All viruses are characterized by these structural characteristics. Each virion is composed of an outer capsid consisting of proteins and an inner core of nucleic acid. Keep it simple. Viruses consist of genetic material contained within a protective protein capsid The protein capsid and nucleic acid core can together be referred to as the nucleocapsid. In some viruses an outer lipid bilayer envelope is also present.

Viruses are categorized based on morphology. The capsid may be helical or icosahedral. Furthermore, enveloped viruses cover themselves in a modified form of cell membrane (which they can pick up from the outer plasma membrane of the infected cell or, in some cases, the nuclear membrane or ER.). Some viruses, lastly, are morphologically complex, such as bacteriophages.

Viruses are also categorized based on the type of genetic material they contain, whether it is single or double stranded, and their mode of replication. Viral genomes may be double stranded DNA, single stranded DNA (during part of the life cycle), or RNA. Retroviruses are RNA viruses that replicate through DNA intermediates.

As you learn viruses, keep in mind the primary means of classifying them. Make it a habit to reflect on any new virus in terms of morphology, type of genetic material, and mode of replication.