Final answer:
In RNA, Uracil is the pyrimidine base that replaces thymine found in DNA, making it the correct answer.
Step-by-step explanation:
The pyrimidine base that is only present in RNA and not DNA is Uracil. In DNA, adenine pairs with thymine (A-T), while in RNA, adenine pairs with uracil (A-U). Therefore, among the options provided, Uracil is the correct answer as it is the base that replaces thymine in RNA. The other bases mentioned, guanine, cytosine, and adenine, are found in both RNA and DNA. Guanine and adenine are purines, having a double carbon-nitrogen ring structure, while cytosine and uracil are pyrimidines, with a single carbon-nitrogen ring structure.
RNA contains the pyrimidine base Uracil which is only present in RNA. The other options listed – Guanine, Cytosine, and Adenine – are present in both RNA and DNA. Uracil pairs with Adenine in RNA, just as Thymine pairs with Adenine in DNA.