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) Bond (a) is a phosphoester bond, and bond (b) is a phosphodiester bond. These bonds connect a phosphate group to the 5′ carbon of one sugar and the 3′ carbon of the adjacent sugar, establishing the sugar-phosphate backbone.
(b) The stick diagram displays the nitrogenous bases A, T, and G. Among these, adenine (A) and guanine (G) are purines. Consequently, there are 2 purines in total.
(c) The chemical structure of the provided polynucleotide chain comprises: 

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

A free hydroxyl group (-OH) at the 3′ terminus. The structure, whether hand-drawn or digital, should illustrate the sequence A-T-G from the 5′ to the 3′ direction, showing appropriate covalent connections between phosphates and deoxyribose, and the bases attached to the 1′ carbon of each sugar.