How does the base sequence of a gene get turned into a protein, and what happens when a mutation changes it?
Protein synthesis as transcription of a gene into mRNA and translation at the ribosome, the role of the triplet code and amino acids, how the order of bases determines the protein made, and how mutations can change a protein and its function.
A focused answer to the OCR Gateway GCSE Biology A topic B5 on protein synthesis, covering transcription of a gene into mRNA, translation at the ribosome, the triplet code and amino acids, how the base order determines the protein made, and how mutations can change a protein and its function.
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What this dot point is asking
OCR wants you (mainly at Higher tier) to describe protein synthesis as two stages, transcription and translation, explain the role of the triplet code and amino acids, show how the order of bases determines the protein made, and explain how mutations can change a protein and its function.
The job a gene does
A gene is a section of DNA that codes for a particular protein. Proteins do most of the work in a cell: they include enzymes, structural proteins (such as collagen) and hormones (such as insulin). The order of bases in a gene fixes the order of amino acids in its protein, and the order of amino acids determines how the protein folds and therefore what it does.
Stage 1: transcription
DNA is too large to leave the nucleus, but proteins are built on ribosomes in the cytoplasm. So a copy of the gene is made:
- The DNA double helix unwinds at the gene.
- The base sequence of the gene is used as a template to build a complementary strand of messenger RNA (mRNA).
- The mRNA is a single strand that carries the code; it leaves the nucleus through a pore and travels to a ribosome.
The mRNA is complementary to the gene, so it carries the same information in a form that can move to the ribosome.
Stage 2: translation
During translation:
- The ribosome moves along the mRNA, reading one triplet at a time.
- Carrier molecules bring the correct amino acid for each triplet to the ribosome.
- The amino acids are joined together in the order set by the mRNA, forming a polypeptide chain.
- The chain folds into a specific 3D shape, becoming the finished protein.
Because the order of bases set the order of triplets, and the triplets set the order of amino acids, the base sequence of the gene determines the protein that is made.
How proteins fold and why shape matters
A protein only works if it has the right shape. Enzymes are a clear example: an enzyme's active site has a shape complementary to its substrate, and that shape depends on the order and folding of its amino acids. Change the amino acids and you can change the shape of the active site, so the enzyme may no longer work. This connects protein synthesis to the enzyme topic in B1.
Mutations and their effect on proteins
A mutation is a random change to the DNA base sequence. Because the bases are read in triplets, a mutation can change the protein in several ways:
- Changing a base may change a triplet so it codes for a different amino acid, which can change how the protein folds and so its shape and function.
- A change in the gene for an enzyme could alter the active site so the substrate no longer fits, stopping the enzyme working.
- Many mutations have no effect: more than one triplet can code for the same amino acid, so a changed triplet may still code for the same one (a "silent" mutation), and mutations in non-coding DNA may not affect any protein.
- Rarely, a mutation changes a protein in a way that is beneficial, giving an advantage that can spread by natural selection.
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 20196 marksDescribe how a protein is made from the information in a gene, naming the two main stages and what happens in each. This is a Higher tier question.Show worked answer →
A Higher tier 6-mark extended response, marked on an ordered, linked account.
Transcription: a gene (a section of DNA) is copied into a molecule of messenger RNA (mRNA) in the nucleus. The DNA double helix unwinds and the base sequence of the gene is used as a template to build a complementary mRNA strand. The mRNA then leaves the nucleus and travels to a ribosome.
Translation: at the ribosome, the mRNA is read in groups of three bases (triplets, or codons). Each triplet codes for one amino acid. Carrier molecules bring the correct amino acids in the order set by the mRNA, and the amino acids are joined together to build a polypeptide chain, which folds into the protein.
Reward the two named stages in order, the idea that DNA is the template for mRNA, that mRNA carries the code to the ribosome, that triplets code for amino acids, and that amino acids are joined in the right order to make the protein.
OCR 20214 marksA mutation changes one base in a gene. Explain how this could change the protein the gene codes for, and why some mutations have no effect on the protein. This is a Higher tier question.Show worked answer →
A Higher tier reasoning question on mutation.
How it can change the protein: a mutation changes the base sequence of the gene. Because the bases are read in triplets, changing one base can change a triplet so that it codes for a different amino acid. A different amino acid in the chain can change the way the protein folds, which can change its shape and so its function (for example, the shape of an enzyme's active site).
Why some have no effect: more than one triplet can code for the same amino acid, so a changed triplet may still code for the same amino acid and the protein is unchanged. A mutation in a non-coding part of the DNA may also have no effect on the protein. Reward the link base change, triplet change, different amino acid, altered folding/shape/function, and a correct reason a mutation can be silent.
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