Answer:
Uracil
Step-by-step explanation:
DNA molecule is made up of monomeric nucleotides, which are of four different types viz: Adenine, Guanine, Cytosine, and Thymine. According to the COMPLEMENTARY BASE PAIRING RULE as proposed by Chargaff, these nucleotides pair in the DNA as follows: Adenine to Thymine (A-T), Guanine to Cytosine (G-C).
However, in the RNA molecule (second nucleic acid), a fifth nucleotide called URACIL is found. This URACIL replaces thymine because thymine is only found in DNA while Uracil is only found in RNA. When DNA is transcribed into mRNA, Adenine nucleotide gets transcribed into Uracil in the RNA instead of thymine i.e A to U.