Genes, proteins and the central dogma
A protein is a long chain of small units called amino acids. There are 20 kinds. The order of amino acids decides the shape of the protein, and the shape decides its job.
A gene is a part of DNA that stores the order for one protein. Using a gene to make its protein is called gene expression.
Information flows one way in most cells: DNA → RNA → protein. Scientists call this the central dogma. It happens in two stages: transcription and translation.
Transcription: copying DNA into mRNA
Transcription happens in the nucleus (in bacteria, which have no nucleus, it happens in the cytoplasm).
- The enzyme RNA polymerase attaches at the start of the gene (the promoter).
- It opens the two DNA strands, like a zip.
- It reads one strand (the template strand) and builds a matching RNA strand: A pairs with U, T with A, G with C, C with G.
- At the end of the gene it lets go. The new mRNA is released and the DNA zips up again.
The mRNA has the same base order as the other DNA strand (the coding strand), except U is used in place of T. In cells with a nucleus, the mRNA is trimmed and edited, then leaves through a nuclear pore.
The genetic code: codons
The mRNA is read in groups of three bases called codons. 4 bases in groups of 3 give 4 × 4 × 4 = 64 codons.
- AUG is the START codon. It also codes for the amino acid methionine (Met).
- UAA, UAG, UGA are STOP codons. They code for no amino acid.
- The other 61 codons code for the 20 amino acids, so most amino acids have more than one codon. The code is redundant (degenerate).
- The code is universal: almost every living thing uses the same table. That is why bacteria can make human insulin.
- The code is non-overlapping: each base belongs to only one codon.
Translation: building the protein at the ribosome
Translation happens on ribosomes in the cytoplasm (free, or on the rough endoplasmic reticulum). Three kinds of RNA work together:
- mRNA carries the message (the codons).
- tRNA carries one amino acid. It has an anticodon of 3 bases that pairs with the codon. Codon AUG pairs with anticodon UAC.
- rRNA (with proteins) makes up the ribosome and joins the amino acids with peptide bonds.
- Start: the ribosome finds AUG; the first tRNA brings Met.
- Grow: the ribosome moves one codon at a time; each new tRNA adds its amino acid to the chain.
- Stop: at a STOP codon the chain is released.
The chain (a polypeptide) then folds into its 3D shape. Many ribosomes can read one mRNA at the same time, so many copies are made quickly.
Mutations and how genes are switched on
A mutation is a change in the DNA bases.
- Silent: the new codon codes for the same amino acid. The protein does not change.
- Missense: one amino acid changes. Sickle-cell anaemia is caused by one such change in the haemoglobin gene.
- Nonsense: the codon becomes STOP, so the protein is cut short.
- Frameshift: adding or removing one base shifts every codon after it. Usually very harmful.
Every body cell has the same DNA, but each cell type uses only some genes. A liver cell and a skin cell differ because different genes are switched on (expressed). Signals like hormones can turn genes on or off.
Try it: decode a gene
Write the coding strand ATG-GGC-TTT-AAA-TGG-TAA on paper. Change every T to U to get the mRNA. Split into codons and look up each one in a codon table: AUG = Met, GGC = Gly, UUU = Phe, AAA = Lys, UGG = Trp, UAA = STOP. Now change the 6th base C to A (GGA): still Gly — silent! Change the 12th base A to G (AAG): still Lys. Check your answers in the 3D free-play step.
Key formulas and definitions
- DNA → (transcription) → mRNA → (translation) → protein
- Base pairing in transcription: A→U, T→A, G→C, C→G
- 1 codon = 3 bases = 1 amino acid
- 4³ = 64 codons: 61 for amino acids + 3 STOP
- START codon = AUG (Met); STOP = UAA, UAG, UGA
- Number of amino acids = (number of mRNA bases before STOP) ÷ 3
Worked examples
1. The template DNA strand reads TAC GGA CTT. Write the mRNA.
Pair each base: T→A, A→U, C→G, G→C. mRNA = AUG CCU GAA.
2. An mRNA reads AUG UUU GGC UAA. How many amino acids are in the protein?
Codons: AUG, UUU, GGC, UAA. UAA is STOP and adds nothing. So 3 amino acids: Met–Phe–Gly.
3. A codon is GCA. What is the tRNA anticodon?
Pair each base: G–C, C–G, A–U. Anticodon = CGU.
4. A gene has 900 bases from the start codon up to and including the stop codon. How many amino acids?
900 ÷ 3 = 300 codons. One is STOP. So 299 amino acids.
5. A DNA coding strand changes from AAA to AAG. What happens to the protein?
mRNA AAA and AAG both code for lysine. The protein is the same: a silent mutation.
6. A base is deleted near the start of a gene. Why is this usually worse than a single base swap?
Every codon after the deletion is read in the wrong groups of 3 (frameshift). Many amino acids change and a STOP codon often appears early.
Common mistakes
- Writing T in mRNA. RNA has U, never T.
- Mixing up the places: transcription is in the nucleus, translation is at ribosomes in the cytoplasm.
- Counting the STOP codon as an amino acid.
- Confusing codon (on mRNA) with anticodon (on tRNA).