Skip to content

Chemistry · Ch 10 — Biomolecules

Chemical Composition of Nucleic Acids

10.5.1

Chemical Composition of Nucleic Acids

What hydrolysis reveals

When a nucleic acid (DNA or RNA) is completely hydrolysed, it breaks down into three distinct types of simpler molecules:

  1. a pentose sugar
  2. phosphoric acid, H3PO4\text{H}_3\text{PO}_4
  3. nitrogen-containing heterocyclic compounds, called bases

These three building blocks — sugar, phosphate, and base — are the entire chemical alphabet from which every nucleic acid is constructed.

The pentose sugar: the DNA/RNA distinction starts here

The identity of the sugar is what fundamentally separates the two nucleic acids:

  • DNA contains β-D-2-deoxyribose
  • RNA contains β-D-ribose

Both are five-membered (furanose) rings made of four ring carbons and one ring oxygen, numbered 1′,2′,3′,4′,5′1', 2', 3', 4', 5' (primed numbers, to keep them distinct from the numbering used on the bases).

Haworth-style furanose ring structures of β-D-ribose (the RNA sugar, with hydroxyl groups at the 1, 2 and 3 positions) and β-D-2-deoxyribose (the DNA sugar, identical except that carbon-2 carries only hydrogen), drawn side by side as printed in NCERT.
Haworth-style furanose ring structures of β-D-ribose (the RNA sugar, with hydroxyl groups at the 1, 2 and 3 positions) and β-D-2-deoxyribose (the DNA sugar, identical except that carbon-2 carries only hydrogen), drawn side by side as printed in NCERT.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your NCERT textbook's own diagram.

Redrawn from the NCERT page with the structures and reagent placement exactly as the textbook prints them. Every element of this display was checked against the printed page during the sweep's blind-judge verification pass, so what you …

β-D-Ribose (the RNA sugar):

PositionSubstituent
C1′C1'−OH-\text{OH} (this is the position where a base attaches)
C2′C2'−OH-\text{OH}
C3′C3'−OH-\text{OH}
C4′C4'ring carbon, bonded to C5′C5'
C5′C5'exocyclic −CH2OH-\text{CH}_2\text{OH} (this is the position where phosphate attaches)

β-D-2-Deoxyribose (the DNA sugar):

PositionSubstituent
C1′C1'−OH-\text{OH}
C2′C2'−H-\text{H} only — no oxygen at this carbon
C3′C3'−OH-\text{OH}
C4′C4'ring carbon, bonded to C5′C5'
C5′C5'exocyclic −CH2OH-\text{CH}_2\text{OH}

The only structural difference between the two sugars is at C2′C2': ribose carries a hydroxyl there, while deoxyribose carries only a hydrogen — it has been "de-oxygenated" at that one carbon. This single missing −OH-\text{OH} is the reason DNA is chemically far more stable than RNA and is the molecule entrusted with long-term storage of genetic information.

The nitrogen bases

Hydrolysis also liberates nitrogen-containing heterocyclic bases. These fall into two structural families:

Important

DNA vs RNA at a glance

DNARNA
Sugarβ-D-2-deoxyribose (no −OH-\text{OH} at C2′C2')β-D-ribose (has −OH-\text{OH} at C2′C2')
BasesAdenine, Guanine, Cytosine, ThymineAdenine, Guanine, Cytosine, Uracil
StrandednessDouble-stranded helixUsually single-stranded (may fold back on itself)

The sugar and the fourth base (thymine vs uracil) are the two features that distinguish DNA from RNA at the molecular level.

Purines — a fused bicyclic ring system (a six-membered ring fused to a five-membered ring):

  • Adenine (A) — carries an −NH2-\text{NH}_2 group; present in both DNA and RNA
  • Guanine (G) — carries an −NH2-\text{NH}_2 group and a ring C=O\text{C=O}; present in both DNA and RNA
Structures of the two purine bases adenine (A) and guanine (G) as NCERT prints them — a fused five- and six-membered ring with every ring atom written out as letters, adenine bearing an -NH2 group at the top of the six-membered ring and guanine bearing a ring C=O plus an -NH2 group.
Structures of the two purine bases adenine (A) and guanine (G) as NCERT prints them — a fused five- and six-membered ring with every ring atom written out as letters, adenine bearing an -NH2 group at the top of the six-membered ring and guanine bearing a ring C=O plus an -NH2 group.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your NCERT textbook's own diagram.

Redrawn from the NCERT page with the structures, printed labels (NH2, CH, HC, NH) and reagent placement exactly as the textbook prints them. Every element of this display was checked against the printed page during the sweep's blind-judge verification pass, so wha …

Pyrimidines — a single six-membered ring:

  • Cytosine (C) — carries an −NH2-\text{NH}_2 group and a ring C=O\text{C=O}; present in both DNA and RNA …
Structures of the three pyrimidine bases cytosine (C), thymine (T) and uracil (U) as single six-membered rings with every ring atom written out — cytosine with an -NH2 group and one ring C=O, thymine with a -CH3 group and two ring C=O groups, and uracil identical to thymine but without the methyl group.
Structures of the three pyrimidine bases cytosine (C), thymine (T) and uracil (U) as single six-membered rings with every ring atom written out — cytosine with an -NH2 group and one ring C=O, thymine with a -CH3 group and two ring C=O groups, and uracil identical to thymine but without the methyl group.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your NCERT textbook's own diagram.

Redrawn from the NCERT page with the structures, printed labels (NH2, HC, NH, H3C) and reagent placement exactly as the textbook prints them. Every element of this display was checked against the printed page during the sweep's blind-judge verification pass, so wha …

Start your 14-day free trial to unlock the full solution →
← Previous · 10.5Nucleic AcidsNext · 10.5.2 →Structure of Nucleic Acids