DNA structure
DNA is a double helix made of two strands of nucleotides. Each nucleotide has a sugar (deoxyribose), a phosphate and one of four bases: adenine (A), thymine (T), guanine (G) or cytosine (C). The strands are held together by hydrogen bonds between bases following base-pairing rules: A pairs with T, and G pairs with C. The two strands run in opposite directions (antiparallel).
Because of this pairing, one strand predicts the other. If one strand reads ATGC, the complementary strand reads TACG.
Replication
Before a cell divides, its DNA is copied. Replication is semiconservative: the helix unwinds, and each old strand acts as a template for a new one. The result is two molecules, each with one old and one new strand. Enzymes: helicase unwinds, DNA polymerase adds nucleotides. Proofreading keeps errors very rare.
The central dogma
DNA → RNA → protein.
RNA differs from DNA: it is usually single-stranded, has ribose sugar, and contains uracil (U) instead of thymine.
Transcription (nucleus, in eukaryotes)
An enzyme, RNA polymerase, reads a DNA template strand and builds a matching mRNA. Base pairing is the same except A on the template pairs with U in RNA. In eukaryotes the new mRNA is processed (introns removed, a cap and tail added) before leaving the nucleus.
Translation (ribosome)
The ribosome reads the mRNA in groups of three bases called codons. Each codon specifies one amino acid (or a stop signal). tRNA molecules carry the amino acids; each has an anticodon that pairs with a codon. The ribosome links the amino acids into a chain, which folds into a protein. The codon AUG is the start signal and codes for methionine.
Worked example
DNA template strand: 3′-TAC GGA-5′
- Transcribe by pairing each base: T→A, A→U, C→G, G→C, G→C, A→U.
- mRNA: 5′-AUG CCU-3′.
- Translate with the genetic code: AUG = methionine (start), CCU = proline.
- The short peptide is Met–Pro.
Mutations
A mutation is a change in DNA sequence. A substitution swaps one base; insertions and deletions add or remove bases and can shift the reading frame, which usually has a large effect. Because the genetic code is redundant (several codons per amino acid), some substitutions are silent, leaving the protein unchanged. Others can change one amino acid or create an early stop. Mutations are the ultimate source of new alleles.
Gene regulation
Not every gene is active in every cell. Cells switch genes on and off, which is why a neuron and a skin cell have the same DNA but different proteins.