2 Influenza viruses
Viruses have very diverse genomes. Whereas the genomes of bacteria, plants and animals are of double-stranded DNA, the genomes of viruses can be constituted from either DNA or RNA and may be double- or single-stranded molecules.
Usually, DNA is a double-stranded molecule with paired, complementary strands (dsDNA) and RNA is a single stranded molecule (ssRNA). However, some viruses have single-stranded DNA genomes (ssDNA) and some have double-stranded RNA genomes (dsRNA). The type of nucleic acid found in the genome depends on the group of viruses involved.
RNA encodes protein in all living things, and the sequence of bases in the RNA determines the sequence of amino acids in the protein. A strand of RNA which has the potential to encode protein is said to be ‘positive sense’ (+). If a strand of RNA is complementary to this, then it is ‘negative sense’ (–). Negative-sense RNA must first be copied to a complementary positive-sense strand of RNA before it can be translated into protein.
The description of the influenza genome as negative-sense ssRNA means that its RNA cannot be translated without copying first. This copying is performed by influenza’s viral RNA polymerase, a small amount of which is packaged with the virus, ready to begin copying the viral genome once it enters a host cell. Viral RNA polymerase consists of three subunits: PB1, PB2, and PA, encoded separately by the first three viral RNA strands.
Understanding the way in which different viruses replicate is important, since it allows the identification of particular points in their life-cycle that may be susceptible to treatment with antiviral drugs.
Viruses are classified into different families, groups and subgroups in much the same way as are species of animals or plants.
As you have already read, the influenza viruses are (–)ssRNA organisms (Baltimore group V) and belong to a family called the Orthomyxoviruses (see Box 1). They fall into three groups: influenza A, B and C.
Type A viruses are able to infect a wide variety of endothermic (warm-blooded) animals, including mammals and birds, and analysis of their viral genome indicates that all strains of influenza A originated from aquatic birds.
By contrast, types B and C are mostly confined to humans. At any one time, a number of different strains of virus may be circulating in the human population.
Box 1 Families, groups and strains of virus
Viruses were originally classified into different groups according to similarities in their structure, mode of replication and disease symptoms. For example, the Orthomyxoviruses include viruses that cause different types of influenza, while Paramyxoviruses include the viruses that cause measles and mumps.
Such large groupings are often called a family of viruses. The families can be subdivided into smaller groups, such as influenza A, B and C. Even within a single such group of viruses there can be an enormous level of genetic diversity, and this is the basis of the different strains. As an example, two HIV particles from the same individual may be 4% different in their genome; compare this with the 1% difference between the genomes of humans and chimpanzees, which are different species.