How DNA stores, copies and expresses genetic information
DNA is a double helix of two antiparallel polynucleotide strands, with adenine pairing with thymine and guanine with cytosine, as described by Watson and Crick. It replicates semi-conservatively, shown by the Meselson–Stahl experiment: each new molecule has one old and one new strand. DNA polymerase builds new strands in the 5′ → 3′ direction, so one strand is made continuously and the other in short Okazaki fragments joined by ligase.
The central dogma states that information flows from DNA to RNA to protein. In transcription, RNA polymerase copies a gene into messenger RNA; in eukaryotes, introns are removed by splicing. In translation, ribosomes read the mRNA in three-letter codons, and transfer RNAs bring the matching amino acids. The genetic code is nearly universal and degenerate, with AUG as the start codon and UAA, UAG and UGA as stop codons. Genes are regulated; the lac operon of E. coli switches on only when lactose is present. The Human Genome Project mapped human DNA, and DNA fingerprinting uses repetitive sequences to identify individuals.
Why it matters
Molecular biology explains how mutations cause diseases such as sickle-cell anaemia, how mRNA vaccines work and how DNA tests identify people in forensics. It is the base of genetic engineering, gene therapy and personalised medicine, and it helps scientists study evolution by comparing genes across species.
Key terms
- Semi-conservative replication
- DNA copying in which each new molecule keeps one original strand and gains one new strand.
- Transcription
- The synthesis of RNA from a DNA template.
- Codon
- A sequence of three nucleotides in mRNA that specifies an amino acid or a stop signal.
- Operon
- A group of genes controlled together by a single promoter and operator, as in bacteria.