DNA

Section 4 of 4

Almost every cell in your body carries the instructions for making your proteins. They are written in DNA, short for deoxyribonucleic acid.

A DNA molecule is two long strands wound round each other into a double helix, like a twisted ladder. The sides of the ladder are the backbones: sugar and phosphate groups joined one after another. Each sugar carries one base, and the bases on the two strands meet in the middle to make the rungs.

DNA's double helix Two strands, the backbones, wind round each other like a twisted ladder. Between them, rungs run across, each rung made of two coloured halves: a base on each strand. Blue adenine always meets amber thymine, and teal guanine always meets red cytosine. A key at the bottom names the four bases. backbone A adenine T thymine G guanine C cytosine
DNA's double helix. The two backbones wind round each other, and each rung is a pair of bases, one from each strand.

Bases pair up

There are four bases: adenine (A), thymine (T), guanine (G) and cytosine (C). They are flat rings of carbon and nitrogen atoms, like the flat benzene ring, with N-⁠H groups, C=O groups and nitrogen atoms with lone pairs along their edges. Across the middle of the helix, A always pairs with T, and G always pairs with C.

The pairs are held together by hydrogen bonds. Nitrogen pulls the shared electrons of an N-⁠H bond toward itself, which leaves the hydrogen slightly positive, and that hydrogen is attracted to a lone pair on an oxygen or nitrogen atom of the base opposite. A and T fit together with 2 hydrogen bonds, and G and C with 3.

DNA base pairs: A with T, G with C Two base pairs drawn as skeletal formulas. Top: adenine, two joined rings, faces thymine, one ring. Two dashed hydrogen bonds join them: an N-H of adenine to an oxygen of thymine, and an N-H of thymine to a nitrogen of adenine. Bottom: guanine, two joined rings, faces cytosine, one ring, joined by three dashed hydrogen bonds: an N-H of cytosine to guanine's oxygen, and two N-H groups of guanine to a nitrogen and an oxygen of cytosine. A short dotted line from each base leads to the backbone. 2 hydrogen bonds NNNNNHH NNOOH A T adenine thymine 3 hydrogen bonds NNNNOHNHH NNONHH G C guanine cytosine
Base pairs as skeletal formulas, with hydrogen bonds as dashed lines: 2 between adenine and thymine, 3 between guanine and cytosine. The dotted lines lead to the backbone.

A big two-ring base always pairs with a small one-ring base, so every rung is the same length and the helix keeps the same width all the way along. Each hydrogen bond is much weaker than a covalent bond. A cell can unzip the two strands to read or copy them, while the covalent bonds along each strand hold firm.

Instructions in four letters

The order of the bases along a strand spells out the instructions. Each group of three bases stands for one amino acid, so a stretch of DNA called a gene lists the amino acids of one protein, in order. Because A always faces T and G always faces C, one strand fixes the order of the other. When a cell divides, it unzips its DNA and builds a new partner for each strand, making two matching copies.

Quick check

One strand of DNA has the bases A, A, G, C in a row. Which bases sit opposite them on the other strand?

That's the end of lesson 10

In this lesson you found out:

  • Glucose, and how it joins into starch and cellulose
  • Fats and oils as big esters, and why oils stay liquid
  • Proteins, made of amino acids joined by peptide bonds
  • How DNA's base pairs fit together and store instructions
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