In the living world, plant viruses and some animal viruses have RNA as the genetic material. Thus, in the living world, most organisms have DNA as the genetic material which can be transmitted from one generation to another. Even though RNA is single stranded, purine:pyrimidine ratio in ribose nucleic acid can sometimes be equal to 1. The 5' end of the DNA strand has a free phosphate group, and the 3' end of the DNA strand has a free -OH group. They do not form phosphodiester bond. Thus, the 3rd carbon atom and the 5th carbon atom of some nucleotides in DNA are involved in the formation of phosphodiester bond. Thymine (monocyclic nitrogen base: pyrimidine) always forms two weak hydrogen bonds with adenine (dicyclic nitrogen base: purine) and cytosine (monocyclic nitrogen base: pyrimidine) always forms three weak hydrogen bonds with guanine (dicyclic nitrogen base: purine) in B-DNA.