Relate genes to polypeptides
A gene is a sequence of nucleotides forming part of DNA; its information can specify the amino-acid sequence of a polypeptide.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain DNA, RNA and protein synthesis. Written practice is self-checked; it does not replace teacher review or practical assessment.
Before you begin
- Recognise that DNA stores genetic information and that proteins are made from amino acids.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a gene in the syllabus definition?
What you will learn
- State that a gene is a sequence of nucleotides that forms part of a DNA molecule
- Relate a gene’s information to the polypeptide it codes for
A gene is part of a DNA molecule
A gene is a sequence of nucleotides that forms part of a DNA molecule. The order of bases in a gene stores information used to make a polypeptide.
A chromosome contains many genes and other DNA sequences. Do not describe a gene as a whole chromosome or as a complete protein.
Base sequence relates to amino-acid sequence
During gene expression, information is transcribed into RNA and read in triplets during translation. The order of codons determines the sequence of amino acids in the polypeptide.
A mutation can alter the base sequence and may change the polypeptide, but a DNA change does not always change the amino-acid sequence. The genetic code’s redundancy helps explain why.
Pause and explain
What does the sequence of codons ultimately specify?
Worked example
Complete the information chain: gene → mRNA → ______. State what a gene is.
Show the worked solution
- A gene is a nucleotide sequence within DNA.
- mRNA codons are translated into amino acids.
Answer A gene is a sequence of nucleotides forming part of a DNA molecule. Its information is transcribed into mRNA and used to determine a polypeptide’s amino-acid sequence.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define a gene and state how its information relates to a polypeptide.
Give me a hint
Include where the gene is found and what its sequence can specify.
Compare my reasoning
- A gene is a sequence of nucleotides within DNA.
- Its information determines a polypeptide’s amino-acid sequence.
A gene is a sequence of nucleotides forming part of a DNA molecule. Its base sequence provides information that determines the amino-acid sequence of a polypeptide.
Look for these in your work
- Definition correct
- Polypeptide relationship stated
A student describes a gene as “a protein stored in the nucleus”. Correct the definition.
Give me a hint
Distinguish the information from its product.
Compare my reasoning
- A gene is DNA, not a protein.
- It is a nucleotide sequence that can code for a polypeptide.
A gene is a sequence of nucleotides that forms part of a DNA molecule. It contains information used to make a polypeptide; it is not itself a protein.
Look for these in your work
- DNA sequence identified
- Gene and product distinguished
A mutation changes one DNA base, but the encoded amino acid remains the same. Explain why this does not contradict the idea that a gene codes for a polypeptide.
Give me a hint
Consider how multiple codons can specify the same amino acid.
Compare my reasoning
- Codons are triplets.
- The genetic code is degenerate, so different codons can specify the same amino acid.
The gene’s sequence still provides the information for the polypeptide. Because the genetic code is degenerate, a base change can create a different codon that specifies the same amino acid, leaving the polypeptide sequence unchanged.
Look for these in your work
- Information relationship retained
- Degeneracy explains unchanged amino acid
Common mistakes
- Calling a gene a protein
- Saying a gene is always the entire DNA molecule
- Assuming every base change necessarily alters the polypeptide
What can you explain now?
What is a gene in the syllabus definition?
Compare with the explanation
A sequence of nucleotides that forms part of DNA
A gene is a DNA sequence.
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Make it stick
What is a gene, and how does its nucleotide sequence specify a polypeptide?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.