In a DNA strand the nucleotides are linked together by: (a) glycosidic bonds (b) phosphodiester bonds (c) pept

NCERT Exemplar · Q1

Q. In a DNA strand the nucleotides are linked together by:

(a) glycosidic bonds

(b) phosphodiester bonds

(d) hydrogen bonds

Rajasthan RbseMCQ· 1mImportance★★★★★

16% · 15/96 Questions

✓ Free question

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.
Recognize the reaction: It's a classic example of disaccharide hydrolysis – water adds across the bond, one OH goes to one sugar, one H to the other.
Remember the formula: Lactose + H₂O → Galactose + Glucose. The water molecule is consumed, so the total number of carbon atoms stays the same (12), but you now have two separate C6 units.

Tip

A quick memory aid: Lactose → Lact (milk) + ose (sugar). Its products are Galactose and Glucose – both start with G, but galactose is the one that sounds like "galactic" (less common), while glucose is the body's main fuel.

Lactose hydrolysis into glucose and galactose is drawn directly from the carbohydrates section of the NCERT Class 12 Chemistry chapter on biomolecules, a frequent source of short-answer and important questions in CBSE board exams. Searches for "lactose hydrolysis products class 12 chemistry" or "disaccharide hydrolysis NCERT" will find this beta-1,4-glycosidic-bond explanation matches the syllabus treatment.

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.
- Left: 12 C, 24 H, 12 O
- Right: 6+6 = 12 C, 12+12 = 24 H, 6+6 = 12 O ✓
Bond energy: The glycosidic bond is an acetal linkage. Water provides the –H and –OH needed to convert it into two hemiacetal (free sugar) forms.
Biological significance: Lactase enzyme in the small intestine catalyzes this specific hydrolysis because the active site is shaped to fit the $\beta(1\to4)$ bond — not $\alpha$ bonds or other linkages.

6. Exam tip — what to remember

Component	Formula	Type
Lactose	$\ce{C12H22O11}$	Disaccharide
Galactose	$\ce{C6H12O6}$	Aldohexose
Glucose	$\ce{C6H12O6}$	Aldohexose

Key takeaway: Hydrolysis of lactose yields one molecule each of D-galactose and D-glucose — not two glucoses, not two galactoses. The bond specificity determines the product identity.

In a DNA strand, the individual nucleotides are joined end-to-end by phosphodiester bonds — the backbone of each strand is a sugar-phosphate chain held together by these covalent links.

The question asks how nucleotides are linked within a single DNA strand. This is a fundamental point about nucleic acid structure, and the answer is straightforward once you picture the chain.

Each nucleotide has three components: a phosphate group, a sugar (deoxyribose in DNA), and a nitrogenous base. When nucleotides polymerise to form a strand, the phosphate group of one nucleotide forms a covalent bond with the sugar of the next nucleotide. Specifically, the bond is between the 5' carbon of one sugar and the 3' carbon of the next sugar. This is called a phosphodiester bond because it involves a phosphate group that is now linked to two sugar molecules via ester linkages.

Note

The term "phosphodiester" tells you exactly what it is: a phosphate (phospho-) making two ester bonds (-diester) with two different sugar molecules.

So option (B) is correct. Let's quickly see why the other options are wrong, because each of those bonds appears elsewhere in biology and the confusion is common.

Glycosidic bonds link a sugar to another molecule. In DNA, the base is attached to the sugar by an N-glycosidic bond — but that's within a single nucleotide, not between nucleotides.
Peptide bonds are the links between amino acids in proteins. They have nothing to do with nucleic acids.
Hydrogen bonds are crucial in DNA, but they hold the two strands together (between complementary bases), not the nucleotides within a single strand.

Important

A single DNA strand is held together by strong covalent phosphodiester bonds. The two strands are held together by weaker hydrogen bonds between bases. These are two completely different types of bonding serving different structural roles.

✓Final answer

In a DNA strand, the nucleotides are linked together by phosphodiester bonds (option B). These covalent bonds form the sugar-phosphate backbone of each strand.

←Previous question5.14 — Briefly describe the following: (a) Transcription (b) Polymorphism (c)…→Next questionQ2 — A nucleoside differs from a nucleotide. It lacks the: (a) base (b) sug…

Q.In a DNA strand the nucleotides are linked together by:

🎓Unlock everything free for 14 days

Unlock everything free for 14 days