Viruses have existed since the existence of life forms on earth. They are tiny organisms with just a genetic code or nucleic acid and a protein cover. Their small size makes them visible only under the electron microscope.

Are viruses living organisms?

There is controversy regarding whether viruses should really be considered as living organisms. Viruses depend entirely on a host cell for their multiplication and functioning.

Types of virus

These pathogens are one of the most widespread of all organisms and are capable of infecting every species of animal from mammals down to insects, protozoa, and even bacteria and plants. In fact there are more species of virus than of all other creatures put together.

Some are harmless while some are extremely dangerous and like HIV causing AIDS or Ebola and Marburg virus etc.

Origin of viruses

Researchers have found resemblance of the genomes of viruses with genes of higher animals that can ‘jump’ from one chromosome to another. Some believes these viruses may have originated from bacterial plasmids, which are little packets of genes lying outside the bacterial chromosomes and capable of being transferred to another bacterium.

Viruses may also have originated from degenerate bacteria that have become obligate parasites.

Viral structure, replication, and function

Viruses vary greatly in size as well as complexity. But some of their features are common among all species of viruses. There are three basic parts:

• nucleic acid
• protein coat
• lipid membrane

Viral nucleic acid

The nucleic acid is the core of the virus. It is either DNA or RNA (deoxyribonucleic acid and ribonucleic acid respectively). The DNA or RNA holds all of the information for the virus and that makes it unique and helps it multiply.

There may be either RNA or DNA and never both and this determines the way in which viruses replicate themselves. Both types of genome however function to make viral proteins and more copies of the viral coat and DNA/RNA.

DNA viruses are the simplest. They use the host cell’s RNA polymerase can make mRNA that translate on the host ribosomes to make viral proteins.

RNA viruses on the other hand need to provide their own RNA polymerase to make mRNA.

In addition, those that are negative-sense RNA viruses need yet another step to make positive-sense RNA.

Protein Coat (capsid)

This is a covering over the nucleic acid that protects it. This has a symmetrical structure and is built of one or more subunits packed like a chemical crystal.

Lipid membrane (envelope)

This covers the capsid. Many viruses do not have this envelope and are called naked viruses. This membrane is usually acquired by the virus from the host cell in the process of leaving the cell. This coat enables the virus to survive outside the cell sufficiently long to spread elsewhere via the blood.

Viral structure

The capsid and entire virus structure can be of four main types:

1. Helical

There is a capsomer coiled around a central axis to form a helical structure. This is a common structure seen in single stranded RNA viruses. Tobacco mosaic virus is a helical virus.

2. Icosahedral

These are near-spherical and this shape is adopted because the coat forms a closed shell. Rota virus has twelve capsomers and appear spherical.

3. Envelope

The virus is covered with a lipid membrane in a modified form of one of the cell membranes. The outer membrane is from the infected host cell and internal membranes from nuclear membrane or endoplasmic reticulum forming a lipid bilayer known as a viral envelope. This membrane is studded with proteins or receptors.

4. Complex

There is a capsid that is neither purely helical, nor purely icosahedral. There may be extra features like protein tails or a complex outer wall. Bacteriophages are examples of this type of viral structure.

Viral receptors

The virus has to enter the cell in order to infect it. Viruses are not taken up by the cell directly. They must attach to a receptor on the cell surface first in order to gain entry. If the receptor is not a necessary one, the cell once infected with the virus, may go on to remove the receptor altogether.

Each virus has its specific receptor and it is a vital component of the cell surface so that the cell cannot get rid of it to avoid the infection. The selectivity of the viruses determines the cell-preference.

For example, rhinoviruses have a preference for cells lining the nose, airways and the lungs, HIV with CD4, CCR5, CXCR4 viruses, Epstein–Barr virus (EBV) with Rabies virus with CR2, Acetylcholine receptor, Influenza virus with Neuraminic acid on red blood cells etc.

HIV infects mainly T lymphocytes and macrophages because only they carry a surface molecule known as CD4 receptor and EBV infects B lymphocytes carrying the complement receptor CR2.