Question

Which base is present in RNA but not in DNA?
A. Uracil
B. Thymine
C. Guanine
D. Cytosine

Answer 1

Nitrogenous bases are the organic compounds that are basic in nature due to the presence of nitrogen atoms. The nitrogen atom present in a nitrogenous base contains a lone pair of electrons. This lone pair of electrons is responsible for the basic nature of a nitrogenous base.

Complete answer:
• A nitrogenous base is an important component that forms nucleotides. Nucleotides are the building blocks of nucleic acids (DNA and RNA). A combination of nitrogenous base, pentose sugar, and phosphate group is called a nucleotide. As nitrogenous bases play an important role in the formation of nucleic acids, they are also known as nucleobases.
• Nitrogenous bases are of two types – purines and pyrimidines.
• Purines include Adenine and Guanine bases. Adenine is present in both DNA and RNA. It is denoted by the letter A. Guanine is also present in both DNA and RNA. It is denoted by the letter G.
• Pyrimidines include Thymine, Cytosine, and Uracil bases which are denoted by letters T, C, and U respectively. Thymine is present in DNA but absent in RNA, while Uracil is present in RNA but absent in DNA. Cytosine is present in both DNA and RNA.

Hence, the correct answer is option (A).

Note: In nucleic acids, the complementary nitrogenous bases form bonds by pairing with each other. In RNA, Adenine (A) pairs with Uracil (U) and Guanine (G) pairs with Cytosine (C). In DNA, Guanine (G) pairs with Cytosine (C) similar to RNA, but Adenine pairs with Thymine (T) as Uracil (U) is absent.