Question

Which of the following is the pyrimidine bases of DNA
(a) Cytosine and Thymine
(b) Cytosine and Uracil
(c) Guanine and Adenine
(d) Adenine and Thymine

Answer 1

Hint: The nitrogenous bases that are different in RNA and DNA. In RNA, a nitrogenous base present in DNA is replaced with 5-methyl uracil.

Complete answer:
A nitrogenous base, a pentose sugar, and a phosphate group are the three components of a nucleotide. The two types of nitrogenous bases present in the nucleotide are purine and Pyrimidine. Adenine and Guanine are purines and Cytosine, Uracil, and Thymine are pyrimidines.
In both DNA and RNA, purines and cytosine are common and in RNA in place of thymine, Uracil is present. Thymine is present only in DNA.

Additional information:
- Chemical structure of polypeptide chains either in DNA or RNA consists of nitrogenous bases, a pentose sugar (in case of RNA-ribose, and DNA- deoxyribose), and a phosphate group.
- Adenine and Guanine are the purines and cytosine, Uracil, and Thymine are pyrimidines.
- A nitrogenous base is linked to the pentose sugar through an N-glycosidic linkage to form a nucleoside, such as adenosine or deoxythymidine.
- A corresponding nucleotide is formed when a phosphate group is linked to 5-OH of nucleoside through a phosphodiester linkage
- Dinucleotide is formed when two nucleotides are linked together through phosphodiester linkage.
- More nucleotides can be joined in such a manner to form a polynucleotide chain.
- The backbone of the polypeptide chain is formed due to sugar and phosphate
So, the correct answer is '(b) Cytosine and Thymine.

Note: Thiamine is found only in DNA whereas, in RNA, uracil is present in place of thymine.
In every RNA nucleotide, it has an additional OH group present at the 2'-position in the ribose.
5-methyl uracil is the chemical name of thymine.