How does the sequence of bases in DNA carry the code to build a protein?
The structure of DNA as a double helix of two strands made of nucleotides, complementary base pairing (A-T, C-G), the gene as a section of DNA coding for a sequence of amino acids, and an overview of protein synthesis using mRNA and ribosomes.
A focused answer to the OCR Gateway GCSE Biology A topic B1 on DNA and protein synthesis, covering the double helix, nucleotides, complementary base pairing, the gene as a code for a protein, and the overview of transcription and translation using mRNA and ribosomes.
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What this dot point is asking
OCR wants you to describe the structure of DNA, state the complementary base pairing rules, explain that a gene codes for a protein, and give an overview of how a protein is made using mRNA and ribosomes. The molecular detail is kept at overview level, but the link from base sequence to protein is essential.
The structure of DNA
The sugar and phosphate groups join up to form the backbone of each strand. The bases stick out from the backbone and pair across the two strands, holding them together. The four bases are adenine (A), thymine (T), cytosine (C) and guanine (G).
Complementary base pairing
The two strands are held together by pairs of bases, and the pairing always follows the same rules:
- A pairs with T
- C pairs with G
This is called complementary base pairing. Because the pairing is fixed, if you know the sequence on one strand you can always work out the sequence on the other. For example, a strand reading A-A-C-G-T pairs with a strand reading T-T-G-C-A.
The gene as a code for a protein
A gene is a section of DNA. The order of bases in a gene is a code for the order of amino acids in a protein. Proteins are made by joining amino acids together in a particular sequence; that sequence is what makes one protein different from another, and it is set by the base sequence of the gene.
A small change in the base sequence can change one amino acid, which can change the shape and function of the whole protein. This is why mutations in genes can matter, a link you meet again in topics B5 and B6.
An overview of protein synthesis
You are not expected to know the full molecular mechanism, but you should know the overview:
- In the nucleus, the base sequence of the gene is copied onto a molecule of mRNA (messenger RNA). This step is sometimes called transcription.
- The mRNA carries the copied code out of the nucleus to a ribosome in the cytoplasm.
- At the ribosome, amino acids are joined together in the order set by the mRNA, building the protein. This step is translation.
- The finished chain of amino acids folds into the protein's specific three-dimensional shape, which gives it its function.
Why structure matters for function
Because the base order codes for the amino acid order, DNA is the cell's set of instructions for making every protein it needs, including enzymes, structural proteins and receptor molecules. This connects directly to enzymes (proteins that catalyse reactions) and to inheritance, where genes are passed from parents to offspring.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksDNA is a polymer made of two strands. Describe the structure of DNA, and explain how the order of bases in a gene determines the protein that is made.Show worked answer →
A B1 structured question worth 4 marks, split between Describe and Explain.
Structure (Describe): DNA is made of two strands coiled into a double helix. Each strand is a polymer of repeating units called nucleotides, each containing a sugar, a phosphate group and a base. There are four bases (A, T, C and G), and they pair up by complementary base pairing, A with T and C with G, holding the two strands together.
Code (Explain): a gene is a section of DNA. The order (sequence) of bases in the gene is a code for the order of amino acids in a protein. A different sequence of bases codes for a different sequence of amino acids, which folds into a different protein. Markers reward the link from base order to amino acid order to protein.
OCR 20213 marksOne strand of a short piece of DNA has the base sequence A-T-G-C-C-A. Write the base sequence of the complementary strand, and state where in the cell proteins are assembled.Show worked answer →
A 3-mark question testing base pairing and the site of protein synthesis.
Complementary strand: use the pairing rules A-T and C-G. Opposite A goes T, opposite T goes A, opposite G goes C, opposite C goes G. So the complementary strand of A-T-G-C-C-A is T-A-C-G-G-T. Markers award the correct full sequence; a single wrong base usually loses the mark.
Site of protein synthesis: ribosomes (found in the cytoplasm). Reward "ribosomes". A strong answer notes that the DNA code is first copied into mRNA in the nucleus, and the mRNA carries the code to the ribosomes where amino acids are joined in order.
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