Question

Answer the following case-based questions on biomolecules: \[ (a)\;\text{Write the name of basic building units of proteins and nucleic acids. How can you differentiate between fibrous and globular proteins on the basis of their structures?} \] \[ (b)(i)\;\text{What products would be formed when a nucleotide from DNA containing thymine is hydrolyzed?} \] OR \[ (b)(ii)\;\text{Write one structural difference between DNA and RNA.} \] \[ (c)\;\text{Give one example each of a fat-soluble vitamin and a water-soluble vitamin.} \]

Hint:

Nucleotide = nitrogenous base + sugar + phosphate group. Protein = polymer of amino acids.

Answer 1

Step 1: Basic building units of proteins and nucleic acids.
Proteins: basic building unit = amino acid. There are 20 standard amino acids, each with an amino group ($-NH_2$) and a carboxyl group ($-COOH$) on the alpha carbon, plus a specific side chain (R group). Nucleic acids (DNA, RNA): basic building unit = nucleotide. Each nucleotide has three parts: (i) a nitrogenous base, (ii) a pentose sugar, (iii) a phosphate group.
Step 2: Differentiate fibrous and globular proteins by structure.
Fibrous proteins: polypeptide chains are elongated, run parallel or wrap around each other forming long fibres or flat sheets. They are held together by intermolecular hydrogen bonds and other forces. Generally insoluble in water. Structural role. Examples: keratin (hair, nails), collagen (tendons), fibroin (silk). Globular proteins: polypeptide chains are folded and coiled upon themselves into a compact, roughly spherical 3D shape. Generally soluble in water. Functional role (enzymes, hormones, transport). Examples: haemoglobin, myoglobin, insulin.
Step 3: Part (b)(i): Products from hydrolysis of a nucleotide.
A nucleotide on complete hydrolysis gives: \[ \text{Nucleotide} \xrightarrow{H_2O} \text{Nitrogenous base} + \text{Pentose sugar} + \text{Phosphoric acid} \] (i) Nitrogenous base: purine (adenine or guanine) or pyrimidine (cytosine, thymine in DNA; cytosine, uracil in RNA). (ii) Pentose sugar: ribose (RNA) or 2-deoxyribose (DNA). (iii) Phosphoric acid ($H_3PO_4$).
Step 4: Key difference between DNA and RNA nucleotides.
DNA nucleotides: 2-deoxyribose sugar; bases A, G, C, T. RNA nucleotides: ribose sugar; bases A, G, C, U (uracil replaces thymine). The $-OH$ at C2 of ribose (RNA) vs. $-H$ at C2 of deoxyribose (DNA) is the chemical distinction.
Step 5: Base pairing in DNA (complementary pairing).
In DNA double helix: A (adenine) pairs with T (thymine) via 2 hydrogen bonds. G (guanine) pairs with C (cytosine) via 3 hydrogen bonds. This complementary base pairing (Watson-Crick) holds the two strands of DNA together.
Step 6: Summarise all answers.
Building units: proteins = amino acids; nucleic acids = nucleotides. Fibrous proteins = elongated/insoluble/structural; globular proteins = compact/spherical/soluble/functional. Hydrolysis of nucleotide gives: nitrogenous base + pentose sugar + phosphoric acid. \[ \boxed{\text{Nucleotide} \rightarrow \text{Nitrogenous base} + \text{Pentose sugar} + H_3PO_4} \]