DNA and protein synthesis
The structure of DNA and how genes code for proteins.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 17.2.1Describe the structure of DNA and explain how a gene codes for a protein.Supplement
6 minute read
DNA and protein synthesis
Structure of DNA
DNA is a double helix: two strands twisted around each other, held together by base pairs. There are four bases, and they pair in a fixed way — complementary base pairing:
- A pairs with T
- C pairs with G
So a strand reading ATGCC has the complementary strand TACGG. Because the pairing is fixed, one strand carries all the information needed to rebuild the other, which is what makes accurate copying possible.
Genes and proteins
A gene is a length of DNA that codes for one protein. The sequence of bases determines the sequence of amino acids, and the amino acid sequence determines the protein's shape and therefore its function. Each amino acid is coded by a sequence of three bases.
How a protein is made
- The gene stays in the nucleus — DNA is too large to leave.
- A copy of the gene is made as mRNA, which is small enough to pass out through a nuclear pore.
- The mRNA travels to a ribosome in the cytoplasm.
- The ribosome reads the base sequence in threes and assembles the matching amino acids in order.
- The chain of amino acids folds into a protein.
Why this matters
Different cells make different proteins because different genes are switched on. A muscle cell and a nerve cell contain identical DNA — their difference lies entirely in which genes are expressed. A mutation — a change in the base sequence — can change one amino acid, altering the protein's shape. In an enzyme, that may change the active site so the substrate no longer fits.
Think of it like this
DNA is a reference library that cannot be taken out. mRNA is a photocopy of a single page, carried to the workshop (ribosome) where the item is built to that specification.
Worked examples
Method, step by step
One strand of DNA reads TACGGA. Write the complementary strand and explain your reasoning.
- 1Base pairing is fixed: A pairs with T, and C pairs with G.
- 2Take each base in turn: T→A, A→T, C→G, G→C, G→C, A→T.
- 3This gives ATGCCT.
ATGCCT — because A always pairs with T and C always pairs with G.
Common misconceptions
- Thinking DNA leaves the nucleus. It does not — mRNA carries the copy instead.
- Believing different cells contain different DNA. They contain the same DNA; different genes are switched on.
- Getting base pairing wrong. A pairs with T and C with G — never A with C.
In the exam
- For "how does a gene control a characteristic", give the chain: base sequence → amino acid sequence → protein shape → function.
- When asked why a mutation matters, link the changed base to a changed amino acid and then to a changed protein shape.