2. Viruses

the basic facts

Questions

What are the basic properties of viruses?

How are viruses classified?

How do viruses replicate?

Viruses are named from the Latin for poison (venenum). They are the smallest and simplest of replicative agents of infection in humans. They are both the commonest organisms found in nature and the most frequent cause of human infection. The following properties distinguish them from living (prokaryotic and eukaryotic) cells:

they are acellular and have a simple organization

they possess either DNA or RNA in the same structure

they cannot replicate independently of host cells.

Viruses can exist extracellularly, as virions with few, if any, enzymes, and intracellularly, when they ‘hijack’ the host biochemical machinery to produce copies of virion components.

Virions are 15–400 nm in diameter and exhibit one of five basic morphologies (Figs 3.2.1 and 3.2.2). They basically consist of a shell, called a capsid, which may be icosahedral or helical in shape. Capsids may be surrounded by an outer membrane, called the envelope. There is a further structure type, termed complex, with a capsid structure that is neither helical nor icosahedral; these complex viruses may also possess an envelope. The capsid is an arrangement of protein subunits, termed protomers or capsomeres. Enclosed within the capsid is the genetic material, which is either RNA or DNA, and may be single or double stranded. If single-stranded RNA, this may be capable of acting as messenger RNA (mRNA), so-called positive sense, or it will have to be made into a complementary copy to do so, termed negative sense. Some genomes are also segmented, such as rotaviruses.

Fig. 3.2.1 Relative sizes of viruses.