IGCSE Biology / Chromosomes, genes and DNA
The double helix and base pairing revision notes
Edexcel International GCSE Biology (4BI1), chapter 16: Chromosomes, genes and DNA, specification point 3.16B, 3.17B. Free lesson.
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DNA is made of two strands wound into a double helix, with bases pointing inwards and pairing in a fixed way. This complementary pairing allows DNA to be copied accurately and helps explain how genes are read to make proteins.
How DNA is arranged
DNA has two strands wound around each other to form a double helix. Each strand has a backbone made of alternating sugar and phosphate groups. The bases stick inwards from the backbones, where bases on opposite strands pair and hold the strands together. The base pairs are joined by hydrogen bonds. There are many hydrogen bonds, so DNA is stable overall. Each bond is weak, which allows the strands to separate when DNA needs to be read or copied. The bases contain the genetic information.
Which bases pair together
DNA has four bases. Their pairing is fixed, so each base on one strand determines the base opposite it. The two strands are therefore complementary. Adenine (A) pairs with thymine (T). Cytosine (C) pairs with guanine (G). A pairs with T, and C pairs with G. No other pairings occur. Because one strand determines the other, DNA can be copied accurately. The base sequence is also used when a gene is read to make a protein.
How to write a complementary strand
To find the matching strand, work through the given sequence one base at a time and replace each base with its partner.
How base percentages are linked
Because A always pairs with T, they must make up equal percentages of the bases. C and G must also have equal percentages. The four percentages together add up to 100%. If adenine is 20%, thymine is also 20%. That leaves 60% for cytosine and guanine. They share it equally, so each is 30%.
How DNA pairing relates to RNA
The pairing rule is also used when writing messenger RNA (mRNA) from a DNA strand. RNA is single-stranded and uses uracil (U) instead of thymine (T), while the other base-pairing relationships stay the same. A group of three bases codes for one amino acid. This is how a gene specifies a protein.