Question

Which of the following elements is not present in a nitrogenous base?
a. Hydrogen
b. Carbon
c. Phosphorus
d. Nitrogen

Answer 1

Organic molecules that contain nitrogen atoms have the chemical properties of a base and are known as nitrogenous base. Their main biological function is to bind nucleic acids together. Nitrogenous bases are derivatives of pyrimidine and purine, which are non-polar due to their aromaticity.

Complete answer:
- The chemical formula for adenine is ${C_5}{H_5}{N_5}$ and the chemical formula for thymine is ${C_5}{H_5}{N_5}{O_2}$. In DNA, the adenine binds with thymine via two hydrogen bonds to stabilize the structure of the nucleic acid. In RNA, adenine binds with uracil (${C_4}{H_4}{N_2}{O_2}$) instead of thymine.
- The nucleotide of guanine is known as guanosine. The chemical formulas for guanine and cytosine are ${C_5}{H_5}{N_5}O$ and ${C_4}{H_5}{N_3}O$, respectively. In DNA and RNA, guanine binds with cytosine via three hydrogen bonds.
- Uracil is synonymous with thymine as they are interchangeable as the third base and they can be distinguished by the presence or absence of a methyl group at the $C_5$ position.
- Purine includes adenine and guanine while pyrimidines include uracil, thymine and cytosine.
- The chemical formula of the five nitrogenous bases suggest that they are made up of carbon, nitrogen, hydrogen and oxygen.

Phosphorus is not present in a nitrogenous base.
Hence, the correct answer is option (C).

Additional information:
In the construction of nucleotides to form DNA, four nitrogenous bases are utilized. The four nitrogenous bases are thymine, cytosine, guanine and adenine. In RNA, thymine is replaced by uracil.

Note: All the nitrogenous bases contain some information in itself and each nitrogenous base is read as a unit in a group of three bases. In humans, each cell contains around six billion individual nitrogenous bases. Apart from storing genetic information, nitrogenous bases are also used to transfer energy and cell signaling.