There are two types of Nucleic acid molecules (DNA and RNA) in the cells of living things. A Nucleic acid molecule is composed of nucleotide units; each nucleotide unit contains pentose sugar, phosphate ion and one of the five nitrogenous bases (Adenine, Guanine, Thymine, Uracil and Cytosine).The bases adenine (A) and guanine (G) are purines with a double ring, and the bases thymine (T) and cytosine (C) are pyrimidines with a single ring.

1. DNA is a nucleic acid molecule composed of the four nitrogenous bases Adenine, Guanine, thymine and cytosine C, each attached with pentose sugar (C₅H₁₀O₅), phosphate ion (PO₄^3-).
2. RNA is a nucleic acid composed of the four nitrogenous bases adenine, guanine, cytosine and uracil, each attached with a pentose sugar (C₅H₁₀O₄) and Phosphate ion (PO₄^3-). There are three types of RNA molecules in the cell cytoplasm.