How many of the following are nucleosides?
Adenosine, Uracil, Deoxyguanosine, Thymine, Cytidine, Guanine
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Three
A nitrogenous base on its own is not a nucleoside. When the base is joined to a pentose sugar (ribose or deoxyribose) by an N-glycosidic linkage, it becomes a nucleoside. Adding a phosphate group to the nucleoside makes a nucleotide. The names help: the bases are adenine, guanine, cytosine, thymine and uracil, while their nucleosides end in “-osine” or “-idine” (adenosine, guanosine, cytidine, thymidine, uridine).
Adenosine — Nucleoside. Adenine joined to ribose.
Uracil — Not a nucleoside. It is a pyrimidine base, found in RNA in place of thymine. Its nucleoside is uridine.
Deoxyguanosine — Nucleoside. Guanine joined to deoxyribose, the sugar of DNA.
Thymine — Not a nucleoside. It is a pyrimidine base of DNA. Its nucleoside is deoxythymidine.
Cytidine — Nucleoside. Cytosine joined to ribose.
Guanine — Not a nucleoside. It is a purine base. Its nucleosides are guanosine and deoxyguanosine.
So three of the six (adenosine, deoxyguanosine and cytidine) are nucleosides.
NCERT Extract:
Structure of Polynucleotide Chain – “… A nucleotide has three components – a nitrogenous base, a pentose sugar (ribose in case of RNA, and deoxyribose for DNA), and a phosphate group. There are two types of nitrogenous bases – Purines (Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine). Cytosine is common for both DNA and RNA and Thymine is present in DNA. Uracil is present in RNA at the place of Thymine. A nitrogenous base is linked to the OH of 1′ C pentose sugar through a N-glycosidic linkage to form a nucleoside, such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine and uridine or deoxythymidine.”