Nucleic acids make up both DNA and RNA. Cytosine (C) and guanine (G) are complimentary base pairs in both DNA and RNA, whereas adenine (A) binds with thymine (T) in DNA and with uracil (U) in RNA. Thymine (T), guanine (G), adenine (A), and cytosine (C) are the links between genotype and phenotype. RNA is a type of nucleic acid that plays a role in protein synthesis. Codons are part of the molecular structure of mRNA, tRNA, and rRNA.