Dna and rna share a number of similarities, but they also differ in certain aspects of their structure. which nitrogenous base is found in rna but is not found in dna?

Uracil

Explanation:

Uracil is the nitrogenous base which exists in RNA but not in DNA.

DNA has thiamine instead of uracil. It is believed that Uracil was replaced by thiamine in DNA because thiamine can be detected by DNA repair machinery more efficiently in case of any DNA damage. So, thiamine was accepted as normal nitrogenous base in DNA.

DNA is a hereditary material therefore it needs to be repaired at any cost because if it will not be repaired on time it will pass on to the next generation in damaged condition which will lead to mutations in the progeny. RNA on the other hand does not serve the function of hereditary material in most of the organisms therefore it does not need to be repaired.

The reason why thiamine did not replace uracil in RNA is because it's synthesis is energetically less expensive than thiamine so nature decided to retain it but only in RNA.