3.4 — Building DNA from nucleotides

Start at the molecular scale. A DNA molecule is a polymer: a large molecule made by joining many repeating smaller units. The repeating units in DNA are nucleotides.

The four bases are adenine (A), thymine (T), cytosine (C) and guanine (G). Many nucleotides join to form a DNA strand. The sugar and phosphate groups repeat along the outside of the strand, while the bases point inwards.

This distinction matters: a base is one component of a nucleotide. A nucleotide contains the base and its sugar and phosphate group.

3.4 — From base pairs to a double helix

One DNA molecule contains two nucleotide strands. A base on one strand pairs with a base on the other, so the paired bases link the two strands like the rungs of a ladder. The pairing is complementary, meaning that each base has one particular partner:

• adenine pairs with thymine: A-T
• cytosine pairs with guanine: C-G

The complementary bases are joined by weak hydrogen bonds. These bonds act across the two strands, between paired bases; they do not make the repeating sugar-phosphate chain along a strand. The two linked strands coil around each other to form a double helix, like a twisted ladder.

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Worked example: applying complementary pairing

Suppose one DNA strand contains this sequence:

A C T G G A

Match each base using A-T and C-G:

first strand:         A C T G G A
complementary strand: T G A C C T

Every position is determined by the same complementary pairing rule.

3.5 — Genes and the genome

The words gene and genome describe DNA at very different scales.

A genome contains many genes, but genome does not mean only the genes. It means all of the organism's DNA. A gene is therefore one defined section within that much larger whole, not a whole DNA molecule and not a whole genome.