Viruses are simple, yet very successful non-living, infectious agents. These efficient parasites make a living commandeering the cells of organisms and turning them into virus factories. A virus undertakes this monumental invasion with very little of its own equipment. So, what does it take to be considered a virus?
* What Are Viruses Made Of? *
Viruses are composed of nucleic acid, proteins, and, in some cases, lipids as well. Their nucleic acid is surrounded by a protective protein coat, called a capsid. Some viruses are also enveloped by an outer membranous layer made of lipid and protein, called an envelope.
* Viral Genetic Material *
Nucleic acid is the genetic material of all living organisms, as well as that of viruses and viroids (bizarre infectious agents made only of nucleic acid and nothing else). The genetic material of both living things and viruses is called its genome. Essentially, it's the blueprint of all information needed to make more of that same organism or acellular particle. The instructions in nucleic acid are read by living cells and ultimately result in the building of specific proteins.
* Viral Genomes are Small *
Viral genomes are tiny, much smaller than the genomes of living organisms. While it is the same kind of nucleotide building blocks that make up a nucleic acid genome, whether one of a living thing or virus, viruses have fewer of these building blocks in their genome, therefore less information. This is because viruses only need to contain enough information to take over a cell and then tell that cell how to make more viruses. Making more of themselves is the singular task of viral parasites.
* Viral Genomes Can Be Either DNA or RNA *
Although the genetic material of living organisms is exclusively DNA (deoxyribonucleic acid), the genetic material of viruses can be either DNA or RNA (ribonucleic acid), although never both. The viral nucleic acid encodes the genetic information that is necessary to make copies of the virus. Although viral genomes are small, they do show more variability than the genetic material of living things, since it is only viral genomes that can be either DNA or RNA.
* Making Sense of Viral RNA Strands *
Not only can viral genomes be made of either DNA or RNA, they can be single-stranded DNA, double-stranded DNA, single-stranded RNA or double-stranded RNA (ssDNA, dsDNA, ssRNA or dsRNA respectively). If made of single stranded RNA, then even the type of RNA, called the positive or negative sense strand, is used in viral classification.