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Molecular Basis of Heredity: Part 2. Genomes

Author(s): Raye L. Alford, PhD

Viral Genomes

Viruses are not cells. They are genetic material encased in a protein coat, called a viral capsid. Some complex viruses also surround themselves with a viral envelope or membrane made up of lipids and glycoproteins. The genetic and biological characteristics of viruses vary widely.

Like bacterial genomes, the genomes of viruses are very small and typically do not contain introns (non-coding sections of DNA). However, despite their small sizes, the genomes of viruses can be quite complex in that the genes of viruses often overlap or lie immediately adjacent to one another. Like bacteria, some viruses encode multicistronic RNA molecules, which are RNA molecules that contain the coding sequences of more than one protein, usually arranged in sequential manner along the length of the RNA. A multicistronic RNA molecule encodes several proteins but is transcribed in a single RNA molecule. When the RNA is translated, one long polypeptide is made that is actually several proteins joined end-to-end. The polypeptide is processed by enzymatic cleavage to liberate the individual proteins from the larger polypeptide.

In addition, many viruses demonstrate temporal regulation of transcription. For example, early and late genes are expressed at different times during the course of a viral infection of a cell.