Nucleic Acids are the molecules that BUILD DNA! Each of these molecules are made up of Nitrogen, Oxygen, Hydrogen, and Carbon atoms. These nucleic acids can bond with each other, but only Adenine bonds to Thymine and Guanine bonds with Cytosine.
Uracil is the 5th wheel of Nucleic Acids. It replaces thymine in a strand of RNA that we will learn about later. But it is still important.