetrical manner and give a specific shape to a particular virus.
Usually they are arranged in two geometric forms:
1. Helical Viruses:
The nucleic acid is coiled like a spring and capsomeres are helically arranged around their string, e.g., Tobacco mosaic virus.
2. Icosahedral Type:
The nucleic acid is packed in unknown manner within a hollow polyhedral head. The capsid is antigenic and protects nucleic acid from un-favourable extra-cellular environment. It also makes easy the entry of nucleic acid into the host cells. The core within capsid is called the nucleocapsid core.
The Envelope:
Some animal viruses e.g., Herpes viruses and pox viruses develop a 10-15 nm thick lipoprotein envelope around their protein coat while it is absent in plant viruses. The lipid is derived from the host while the protein is of viral origin. This envelope is also known as mantle or limiting membrane and is covered with projections or spikes. It is made up of several subunits called peplomers.
Enzymes and Other Contents:
In addition to the nucleoprotein, viruses have enzymes, water and carbohydrates. The enzymes are collectively called “transcriptase’s”. These enzymes are essential for infection and growth of viruses. However, the function of carbohydrates is not known.