Chemistry · Ch 5 — Biomolecules
Nucleic Acids: Composition and Structure of DNA and RNA
Nucleic Acids: Composition and Structure of DNA and RNA
Nucleic acids -- deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) -- are polymers built up from repeating monomer units called nucleotides. Each nucleotide is itself made of three distinct component parts joined together: a nitrogenous base, a pentose sugar, and a phosphate group. (A sugar joined to a base, but without the phosphate group, is called a nucleoside; adding a phosphate group to a nucleoside gives the complete nucleotide.)
The nitrogenous bases fall into two chemical families: the two-ring purine bases, adenine (A) and guanine (G), and the single-ring pyrimidine bases, cytosine (C), and either thymine (T) or uracil (U). DNA and RNA differ in exactly which pentose sugar and which set of bases they use. DNA's sugar is 2-deoxyribose, so called because it lacks the group at the 2 position of the ribose ring that RNA's sugar, plain ribose, retains; and DNA uses thymine as its fourth base, whereas RNA uses uracil in thymine's place (both DNA and RNA use adenine, guanine and cytosine). Within a nucleotide, the nitrogenous base is attached to C1 of the sugar, and successive nucleotides are linked together by their phosphate groups, which bridge the 3 carbon of one sugar to the 5 carbon of the next, building up the repeating sugar-phosphate backbone of the polynucleotide chain, with the nitrogenous bases projecting outward from that backbone at each sugar unit. …
What this figure shows. Two antiparallel sugar-phosphate backbones, drawn as two ribbon-like strands winding around each other as a right-handed double helix, with the nitrogenous bases projecting inward from each backbone and meeting at the centre in horizontal, ladder-rung base pairs: an adenine (A) base on one strand connected to a thymine (T) base on the other strand by two short dashed lines representing two hydrogen bonds, and, at a different rung, a guanine (G) base connected to a cytosine (C) base by three short dashed lines representing three hydrogen bonds -- illustrating that A pairs only with T and G pairs only with …
Comparing DNA and RNA
DNA and RNA are both polymers of nucleotides, and they share several features in common -- both use adenine, guanine and cytosine among their bases, and both are built on a repeating sugar-phosphate backbone -- but they differ in three respects that are worth setting out side by side before moving on: the identity of the pentose sugar each one uses, the fourth nitrogenous base each one uses in place of the other's, and how many polynucleotide strands each one is normally built from.
The pentose sugar is deoxyribose in DNA (lacking the group at C2 that gives deoxyribonucleic acid its name) and ribose in RNA (which retains that group). Both nucleic acids use adenine, guanine and cytosine, but DNA's fourth base is thymine, whereas RNA substitutes uracil in thymine's place. Finally, DNA is normally found as a double helix of two complementary, hydrogen-bonded strands, while RNA is normally found as a single strand. …
| Feature | DNA | RNA |
|---|---|---|
| Pentose sugar | 2-Deoxyribose (no at C2) | Ribose (has at C2) |
| Purine bases | Adenine (A), Guanine (G) | Adenine (A), Guanine (G) |
| Pyrimidine bases | Cytosine (C), Thymine (T) | Cytosine (C), Uracil (U) |
| Base unique to it | Thymine (T) | Uracil (U) |
| Number of strands | Two, wound as a double helix | Usually one (single-stranded) |