Question Video: Recalling the Two Categories of Nitrogenous Bases Biology • Third Year of Secondary School

Video Transcript

Nitrogenous bases present in the DNA can be grouped into two categories on the basis of the nitrogenous rings that they contain. What are these two categories of bases called? (A) Purines and pyrimidines, (B) guanines and adenines, (C) thymines and cytosines, or (D) riboses and deoxyriboses.

DNA is usually made up of two strands that are twisted around each other to form a double-helix shape. The black line indicates the sugar phosphate backbone of each strand of DNA. And these colored lines indicate the nitrogenous bases between these two strands. There’s four different nitrogenous bases in DNA: cytosine shown in blue, thymine shown in pink, adenine shown in green, and guanine shown in orange.

As you can see, cytosine and thymine have a single-ring structure, while adenine and guanine have a double-ring structure. Cytosine and thymine are known as pyrimidines, while adenine and guanine are known as purines. Therefore, the two categories of these bases are called purines and pyrimidines.