Question

Study the given below single strand of deoxyribonucleic acid depicted in the form of a “stick” diagram with 5′ – 3′ end directionality, sugars as vertical lines and bases as single letter abbreviations and answer the questions that follow.

Name the covalent bonds depicted as (a) and (b) in the form of slanting lines in the diagram.
How many purines are present in the given “stick” diagram?
Draw the chemical structure of the given polynucleotide chain of DNA.

Hint:

Remember: Purines are double-ringed nitrogenous bases (A and G), while pyrimidines are single-ringed (T and C in DNA). The phosphodiester bond links the 3′ carbon of one sugar to the 5′ phosphate of the next.

Answer 1

(a) The covalent bond labeled as (a) is a phosphoester bond, and (b) is a phosphodiester bond. These bonds link the phosphate group to the 5′ carbon of one sugar and the 3′ carbon of the next sugar, forming the sugar-phosphate backbone.
(b) In the stick diagram, the nitrogenous bases shown are A, T, and G. Out of these, adenine (A) and guanine (G) are purines. Therefore, the total number of purines present is 2. 
(c) The chemical structure of the given polynucleotide chain includes: 

• A phosphate group at the 5′ end (indicated by P),
• A sugar-phosphate backbone,
• Three deoxyribose sugars each attached to a nitrogenous base: Adenine (A), Thymine (T), and Guanine (G),

A free hydroxyl group (-OH) at the 3′ end. A hand-drawn or digital structure should depict the sequence A-T-G from 5′ to 3′, with proper covalent linkages between phosphate and deoxyribose, and the bases attached to the 1′ carbon of each sugar.