From gene to protein
Genes are instructions for proteins. The central dogma describes how the message flows: DNA to RNA to protein.
The central dogma
Genetic information flows from DNA to RNA (transcription) to protein (translation). Proteins then do most of the cell's work: enzymes speed reactions, structural proteins build tissues, and signalling proteins carry messages. Retroviruses are a notable exception, copying RNA back into DNA.
RNA
RNA differs from DNA: it is usually single-stranded, uses the sugar ribose, and has uracil (U) instead of thymine. Messenger RNA (mRNA) carries the gene's message; transfer RNA (tRNA) brings amino acids; ribosomal RNA (rRNA) forms the core of ribosomes.
Transcription
In the nucleus, RNA polymerase binds a promoter at the start of a gene, unwinds the DNA, and builds an mRNA copy of one strand, using base pairing with U in place of T. In eukaryotes the mRNA is processed: non-coding introns are spliced out, leaving exons, and a cap and tail are added before it leaves the nucleus.
The genetic code
mRNA is read in triplets called codons, each coding for one amino acid. With 4 bases, there are 64 possible codons for 20 amino acids, so most amino acids have several codons. AUG codes for methionine and is the start codon; UAA, UAG, and UGA are stop codons. The code is nearly universal across all life.
Translation
At a ribosome, mRNA codons are matched by tRNA molecules with complementary anticodons, each carrying a specific amino acid. The ribosome links the amino acids by peptide bonds into a chain. When it reaches a stop codon, the chain is released and folds into a working protein.
Protein folding
A protein's sequence of amino acids determines how it folds into its three-dimensional shape, and shape determines function. Changing one amino acid can change the shape: in sickle cell disease, one substitution in haemoglobin makes the molecules stick together and distort red blood cells.
Mutations and proteins
A silent mutation changes a codon to another for the same amino acid, with no effect. A missense mutation swaps one amino acid. A nonsense mutation creates an early stop codon, cutting the protein short. Frameshifts change every codon after the mutation.
Worked example
A DNA template strand reads TAC GGA TTT ACT. The mRNA is AUG CCU AAA UGA. Translated: AUG = methionine (start), CCU = proline, AAA = lysine, UGA = stop. The protein is Met-Pro-Lys.
Key takeaways
DNA is transcribed to mRNA, which is translated into protein. RNA uses ribose and uracil. Eukaryotic mRNA has introns removed. Codons are base triplets; AUG starts, three codons stop, and the code is nearly universal. tRNA anticodons match codons at ribosomes. Protein shape follows sequence, so mutations can change function.
Flashcard
DNA -> RNA -> protein
Check your understanding
An mRNA codon reads GCU. What is the anticodon on the matching tRNA?