AQA · GCSE · Biology

    Course version: 8461 · Version from Sept 2018

    A gene is a small section of DNA on a chromosome. Each gene codes for a…

    Full specification wording

    DNA and the genome: A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.

    DNA is a long polymer stored in chromosomes.

    Checked against the AQA 8461 specification (opens in a new tab) on .

    • Point 5 of 9
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    DNA is a long polymer stored in chromosomes. A gene is one short stretch of that DNA, so a chromosome carries many genes. The base sequence within a gene is read in triplets; each triplet specifies one amino acid, and the order of triplets fixes the order of amino acids. That order determines how the chain folds, so each gene makes one particular protein with a specific job, such as an enzyme or a structural protein. A useful model is a recipe book: the chromosome is the book, a gene is one recipe, bases are the letters, and amino acids are the ingredients added in the written order. Because the sequence is specific, a change in it can alter the protein produced.

    What earns the marks

    • State that a gene is a section of DNA located on a chromosome, and that a chromosome carries many genes.
    • Explain that the base sequence of a gene determines the sequence of amino acids in the protein it codes for.
    • Link the amino acid sequence to protein folding and function, giving a named example such as an enzyme.
    • Use the idea that each gene codes for one particular protein, so different genes produce different proteins.
    • Apply the concept to a simple scenario, such as explaining why a mutation in one gene affects only one protein.

    Common mistakes and how to put them right

    • Saying a gene is a whole chromosome.

      Put it right by describing a gene as a small section of DNA on a chromosome.

    • Confusing amino acids with bases.

      Put it right by stating that bases in DNA code for amino acids, which are joined to form a protein.

    • Claiming one gene codes for many unrelated proteins.

      Put it right by stating that each gene codes for a particular sequence of amino acids making a specific protein.

    Exam tips

    • Use the phrase 'sequence of amino acids' rather than 'sequence of proteins' when describing what a gene codes for.
    • If asked to explain, work through the chain: gene base sequence, amino acid sequence, protein folding, protein function.
    • Sketch a quick labelled diagram of chromosome, DNA and gene to organise your answer before writing.

    Try the lesson

    The start of the DNA and the genome: A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which of these best describes a gene?

    Listen

    DNA is a long polymer stored in chromosomes. A gene is one short stretch of that DNA, so a chromosome carries many genes. The base sequence within a gene is read in triplets; each triplet specifies one amino acid, and the order of triplets fixes the order of amino acids. That order determines how the chain folds, so each gene makes one particular protein with a specific job, such as an enzyme or a structural protein. A useful model is a recipe book: the chromosome is the book, a gene is one recipe, bases are the letters, and amino acids are the ingredients added in the written order. Because the sequence is specific, a change in it can alter the protein produced. A strong answer earns marks for these points. State that a gene is a section of DNA located on a chromosome, and that a chromosome carries many genes. Explain that the base sequence of a gene determines the sequence of amino acids in the protein it codes for. Link the amino acid sequence to protein folding and function, giving a named example such as an enzyme. Use the idea that each gene codes for one particular protein, so different genes produce different proteins. A common slip: Saying a gene is a whole chromosome. Instead, describing a gene as a small section of DNA on a chromosome. Exam tip: Use the phrase 'sequence of amino acids' rather than 'sequence of proteins' when describing what a gene codes for.

    See it

    A top-down flowchart with seven boxes joined by arrows. The first box, Chromosome, points down to DNA. DNA points down to a box labelled Gene: a small section of DNA. The gene box points down to Base sequence of the gene, which points down to Particular sequence of amino acids. That points down to Specific protein, which finally points down to Protein function, e.g. an enzyme.
    It shows how a gene, a small section of DNA on a chromosome, codes for a particular sequence of amino acids that makes a specific protein with a specific function.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Protein function, e.g. an enzyme
    2. 2. Specific protein
    3. 3. Gene: a small section of DNA
    4. 4. Particular sequence of amino acids
    5. 5. Chromosome
    6. 6. DNA
    7. 7. Base sequence of the gene

    Key words

    Say what each word means before you turn the card.

    Gene

    Try answering aloud or in your head before turning the card.

    Base sequence

    Try answering aloud or in your head before turning the card.

    Amino acid sequence

    Try answering aloud or in your head before turning the card.

    Protein function

    Try answering aloud or in your head before turning the card.

    See every definition
    Gene
    A small section of DNA on a chromosome that codes for a particular sequence of amino acids to make a specific protein.
    Base sequence
    The order of bases within a gene, read in triplets, which fixes the order of amino acids.
    Amino acid sequence
    The order of amino acids coded for by a gene, which determines how the chain folds.
    Protein function
    The particular job of a protein, such as an enzyme or a structural protein, which depends on how the chain folds.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart with seven boxes joined by arrows. The first box, Chromosome, points down to DNA. DNA points down to a box labelled Gene: a small section of DNA. The gene box points down to Base sequence of the gene, which points down to Particular sequence of amino acids. That points down to Specific protein, which finally points down to Protein function, e.g. an enzyme.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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