RNA is a single-stranded polymer of ribonucleotides. RNA is composed of ribose sugar, a phosphate residue, and a purine or pyrimidine base:
Purines in RNA
adenine (A) and guanine (G)
These are the same as DNA purine bases
Pyrimidines in RNA
cytosine (C) and uracil (U)
Uracil is incorporated into RNA rather than thymine, which is one of the two pyrimidine bases that is incorporated into DNA.
RNA exists in three primary functional forms: