DNA is a polymer of nucleotides which are linked to each other by 3'-5' phosphodiester bond. To prevent polyme

NCERT Exemplar · Q13

Q. DNA is a polymer of nucleotides which are linked to each other by 3'-5' phosphodiester bond. To prevent polymerisation of nucleotides, which of the following modifications would you choose?

(a) Replace purine with pyrimidines

(b) Remove/Replace 3' OH group in deoxyribose

(d) Both 'b' and 'c'

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Concept understanding — Lactose Hydrolysis Products

Lactose Hydrolysis Products – From Intuition to Precision

Imagine you have a glass of milk. That slightly sweet taste comes from a sugar called lactose. But lactose is a disaccharide – it's actually two smaller sugar units joined together. If you could "unstick" those two units, you'd get two simpler sugars. That unsticking process is hydrolysis (water + breaking), and the two simpler sugars you get are the hydrolysis products.

The Intuition: Breaking a Sugar Chain

Think of lactose as a train with exactly two carriages. The coupling between them is a chemical bond. When you add water and the right conditions (like an enzyme called lactase, or an acid), that bond snaps. The train splits into two separate carriages. Each carriage is now a free, smaller sugar molecule.

So the hydrolysis products are simply the two individual sugar units that were originally linked to form lactose.

The Precise Statement

Lactose ( $\text{C}_{12}\text{H}_{22}\text{O}_{11}$ ) is a disaccharide composed of one molecule of D-galactose and one molecule of D-glucose linked by a $\beta(1 \rightarrow 4)$ glycosidic bond. Upon hydrolysis (reaction with water), this bond is cleaved, yielding the two monosaccharides:

\text{Lactose} + \text{H}_2\text{O} \xrightarrow{\text{lactase or acid}} \text{D-Galactose} + \text{D-Glucose}

The two hydrolysis products are:

D-Galactose – a monosaccharide (aldohexose, C6H12O6)
D-Glucose – a monosaccharide (aldohexose, C6H12O6)

Both are reducing sugars, and both have the same molecular formula ( $\text{C}_6\text{H}_{12}\text{O}_6$ ) but differ in the arrangement of the hydroxyl group on carbon 4 (they are C-4 epimers).

Note

In the body, the enzyme lactase (present in the small intestine) performs this hydrolysis so that the resulting glucose and galactose can be absorbed into the bloodstream. Lactose intolerance occurs when lactase activity is low, leaving lactose undigested.

Why This Matters for Exams

Always name both products: galactose and glucose. Never just "sugars" or "monosaccharides."
Know the bond: $\beta(1 \rightarrow 4)$ glycosidic linkage. Hydrolysis breaks this specific bond. …

Why this formula?

Lactose Hydrolysis Products — Understanding the Why

Lactose is a disaccharide composed of two monosaccharides linked by a glycosidic bond. When it undergoes hydrolysis, the bond is broken, yielding specific products. Let's build the reasoning step by step.

1. What is lactose chemically?

Lactose = galactose $\beta(1 \to 4)$ glucose
The bond is between:
- Carbon-1 of galactose (in $\beta$ configuration)
- Carbon-4 of glucose

So the structural formula is:

\text{Galactose} - O - \text{Glucose}

2. What does hydrolysis do?

Hydrolysis means "splitting with water." The reaction is:

\text{Lactose} + \text{H}_2\text{O} \xrightarrow{\text{lactase or acid}} \text{Galactose} + \text{Glucose}

The water molecule adds across the glycosidic bond:

The H from water attaches to the oxygen of the galactose (forming a free –OH on galactose)
The OH from water attaches to the carbon-1 of glucose (forming a free –OH on glucose)

3. Why are the products exactly galactose and glucose?

Because the glycosidic bond is between specific carbons:

Galactose contributes its anomeric carbon (C1)
Glucose contributes its C4

When the bond breaks, each sugar regains its free anomeric carbon (in the case of galactose) or free hydroxyl at C4 (in the case of glucose). No rearrangement occurs — the monosaccharides are released as they were originally linked.

4. Key formula — the hydrolysis equation

The balanced chemical equation:

\ce{C12H22O11 + H2O -> C6H12O6 + C6H12O6}

Lactose: $\ce{C12H22O11}$
Water: $\ce{H2O}$
Products: two molecules of $\ce{C6H12O6}$ (one galactose, one glucose)

Why the same molecular formula?

Both galactose and glucose are aldohexoses — they have the same molecular formula $\ce{C6H12O6}$ but differ in the arrangement of –OH groups (epimers at C4).

5. The "why" behind the formula

Mass conservation: The total number of C, H, O atoms before and after must match. …

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To prevent DNA polymerisation, the crucial 3'-hydroxyl group on the deoxyribose sugar must be removed or replaced, as it is essential for forming the phosphodiester bond with the next nucleotide.

DNA, or deoxyribonucleic acid, is a remarkable biological polymer built from repeating units called nucleotides. Each nucleotide consists of three components: a nitrogenous base (adenine, guanine, cytosine, or thymine), a deoxyribose sugar, and a phosphate group. The process of DNA polymerisation involves linking these individual nucleotides together to form a long, continuous strand. This linkage is highly specific and occurs through the formation of a 3'-5' phosphodiester bond.

Understanding this bond is key. When a new nucleotide is added to a growing DNA strand, the phosphate group attached to the 5' carbon of the incoming nucleotide forms a bond with the hydroxyl (-OH) group present on the 3' carbon of the deoxyribose sugar of the previously added nucleotide. This reaction releases a molecule of water and creates the phosphodiester linkage, extending the DNA chain. Essentially, the 3'-OH group acts as the point of attachment for the next nucleotide.

Let's examine the given options in light of this mechanism:

** (A) Replace purine with pyrimidines:** Purines (adenine and guanine) and pyrimidines (cytosine and thymine) are the nitrogenous bases. While changing the base composition would alter the genetic information carried by the DNA, it does not directly interfere with the formation of the sugar-phosphate backbone. The deoxyribose sugar and phosphate groups would still be available to form phosphodiester bonds, allowing polymerisation to continue.
** (B) Remove/Replace 3' OH group in deoxyribose:** As explained, the 3'-hydroxyl group on the deoxyribose sugar is absolutely critical for the formation of the phosphodiester bond. It is the site where the incoming nucleotide's 5'-phosphate group attaches. If this 3'-OH group is removed or replaced with another group that cannot participate in this reaction, the formation of the phosphodiester bond will be blocked. Consequently, no new nucleotides can be added, and polymerisation will cease.

Important

The 3'-hydroxyl group of the deoxyribose sugar is indispensable for the formation of the 3'-5' phosphodiester bond, which links nucleotides together in a DNA strand. …

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Q.DNA is a polymer of nucleotides which are linked to each other by 3'-5' phosphodiester bond. To prevent polymerisation of nucleotides, which of the following modifications would you choose?

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