Question

Adenosine is an example of:
A. Nucleotide
B. Purine base
C. Pyrimidine base
D. Nucleoside

Answer 1

Hint: When DNA is completely hydrolysed it yields a pentose sugar, phosphoric acid and bases.In DNA molecule, the sugar moiety present is $\beta$-D-2-deoxyribose whereas in RNA molecule, it is $\beta$-D ribose.

Complete answer:
There are two sugar moieties present in DNA.
DNA contains four bases: adenine, guanine, cytosine, thymine

Now let us discuss nucleoside and nucleotide.
When a base is attached to the 1’ position of sugar it is known as nucleoside. In nucleosides the sugar carbons are numbered as 1’, 2’, 3’, etc. In order to distinguish these from the bases.
And when nucleoside is linked to phosphoric acid at 5’- position of sugar moiety we get a nucleotide.

Nucleotides are joined together by phosphodiester linkage between 5’ and 3’ carbon atoms of the pentose sugar.
Now we are clear with nucleoside and nucleotide.
This is adenosine which is among four units of RNA and it is essential for life.
Here we can clearly see that a base is attached to 1’ position of sugar. So it is clear that adenosine is a nucleoside.

Hence our correct option is: D


Note:
If we have knowledge of nucleoside, nucleotide structure and also of adenosine then it is easy to solve this type of question.There are so many similar looking structures that we might get confused while drawing the structure of these biomolecules, therefore attention to minute details is required.