Question

Which carbon atom of ribose sugar is joined to nitrogen base to form nucleoside?

• A. C $-$ 2'
• B. C $-$ 5'
• C. C $-$ 3'
• D. C $-$ 1'

Hint:

To easily remember nucleoside/nucleotide structure, use the "1-3-5" rule for the sugar attachments:
The Base is always at position 1', the next nucleotide connects at position 3', and the Phosphate group is at position 5'.

Answer 1

The Correct Option is D

Step 1: What is a nucleoside.
A nucleoside is made of just two pieces, a sugar and a nitrogen base joined together. In RNA the sugar is ribose.
Step 2: How the carbons are named.
The five carbons of the sugar ring are numbered with a small dash mark, so we call them 1', 2', 3', 4' and 5'. The dash keeps them separate from the numbers used on the base.
Step 3: Find the special carbon.
One carbon of the sugar is special. It is the one that used to carry the carbonyl group in the open chain form, and it is the most reactive. This is the C-1' carbon, also called the anomeric carbon.
Step 4: Where the base attaches.
The nitrogen base always joins the sugar at this C-1' carbon. The link formed is called an N-glycosidic bond.
Step 5: Rule out the others.
C-2', C-3' and C-5' do other jobs. For example, C-5' and C-3' carry the phosphate links in a chain, but they do not hold the base.
Step 6: State the answer.
So the base joins at C-1', which is option 4.
\[ \boxed{\text{C-1}'} \]