Uracil. There are five bases in RNA/DNA. They are Adenine, Thymine, Guanine, Cytosine and Uracil. In DNA: Adenine pairs with Thymine, and Guanine pairs with Cytosine In RNA: Adenine pairs with Uracil and Guanine pairs with Cytosine
In RNA, there are four kinds of base: adenine, guanine, cytosine and uracil. There are no thymine bases. Therefore, there are no thymine and adenine base pairs as there are in DNA so adenine pairs with uracil.
A purine will always pair with a pyrimidine. Examples of purines are adenine (pairs with thymine or uracil) and guanine (pairs with cytosine). Examples of pyrimidines are thymine (pairs with adenine), uracil (pairs with adenine), and cytosine (pairs with guanine).