Question

Which of the following is a correct pair of pyrimidine bases?
A. Adenine and Thymine
B. Adenine and Guanine
C. Thymine and Cytosine
D. Guanine and Cytosine

Answer 1

Hint: The nucleotide has three components – a pentose sugar, a nitrogenous base and a phosphate group. There are two types of nitrogenous bases – pyrimidine and purines.

Complete answer:
The DNA molecule has two strands which bind with each ladder like a twisted ladder.
The backbone of each strand is made up of pentose sugar and phosphate groups. However, one of the nitrogen bases (adenine, cytosine, thymine, and guanine) is attached to each sugar group.
The two strands of the DNA are held together by a hydrogen bond between these nitrogen bases. The pairing is always done between the purines bases and pyrimidine bases
Each nitrogen base has specific hydrogen bond properties and shapes. Therefore the adenine only pairs with thymine and cytosine always pair with guanine. This pairing of nitrogen bases is known as complementarity. It is also known as Watson –Crick base pairing rule.
The adenine and thymine have two hydrogen bonds between them, while, cytosine and guanine have three hydrogen bonds.

Therefore the correct pair of pyrimidine bases is thymine and cytosine, so the correct answer is option C.

Note: The two types of nitrogenous bases are purines and pyrimidine. The purines include adenine and guanine nitrogen bases while pyrimidine includes cytosine and thymine or uracil nitrogen bases. Cytosine is common for both the RNA and DNA. Thymine is present in case of DNA while in RNA uracil is present in place of thymine.