1. Structure of Viruses
- Acellular organization: Viruses are non-cellular, ultra-microscopic infectious agents. They do not have a typical cell structure, cytoplasm or organelles.
- Core (genetic material): Each virus has a core of nucleic acid, which may be either DNA or RNA, but never both together in the same virion.
- Capsid (protein coat): The nucleic acid core is surrounded by a protective protein coat called the capsid, made up of many repeating subunits called capsomeres.
- Shape and symmetry: Arrangement of capsomeres can give different shapes to viruses, such as helical, polyhedral (icosahedral), or complex (e.g., bacteriophages with head and tail).
- Envelope (in some viruses): Certain animal viruses like HIV and influenza have an outer lipoprotein envelope with surface projections (spikes or peplomers) that help in attachment to host cells.
2. Nature of Genetic Material in Viruses
- The genetic material of a virus is a single type of nucleic acid: either DNA or RNA, which may be single-stranded or double-stranded.
- Most plant viruses have single-stranded RNA as their genetic material (for example, Tobacco mosaic virus).
- Many animal viruses may have single- or double-stranded RNA, or double-stranded DNA as genetic material (for example, adenoviruses, influenza virus, HIV).
- Bacteriophages (viruses infecting bacteria) typically possess double-stranded DNA enclosed in a head with a tail structure.
- The viral genome carries information for synthesis of viral proteins and for replication of the virus inside a suitable host cell.
3. Four Common Viral Diseases
- Influenza (flu)
- Mumps
- Smallpox (historically important, now eradicated in nature)
- AIDS (Acquired Immunodeficiency Syndrome, caused by HIV)
Short Exam-style Answer
Viruses are submicroscopic, acellular infectious agents consisting of a nucleic acid core (either DNA or RNA, single- or double-stranded) enclosed in a protein coat called capsid made of capsomeres. In some viruses, an outer lipoprotein envelope with surface spikes is also present. Their genetic material may be RNA (common in plant and many animal viruses) or DNA (common in bacteriophages and some animal viruses), but never both together. Four common viral diseases are influenza, mumps, smallpox and AIDS.