Introduction:
RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) are two types of nucleic acids that play crucial roles in the storage and transmission of genetic information in living organisms. While they have many similarities, there are also some key differences between RNA and DNA. One such difference is the presence of a specific base in RNA that is not found in DNA.

Uracil:
The base that is present in RNA but not in DNA is called uracil. Uracil is one of the four nitrogenous bases that make up the building blocks of RNA. The other three bases found in both RNA and DNA are adenine (A), guanine (G), and cytosine (C). In DNA, thymine (T) replaces uracil as one of the bases.

Differences between RNA and DNA:
To understand why uracil is present in RNA but not in DNA, let's briefly discuss the differences between these two nucleic acids:

1. Sugar Backbone: RNA contains ribose sugar, while DNA contains deoxyribose sugar. The difference lies in the presence of an oxygen atom in the ribose sugar of RNA, which is absent in the deoxyribose sugar of DNA.

2. Helical Structure: RNA is typically single-stranded, although it can fold onto itself to form secondary structures. DNA, on the other hand, is typically double-stranded and forms a double helix structure.

3. Stability: DNA is more stable compared to RNA due to the presence of an extra hydroxyl group in the ribose sugar. This hydroxyl group makes RNA more prone to hydrolysis and degradation.

Role of Uracil:
Uracil plays a crucial role in RNA by pairing with adenine during transcription. During the synthesis of RNA, uracil temporarily replaces thymine in DNA. This pairing is achieved through hydrogen bonds, where uracil forms two hydrogen bonds with adenine. This base pairing specificity is vital for the accurate transcription of genetic information from DNA to RNA.

The presence of uracil in RNA allows for greater flexibility and versatility compared to DNA. It enables RNA to perform a variety of functions, including carrying genetic information, catalyzing chemical reactions (as in ribozymes), and participating in protein synthesis (as in messenger RNA).

Conclusion:
In summary, uracil is a nitrogenous base that is present in RNA but not in DNA. This difference in base composition between RNA and DNA contributes to their distinct properties and functions. Uracil's presence in RNA allows for greater flexibility and versatility in carrying out various biological processes.