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Exam questions · Biology · DNA and the Genome

Mutations and Non-coding DNA (Higher tier)

  • 7 exam questions
  • 20 marks
  • 8 quick checks
  1. 1 State [1 mark]

    What is a mutation?

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    Model answer

    A change in the base sequence of DNA.

    Mark scheme

    • A change in the base sequence (structure) of DNA — 1 mark
  2. 2 Explain [2 marks]

    Most mutations do not change the way an organism looks or works. Give two reasons why.

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    Model answer

    Most mutations do not alter the protein, or only alter it slightly, so its function is not changed. Some happen in non-coding DNA, or the changed triplet codes for the same amino acid.

    Mark scheme

    • Protein not altered, or altered only slightly — 1 mark
    • A further correct reason, such as the mutation being in non-coding DNA — 1 mark
  3. 3 Explain [4 marks]

    The diagram shows a normal gene and the same gene after a mutation. Explain how the mutation has stopped the enzyme working.

    A normal gene making an enzyme that fits its substrate, and a mutated gene making an enzyme that does not.
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    Model answer

    One base in the gene has changed, so the order of the amino acids is different. The different amino acid makes the protein fold into a different shape. The active site no longer fits the substrate, so the enzyme cannot catalyse the reaction.

    Mark scheme

    • One base has changed — 1 mark
    • The amino acid sequence is different — 1 mark
    • The enzyme has a different shape — 1 mark
    • The substrate no longer fits the active site — 1 mark
  4. 4 Calculate [3 marks]

    A gene has the base sequence CAT GGA TTC. The first base of the gene is deleted. Write the new sequence in groups of three and state how many of the original three triplets are now different.

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    Model answer

    ATG GAT TC. All three of the original triplets are different (the last group is also incomplete).

    Mark scheme

    • Deletion gives ATGGATTC — 1 mark
    • Regrouped as ATG GAT TC — 1 mark
    • All the triplets after the deletion are changed — 1 mark
  5. 5 Explain [3 marks]

    A mutation in the gene for a structural protein can make the protein weaker. Explain how.

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    Model answer

    The mutation changes the sequence of bases, so the order of the amino acids is different. The protein may fold into a different shape. The altered shape can mean the protein loses its strength.

    Mark scheme

    • The base sequence changes so the amino acids change — 1 mark
    • The protein folds into a different shape — 1 mark
    • The altered shape makes the protein lose its strength — 1 mark
  6. 6 Describe [3 marks]

    Not all parts of DNA code for proteins. Describe how variants in non-coding DNA can affect an organism.

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    Model answer

    Non-coding parts of DNA can switch genes on and off. A variation in a non-coding part can therefore alter how genes are expressed, so the amount of a protein that is made can change even though the protein itself is the same.

    Mark scheme

    • Non-coding DNA does not code for proteins — 1 mark
    • It can switch genes on and off — 1 mark
    • A variant alters how genes are expressed — 1 mark
  7. 7 Evaluate [4 marks]

    Mutations occur continuously, yet most people are healthy. Explain why mutations do not always cause a problem, and why a few do.

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    Model answer

    Most mutations do not alter the protein, or only alter it slightly, so the function does not change. Many happen in non-coding DNA. A few mutations code for an altered protein with a different shape. An enzyme may then no longer fit its substrate, or a structural protein may lose strength, which can cause a problem.

    Mark scheme

    • Most mutations do not alter the protein or alter it slightly — 1 mark
    • Some are in non-coding DNA — 1 mark
    • A few give a protein with a different shape — 1 mark
    • Enzyme and substrate no longer fit, or structural protein weaker — 1 mark

Quick check

  1. 1

    How often do mutations occur?

    1. AOnly before birth
    2. BOnly in old age
    3. CContinuously
    4. DOnly after an infection
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    C: Continuously

    Mutations occur continuously.

  2. 2

    What do most mutations do to the protein that a gene codes for?

    1. ANothing, or only a slight change
    2. BDestroy it completely
    3. CTurn it into a different enzyme
    4. DMake it larger
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    A: Nothing, or only a slight change

    Most mutations do not alter the protein, or alter it only slightly, so its function is unchanged.

  3. 3

    What can happen to an enzyme made from a mutated gene?

    1. AIt becomes a hormone
    2. BIt always works faster
    3. CIt leaves the cell
    4. DThe substrate may no longer fit its active site
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    D: The substrate may no longer fit its active site

    The enzyme may have a different shape, so the substrate no longer fits the active site.

  4. 4

    Which mutation is most likely to change every amino acid after the mutation?

    1. ASwapping one base for another
    2. BDeleting one base
    3. CChanging the colour of the cell
    4. DDoubling the chromosomes
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    B: Deleting one base

    Inserting or deleting a base shifts the grouping of every triplet after it.

  5. 5

    What can non-coding DNA do?

    1. ASwitch genes on and off
    2. BJoin amino acids together
    3. CCarry amino acids
    4. DDigest proteins
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    A: Switch genes on and off

    Non-coding parts of DNA can switch genes on and off.

  6. 6

    A structural protein is changed by a mutation. What is a likely effect?

    1. AIt becomes a gene
    2. BIt turns into DNA
    3. CIt loses strength
    4. DIt becomes an enzyme
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    C: It loses strength

    A structural protein with an altered shape may lose its strength.

  7. 7

    A gene reads ATG CCA GTT. A substitution changes the second triplet to CCG. How many triplets are changed?

    1. ANone
    2. BOne
    3. CTwo
    4. DAll three
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    B: One

    A substitution changes one base, so only the triplet containing that base is changed.

  8. 8

    How can a variant in non-coding DNA affect an organism?

    1. AIt changes the order of amino acids in every protein
    2. BIt deletes a chromosome
    3. CIt makes the organism immune to mutations
    4. DIt can alter how genes are expressed
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    D: It can alter how genes are expressed

    It can alter how genes are expressed.