Transcription and translation in DNA for A-Level Biology

Transcription is the process where a section of DNA, a gene, is copied into a complementary strand of messenger RNA (mRNA) by the enzyme RNA polymerase inside the nucleus. Translation is the process where that mRNA is read by a ribosome in the cytoplasm and used to assemble a chain of amino acids, building a polypeptide. Together they make up protein synthesis, the central process described by the central dogma of molecular biology.

This guide walks through each stage in the detail AQA expects at A-Level: The enzymes, the base pairing rules, the structure of mRNA and tRNA, and the worked examples that show up in Paper 2 every year.

The central dogma states that genetic information flows from DNA to mRNA to protein. The DNA stays in the nucleus, the mRNA acts as a portable copy of the gene, and the protein is built in the cytoplasm. Splitting the job in two protects the DNA from damage and lets the cell make many copies of a protein from a single gene.

AQA expects you to be able to name the molecules at each step and the enzymes involved. The two stages, transcription and translation, are tested together every year, often through a worked example that asks you to read a sequence and predict the next step.

Transcription begins when RNA polymerase binds to a promoter region of DNA at the start of a gene. The enzyme unwinds the DNA double helix at that point and exposes the two strands of bases.

Only one strand, the template strand (also called the antisense strand), is read. RNA polymerase moves along the template strand from 3' to 5', adding complementary RNA nucleotides to build a new mRNA strand. Adenine on the DNA pairs with uracil on the RNA, thymine pairs with adenine, cytosine pairs with guanine, and guanine pairs with cytosine.

The RNA polymerase stops when it reaches a stop signal called a terminator sequence. The completed mRNA strand detaches and the DNA rewinds back into its double helix shape.

Translation begins when a mature mRNA strand leaves the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm. The ribosome reads the mRNA in groups of three bases called codons, starting at the start codon AUG.

For each codon, a transfer RNA (tRNA) molecule arrives carrying the matching amino acid. The tRNA has an anticodon that is complementary to the codon, so codon and anticodon pair up by base pairing. The ribosome holds two tRNA molecules side by side, forms a peptide bond between their amino acids, and then moves one codon along the mRNA.

The ribosome continues until it reaches a stop codon (UAA, UAG or UGA). At that point no tRNA matches, the polypeptide chain detaches from the ribosome, and translation ends. The polypeptide then folds into its functional 3D shape, often with help from chaperone proteins.

AQA Paper 2 often asks you to compare the two stages directly. The table below summarises everything you need to remember for a comparison answer.

Suppose the template strand of a gene reads 3'–TAC–CGT–AAA–ATC–5'. Work out the mRNA sequence and the amino acids produced.

Step 1: Transcribe the template strand into mRNA. The bases pair A–U, T–A, C–G, G–C. The mRNA reads 5'–AUG–GCA–UUU–UAG–3'.

Step 2: Read the mRNA in codons. The first codon AUG is the start codon and codes for methionine. The second codon GCA codes for alanine. The third codon UUU codes for phenylalanine. The fourth codon UAG is a stop codon, so translation stops.

Step 3: The polypeptide produced is methionine–alanine–phenylalanine, three amino acids long. The stop codon is not translated into an amino acid.

Transfer RNA is a small, single-stranded RNA molecule that folds into a clover-leaf shape. At one end it has an anticodon, three bases that match a specific mRNA codon. At the other end it has an amino acid binding site that holds one specific amino acid.

This pairing is what allows the genetic code to be read accurately. Each tRNA molecule connects a specific codon to a specific amino acid, so the order of codons on the mRNA dictates the order of amino acids in the polypeptide. AQA expects you to label the anticodon and the amino acid binding site on a diagram.

Examiner reports on AQA A-Level Biology Paper 2 flag the same pattern of mistakes every year. Most are about precision rather than understanding, and they are easy to fix once you know what to watch for.