viruses are composed of nucleic acid and
| Share | Cite. virus, parasite with a noncellular structure composed mainly of nucleic acid within a protein coat. Most viruses are too small (100â2,000 Angstrom units) to be seen with the light microscope and thus must be studied by electron microscopes. Share | Cite. virus, parasite with a noncellular structure composed mainly of nucleic acid within a protein coat. Most viruses are too small (100â2,000 Angstrom units) to be seen with the light microscope and thus must be studied by electron microscopes.