Question

(a) Write two structural differences between DNA and RNA.

(b) Which component of starch is a branched chain polymer of \( \alpha \)-D-(+)-glucose ? Write its one characteristic feature.

(c) Which types of bonds hold the polypeptide chains in fibrous proteins ?

Hint:

Think sugar and base for DNA versus RNA, amylopectin for the branched part of starch, and hydrogen and disulphide bonds in fibrous proteins.

Answer 1

Step 1: Understanding the Concept:
The question has three parts from the chapter on biomolecules. They cover nucleic acids, carbohydrates and proteins.

Step 2: Part (a), two differences between DNA and RNA:
1. Sugar: DNA has 2-deoxy-D-ribose. RNA has D-ribose, which has one more OH group at C2.
2. Bases: DNA has adenine, guanine, cytosine and thymine. RNA has uracil in place of thymine.
(Another accepted difference: DNA is a double helix, while RNA is a single strand.)

Step 3: Part (b), the branched component of starch:
Starch has two parts. Amylose is the long unbranched chain. Amylopectin is the branched chain.
Both are made from \(\alpha\)-D-glucose units. So the answer is amylopectin.

Step 4: Part (b), one characteristic feature:
Amylopectin has \(\mathrm{C1-C4}\) glycosidic links in the main chain and \(\mathrm{C1-C6}\) links at the branch points.
It makes up about 80 to 85 percent of starch, and it is insoluble in water.

Step 5: Part (c), bonds in fibrous proteins:
In fibrous proteins, the polypeptide chains run side by side, as threads or sheets.
They are held together by hydrogen bonds and disulphide bonds.
This is why keratin and myosin are insoluble in water.

Final Answer:
(a) Sugar is deoxyribose in DNA and ribose in RNA; thymine in DNA and uracil in RNA. (b) Amylopectin, which has C1-C6 branch links. (c) Hydrogen bonds and disulphide bonds.
\[ \boxed{\text{(b) Amylopectin;\ (c) hydrogen bonds and disulphide bonds}} \]