Q.A nucleoside differs from a nucleotide. It lacks the:

A polynucleotide chain has three repeating components: a phosphate group, a pentose sugar, and a nitrogenous base. Understanding how these connect is fundamental to DNA and RNA structure.

The sugar-phosphate backbone forms the structural spine of nucleic acids, with phosphodiester bonds linking one sugar's 5′ carbon to the next sugar's 3′ carbon through a phosphate group. But the bases — the information-carrying units — must attach to this backbone somehow. That attachment is what we're identifying here.

The N-glycosidic linkage is a covalent bond between the anomeric carbon (C1′) of the pentose sugar and a nitrogen atom of the nitrogenous base. In purines (adenine, guanine), this nitrogen is N9; in pyrimidines (cytosine, thymine, uracil), it's N1. This bond is called "glycosidic" because it resembles the linkage in carbohydrates, and "N-" specifies that it involves a nitrogen atom rather than oxygen.

Let's eliminate the other options systematically:

1. Hydrogen bonds (A) are weak, non-covalent interactions that hold complementary base pairs together across the two strands of a DNA double helix (A–T, G–C). They do NOT attach bases to the sugar within a single strand. The schematic shows a single polynucleotide chain, not inter-strand pairing.

2. Peptide bonds (B) link amino acids in proteins through a C–N bond between a carboxyl group and an amino group. Nucleic acids contain no peptide bonds — this is a protein-specific linkage.

3. Phosphodiester bonds (D) connect adjacent nucleotides by joining the 3′-OH of one sugar to the 5′-phosphate of the next. These form the sugar-phosphate backbone (the S–P–S connections), not the sugar-to-base attachment shown by the dotted line.

4. N-glycosidic linkage (C) is precisely the bond between the pentose sugar and the nitrogenous base. This is a strong covalent bond that defines a nucleoside (base + sugar); adding a phosphate group to a nucleoside creates a nucleotide.

Why DNA strands must be both anti-parallel and complementary

The architecture of DNA isn't arbitrary—it's dictated by the chemistry of how nucleotides bond and how bases recognize each other. Understanding these two properties separately, then seeing why they must coexist, reveals the elegance of the double helix.

The complementarity principle

When Watson and Crick solved DNA's structure in 1953, the breakthrough was recognizing that bases pair in a specific way: adenine (A) always pairs with thymine (T) through two hydrogen bonds, while guanine (G) pairs with cytosine (C) through three hydrogen bonds. This isn't random preference—it's geometric necessity.

The purine bases (A and G, with their double-ring structure) are larger than the pyrimidine bases (T and C, single-ring). If two purines tried to pair, they'd be too bulky and distort the helix. If two pyrimidines paired, they'd be too small to bridge the gap. Only purine-pyrimidine pairs maintain the uniform diameter of the helix (about 2 nm).

Beyond size, the hydrogen-bonding patterns are specific:

• A and T have exactly the right donor and acceptor groups to form two stable H-bonds
• G and C form three H-bonds in perfect alignment
• Other combinations either can't form enough bonds or have steric clashes

This means if one strand reads 5'-ATGC-3', the other must read 3'-TACG-5' to satisfy base-pairing rules. The strands are complementary, not identical.

The anti-parallel orientation

Now consider the sugar-phosphate backbone. Each nucleotide has a deoxyribose sugar with a phosphate group attached to its 5' carbon and the next nucleotide's sugar attached via its 3' carbon. This creates directionality: one end of a strand has a free 5' phosphate, the other a free 3' hydroxyl.

In the double helix, the two strands run in opposite directions—one goes 5' → 3' while its partner goes 3' → 5'. This anti-parallel arrangement is required because:

1. The geometry of base pairing only works when the glycosidic bonds (connecting base to sugar) are positioned correctly relative to each other
2. The major and minor grooves of the helix form only when strands are anti-parallel
3. The hydrogen bonds between bases align properly only in this configuration

If the strands were parallel (both 5' → 3'), the bases couldn't pair—the geometry would be all wrong, with the glycosidic bonds pointing in incompatible directions.

Evaluating the options

Let's see why the other choices fail:

(A) Identical and complementary — Contradiction. If the strands were identical (same sequence), they couldn't be complementary. An A on one strand would face an A on the other, violating base-pairing rules.

(B) Identical and non-complementary — This describes two separate, unrelated DNA molecules, not the paired strands of a double helix.

(D) Anti-parallel and non-complementary — Anti-parallel is correct, but without complementarity, the bases wouldn't pair and the double helix couldn't form. You'd just have two strands lying near each other with no hydrogen bonds holding them together.

(C) Anti-parallel and complementary — Both properties are present and necessary. The strands run in opposite directions and their bases pair according to Watson-Crick rules.

Part (a)

Glycosidic linkage. It forms when the anomeric carbon of a sugar reacts with an $-OH$ of another molecule, eliminating water. Between two monosaccharides it appears as an oxygen bridge $C-O-C$, and it joins the units of di- and polysaccharides (maltose, starch, cellulose). It is cleaved by glycosidases.

Peptide linkage. It is the amide bond formed between the $\alpha$-carboxyl group of one amino acid and the $\alpha$-amino group of the next, with loss of water, written $-CO-NH-$. It is the backbone bond of every polypeptide/protein and is cleaved by proteases.

So the two differ in the atoms bonded ($C-O-C$ vs $C-N$ amide), the monomers joined (sugars vs amino acids) and the class of biomolecule (carbohydrates vs proteins).

Part (b)

Nucleoside. It has two components — a nitrogenous base (purine or pyrimidine) linked to a pentose sugar (ribose/deoxyribose) by an $N$-glycosidic bond at C-1$'$. It carries no phosphate (e.g. adenosine, cytidine).

Nucleotide. It has three components — a nucleoside plus one or more phosphate groups esterified at the 5$'$-$OH$ of the sugar (base + sugar + phosphate). The phosphate makes it acidic and enables nucleotides to polymerise into nucleic acids (e.g. AMP, ATP).

Thus a nucleotide is simply a nucleoside to which a phosphate has been added.