Final answer:
Uracil is the nitrogen base present in RNA instead of thymine which is found in DNA. This substitution is a key distinction between RNA and DNA.
Step-by-step explanation:
The nitrogen base that is present in RNA but not in DNA is uracil. While both RNA and DNA contain the bases adenine, guanine, and cytosine, DNA has thymine as its fourth base, whereas RNA replaces thymine with uracil. This difference in bases is part of what distinguishes RNA from DNA, along with RNA being often single-stranded and containing ribose sugar instead of deoxyribose.