Nucleic Acids — DNA & RNA Structure

What Are Nucleic Acids

Every generation of a species resembles its ancestors. The particles in the nucleus of a living cell responsible for this transmission of inherited characters are the chromosomes, which are built from proteins and a second class of biomolecule — the nucleic acids. There are two kinds: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). They are named for the cell nucleus from which they were first isolated and for their acidic character.

Nucleic acids are long-chain polymers of repeating units called nucleotides, which is why they are also termed polynucleotides. Complete hydrolysis of either DNA or RNA gives three classes of product: a pentose sugar, phosphoric acid, and nitrogen-containing heterocyclic compounds called bases. The whole topic reduces to understanding how these three pieces assemble into a unit, how units link into a strand, and how strands pair into a helix.

The Three Components

On hydrolysis a nucleic acid yields a sugar, a phosphate group and a set of bases. The chemistry of DNA and RNA diverges at the very first component — the sugar.

The pentose sugar

Both sugars are aldopentoses. In DNA the sugar moiety is β-D-2-deoxyribose; in RNA it is β-D-ribose. The two are identical except at carbon-2: ribose carries an –OH there, whereas 2-deoxyribose carries only an –H. The prefix "2-deoxy" simply records that the oxygen at C-2 is missing.

The phosphate group

The acidic character of nucleic acids comes from phosphoric acid, $\ce{H3PO4}$. A phosphate group esterifies the sugar and, in the assembled chain, bridges adjacent sugars. It is this phosphate that links one nucleotide to the next.

The nitrogenous bases

DNA contains four bases: adenine (A), guanine (G), cytosine (C) and thymine (T). RNA shares the first three but replaces thymine with uracil (U). So the only base difference between the two nucleic acids is T (DNA) versus U (RNA).

Purines vs Pyrimidines

The five bases fall into two structural families. Purines are built on a fused two-ring (bicyclic) skeleton; pyrimidines are built on a single six-membered ring. This shape difference is what governs how the bases pair in the double helix, so it is worth fixing firmly.

Nucleoside vs Nucleotide

This is the single most heavily tested distinction in the subtopic, and it turns on counting components. To keep the sugar carbons distinct from the base atoms, the sugar carbons are numbered 1′, 2′, 3′ and so on (read "one-prime", "two-prime").

A nucleoside is formed by the attachment of a base to the 1′ position of the sugar. It therefore has two components — base + sugar. A nucleotide is formed when a nucleoside is linked to phosphoric acid at the 5′ position of the sugar moiety. It therefore has three components — base + sugar + phosphate. In one line:

Nucleoside = base + sugar (linked at 1′).  Nucleotide = nucleoside + phosphate (linked at 5′) = base + sugar + phosphate.

Phosphodiester Linkage

Individual nucleotides are joined into a chain by a phosphodiester linkage formed between the 5′ and 3′ carbon atoms of neighbouring pentose sugars. The phosphate of one nucleotide esterifies the 3′-hydroxyl of one sugar and the 5′-hydroxyl of the next, so each phosphate forms two ester bonds — hence "di-ester". Repetition of this linkage produces the alternating sugar–phosphate backbone, with the bases projecting from the sugars.

A simplified picture of the chain is therefore a regular run of sugar–phosphate–sugar–phosphate units, repeated n times, with one base hanging off each sugar. The sequence in which these nucleotides occur is termed the primary structure of the nucleic acid.

The DNA Double Helix

Beyond the primary sequence, nucleic acids have a secondary structure. James Watson and Francis Crick proposed that DNA takes the form of a double helix — a structure resembling a gently twisted ladder. The two rails are the alternating phosphate and deoxyribose backbones; each rung is a pair of bases. For this work Watson and Crick, with Maurice Wilkins, shared the 1962 Nobel Prize in Physiology and Medicine.

Two nucleic-acid chains are wound about each other and held together by hydrogen bonds between pairs of bases. The pairing is specific and complementary: adenine pairs with thymine and cytosine pairs with guanine. Because a purine always pairs with a pyrimidine, the rungs are of uniform width and the ladder stays even. The two strands run in opposite directions — they are antiparallel, one running 5′→3′ while its partner runs 3′→5′.

Because pairing is fixed, the two strands are not identical but complementary: wherever one strand reads A, the partner must read T; wherever one reads G, the partner reads C. This complementarity is the chemical basis of faithful replication — when the strands separate during cell division, each acts as a template that specifies its new partner, so two identical DNA molecules result, each carrying one old and one new strand.

RNA and Its Three Types

RNA is built from the same kind of nucleotide units, but its secondary structure is different. In the secondary structure of RNA a single-stranded helix is present, which sometimes folds back on itself to form short double-helical regions wherever the base sequences happen to be complementary. RNA molecules occur in three types, each performing a distinct role in protein synthesis.

The division of labour is worth stating plainly: DNA holds the coded message for which protein to make, while the RNA molecules actually carry out the synthesis of that protein in the cell. The information flows from DNA to mRNA, and tRNA delivers the building blocks.

DNA vs RNA Compared

The structural and functional contrasts between the two nucleic acids recur in NEET as direct comparison questions. The table consolidates every distinction made in the NCERT and NIOS texts.