Question

The nitrogen base found only in DNA is also called
(a)5-methyl uracil
(b)${ NH }_{ 4 }Cl$
(c)Uracil
(d)Guanine

Answer 1

Hint: DNA and RNA are both nucleic acids but they differ in some fundamental aspects such as the sugar that makes up the nucleotide is different. In RNA, ribose sugar is used whereas DNA is made up of deoxyribose sugar. Similarly, thymine is found in DNA which gets replaced by uracil in RNA.

Complete answer:
The nitrogen base found only in DNA is known as thymine which is also called 5-methyl uracil as thymine is a derivative of uracil. Both thymine and uracil are pyrimidines and thus are made up of one cyclic nitrogenous ring whereas purines are made up of two cyclic nitrogenous rings. Thymine is found in DNA bonded to adenine with two hydrogen bonds but is replaced by uracil during transcription when mRNA is formed. This occurs because uracil is energetically less expensive to produce. Uracil is not seen in DNA as it is highly reactive and thus can cause mutations in DNA.

Additional Information: Let us look at the structure of DNA in detail.
-DNA is a long chain polymer made up of the monomer known as a nucleotide.
-A nucleotide is made up of a nitrogenous base (purine or pyrimidine), sugar, and a phosphate group.
-Purines found in nucleic acids consist of adenine (A) and guanine (G) whereas pyrimidines found are cytosine (C), thymine (T), and uracil (U).
-When a nitrogenous base bond with sugar through the N-glycosidic bond, the resulting structure is known as a nucleoside.
-A nucleoside bonds with a phosphate group through a phospho-ester bond resulting in a nucleotide.
-Nucleotides are attached to one another through phosphodiester bonds.
So, the correct option is ‘5-methyl uracil’.

Note: -The sugar that makes up DNA is known as deoxyribose as it is missing -OH group at 2’ position which is present in the ribose sugar.
-This missing -OH group provides more stability to DNA as there is no free group to react.
-The phosphate group attached to DNA makes DNA acidic in nature.