What you'll learn
Your DNA carries the instructions for making proteins, but those instructions have to be read and turned into an actual protein — a two-stage process called protein synthesis. For AQA GCSE Biology (higher tier) you need to understand how the base sequence of a gene codes for a protein, the roles of transcription and translation, and how mutations can change a protein. This guide covers the structure of the gene and mRNA, the two stages of protein synthesis, how the amino acid chain folds into a working protein, and the effect of mutations. By the end you should be able to describe transcription and translation in order and explain how the sequence of DNA determines the protein made.
Key terms and definitions
Protein synthesis — The process of making a protein from the instructions in a gene.
Gene — A section of DNA that codes for a particular protein.
Transcription — Copying the gene's base sequence into a molecule of mRNA.
Translation — Using the mRNA code to join amino acids in the correct order.
mRNA (messenger RNA) — A copy of a gene that carries the code from the DNA to the ribosome.
Ribosome — The structure where amino acids are joined together to make a protein.
Amino acid — The building block of a protein; the order of amino acids is set by the gene.
Mutation — A change in the DNA base sequence that may change the protein.
Core concepts
DNA, genes and proteins
DNA is made of two strands twisted into a double helix, with a sequence of four bases (A, T, C and G). A gene is a section of DNA that codes for a particular protein. The order of the bases in the gene determines the order of amino acids in the protein, and it is this order of amino acids that decides which protein is made and how it works. So the sequence of bases is a code, read in groups, for building a protein.
Why proteins matter
Proteins do most of the work in the body. They include enzymes (biological catalysts), hormones (chemical messengers), structural proteins (such as collagen), and antibodies. Because each protein has a specific job, it must have exactly the right sequence of amino acids, which is why the DNA code must be read accurately.
Stage 1: Transcription
Protein synthesis happens in two stages. The first is transcription, which takes place in the nucleus:
- The two DNA strands separate over the gene being copied.
- A complementary copy of the gene's base sequence is made as a molecule of mRNA (messenger RNA).
- The mRNA is a short, single-stranded copy of the gene.
The mRNA then leaves the nucleus and travels to a ribosome in the cytoplasm. Transcription is needed because DNA cannot leave the nucleus, so a mobile copy — the mRNA — carries the code to where the protein is made.
Stage 2: Translation
The second stage is translation, which takes place at the ribosome:
- The mRNA attaches to the ribosome.
- The ribosome reads the mRNA code in groups of three bases; each group codes for a specific amino acid.
- Molecules carry the correct amino acids to the ribosome, matching the code.
- The amino acids are joined together in the order set by the mRNA, forming a chain.
The result is a chain of amino acids in a specific order — the primary structure of the protein.
Folding into a working protein
Once the chain of amino acids is complete, it folds into a unique three-dimensional shape. This shape is determined by the order of amino acids, and it is essential to the protein's function. For example, an enzyme folds so that its active site has exactly the right shape to fit its substrate. If the shape is wrong, the protein cannot do its job. So the base sequence of the gene ultimately determines the shape and function of the protein.
Mutations and their effects
A mutation is a change in the DNA base sequence. Because the base sequence codes for the amino acid sequence, a mutation can change the amino acids in the protein and therefore its shape and function:
- Many mutations have little or no effect, because they do not change the amino acid, or change one in a way that does not alter the protein's function.
- Some mutations change the shape of the protein. In an enzyme, a change to the active site could stop the substrate binding, so the enzyme no longer works.
- In non-coding DNA, mutations can affect how genes are switched on or off, changing how much protein is made.
This is why mutations are the source of new variation, and why some can cause problems.
Non-coding DNA and switching genes on and off
Not all of the DNA in a cell codes for proteins. Some sections are non-coding, and these have important jobs too. Some non-coding regions are involved in switching genes on or off, controlling whether a gene is transcribed and therefore whether its protein is made. This is why different cells in the body — such as muscle cells and nerve cells — can contain the same genes but make different proteins: different genes are switched on in different cells. A mutation in a non-coding region can affect how a gene is controlled, changing how much of a protein is produced, even if it does not change the protein itself. This shows that the control of protein synthesis is as important as the code itself.
Why every cell can make the right proteins
Every cell in the body carries a full copy of the DNA, so it has the instructions for every protein. What makes cells different is which genes are actually transcribed and translated. During transcription, only the gene that is needed is copied into mRNA, so only the required protein is made. This selective process means a single set of instructions can produce hundreds of different proteins in different cells at different times, which is essential for building and running a complex organism. Understanding this helps explain how one genome can produce so much variety within a single body.
Worked examples
Example 1: Ordering the stages
Put these in the correct order for protein synthesis: translation at the ribosome, mRNA leaves the nucleus, transcription in the nucleus. The correct order is: transcription in the nucleus, mRNA leaves the nucleus, translation at the ribosome. The mRNA carries the code from the DNA to the ribosome.
Example 2: Why mRNA is needed
Explain why a molecule of mRNA is needed in protein synthesis. DNA is too large to leave the nucleus, but proteins are made at ribosomes in the cytoplasm. mRNA is a small, mobile copy of the gene that can leave the nucleus and carry the code to the ribosome, so the protein can be made.
Example 3: From bases to protein
Explain how the base sequence of a gene determines the protein made. The order of bases in the gene sets the order of amino acids in the protein. The amino acid order determines how the chain folds into a specific 3D shape, and this shape determines the protein's function.
Example 4: The effect of a mutation
Explain how a mutation could stop an enzyme working. A mutation changes the base sequence, which can change an amino acid in the enzyme. This can change the shape of the active site, so the substrate no longer fits, and the enzyme can no longer catalyse its reaction.
Common mistakes and how to avoid them
A very common error is mixing up transcription and translation. Transcription happens in the nucleus and makes mRNA; translation happens at the ribosome and joins amino acids. Learn which stage does what and where.
Students often forget that the code is read in groups of three bases, each coding for one amino acid. This is a key detail for full marks.
Another mistake is saying the protein's function depends only on which amino acids are present. It depends on their order, which determines the folding and 3D shape. Always mention the shape.
When explaining mutations, do not say every mutation is harmful. Many have no effect, some are harmful, and occasionally one is beneficial and provides useful variation. Give a balanced answer.
Finally, remember that mRNA leaves the nucleus because DNA cannot — this is the reason a copy is made rather than using the DNA directly.
Exam technique for "Protein synthesis: transcription and translation"
For process questions, describe the two stages in order and in the correct place: transcription in the nucleus making mRNA, then translation at the ribosome joining amino acids read in threes. A clear sequence earns the marks.
When asked how the gene determines the protein, build the chain: base sequence → amino acid order → folding → 3D shape → function. This links the DNA to the working protein and is a common higher-mark question.
For mutation questions, explain that a change in the base sequence can change an amino acid and therefore the shape and function of the protein, and note that many mutations have little or no effect. Use precise terms — transcription, translation, mRNA, ribosome, amino acid — throughout.
Quick revision summary
- A gene is a section of DNA; the order of its bases determines the order of amino acids in a protein.
- Transcription (in the nucleus): the gene is copied into mRNA, which leaves the nucleus.
- Translation (at the ribosome): the mRNA is read in groups of three bases, each coding for one amino acid, which are joined into a chain.
- The amino acid chain folds into a specific 3D shape that determines the protein's function (e.g. an enzyme's active site).
- A mutation changes the base sequence and can change the protein's shape and function; many have little or no effect.
- mRNA is needed because DNA cannot leave the nucleus but proteins are made at ribosomes in the cytoplasm.