The structure of DNA
DNA (deoxyribonucleic acid) is the molecule that stores genetic information. It is a double helix — two strands twisted together — built from units called nucleotides, each a sugar, a phosphate and one of four bases: adenine (A), thymine (T), guanine (G) and cytosine (C). The two strands are held together by the bases, which pair by a strict rule: A always pairs with T, and G with C (complementary base pairing).
A gene is a length of DNA that codes for one protein; DNA is packaged with protein into chromosomes in the nucleus.
DNA replication
Before a cell divides it must copy its DNA. The double helix unzips as the base pairs separate, and each old strand acts as a template: free nucleotides pair up against it by the base-pairing rules, building a new complementary strand. The result is two identical DNA molecules, each with one old and one new strand — this is called semi-conservative replication, and it ensures each daughter cell gets an exact copy.
Base pairing is the key
Because A only pairs with T and G only with C, one strand exactly specifies the other. That single rule is what lets DNA be copied faithfully and read to make proteins.
The genetic code and protein synthesis
The sequence of bases is a code: each group of three bases (a triplet, or codon) specifies one amino acid. To make a protein, the gene is first copied into a messenger molecule, RNA (transcription), which carries the code out of the nucleus to a ribosome. There the code is read three bases at a time and the matching amino acids are joined in order (translation) to build the protein.
So the order of bases in a gene sets the order of amino acids in a protein — and hence the protein's shape and job.
Mutations
A mutation is a change in the base sequence of DNA. It may have no effect, or it may change an amino acid and so the protein — sometimes harmfully (causing disease), occasionally usefully (providing variation for evolution). Mutations happen naturally at a low rate, but their frequency is raised by mutagens such as ionising radiation, ultraviolet light and some chemicals.
A worked example
One strand of DNA reads T-A-C-G-G-A. Write the sequence of the complementary strand, and explain how you worked it out.
- Apply the base-pairing rules: A pairs with T, G pairs with C.Bases pair only in these fixed combinations.
- T→A, A→T, C→G, G→C, G→C, A→T.Replace each base with its partner.
- So the complementary strand is A-T-G-C-C-T.Reading off each paired base.
A-T-G-C-C-T — found by pairing A with T and G with C along the given strand.
DNA technology and genetic engineering
Understanding DNA lets us work with it directly. Special enzymes cut DNA at exact sequences, so a useful gene can be removed and inserted into another organism's DNA — the basis of genetic engineering (as when bacteria are given the human insulin gene). The polymerase chain reaction (PCR) makes millions of copies of a tiny DNA sample, and DNA profiling reads the pattern of an individual's DNA — used in forensic science and to check relationships. All of it rests on the base-pairing rules.
The genome and genetic screening
An organism's complete set of DNA is its genome, and the entire human genome has now been sequenced. This makes genetic screening possible — testing a person's DNA for alleles that cause inherited disorders, whether before birth, in newborns, or to assess the risk to future children (genetic counselling). The power to read our own genetic information brings clear medical benefits but also ethical questions about privacy, insurance and how the information is used.
PCR: a molecular photocopier
PCR repeatedly copies a chosen stretch of DNA, doubling it each cycle, so a trace amount — from a crime scene or a single cell — becomes enough to analyse.