Appeared in: 2025 (3 sets)
Ribose and 2-deoxyribose have an important role in biology. Among the most important derivatives are those with phosphate groups attached at the 5 position. Mono-, di- and tri-phosphate forms are important, as well as 3-5 cyclic monophosphates. Purines and pyrimidines form an important class of compounds with ribose and deoxyribose. When these purine and pyrimidine derivatives are coupled to a ribose sugar, they are called nucleosides.
Answer the following questions :
(a) What products would be formed when DNA is hydrolysed ? How is DNA different from RNA with reference to a structure ?
(b) Differentiate between nucleotide and nucleoside.
(c) (i) Mention two important functions of nucleic acid.
OR
(c) (ii) Name the linkage which joins two nucleotides. Name the base that is found in nucleotide of RNA but not in DNA.
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Official Evaluation Steps & EL10 Insight
CBSE Class 12 Chemistry: Biomolecules - Marking Scheme Solution
(a) Hydrolysis products and Structural differences: [1 Mark]
- Hydrolysis Products: Complete hydrolysis of DNA yields a pentose sugar (2-deoxy-D-ribose), phosphoric acid ($H_3PO_4$), and nitrogenous bases (Adenine, Guanine, Cytosine, and Thymine).
- Structural Difference: DNA contains 2-deoxy-D-ribose sugar and has a double-stranded helical structure, whereas RNA contains D-ribose sugar and is typically single-stranded.
(b) Nucleotide vs. Nucleoside: [1 Mark]
- Nucleoside: Formed by the attachment of a nitrogenous base to the $1'$ position of a pentose sugar. (Base + Sugar).
- Nucleotide: Formed when a phosphate group is attached to the $5'$ position of a nucleoside. (Base + Sugar + Phosphate).
(c) (ii) Linkage and Specific Base: [2 Marks]
Step 1 (Identification of Linkage): [1 Mark]
The linkage that joins two nucleotides through the phosphate group is known as the phosphodiester linkage. This occurs between the $3'$ carbon of one sugar and the $5'$ carbon of the next sugar.
Step 2 (Identification of Base): [1 Mark]
The nitrogenous base found in RNA but not in DNA is Uracil ($C_4H_4N_2O_2$).
💡 EL10 Concept Insight:
Think of a nucleoside as a "sugar-base" sandwich, and a nucleotide as that same sandwich with a "phosphate" toothpick holding it together; these toothpicks then link up to form the long, twisted ladder we call DNA!