 However, the two capsids making up the virus are fused; divided, the capsids would be 15 nm in length.Other environmentally characterized ssDNA viruses such as CRESS DNA viruses, among others, can have genomes that are considerably less than 2,000 nucleotides.The smallest RNA viruses in terms of genome size are small retroviruses such as rous sarcoma virus with genomes of 3.5 kilo base pairs (kb) and particle diameters of 80 nanometres (nm).