Mutations and Their Effect on Proteins · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Small DNA changes, big protein effects

Science · Genetics & Evolution · ages 15-16
Name ______________________   Date ____________

  1. Why can deleting one base after the start codon change later amino acids?

    • Later bases regroup into different triplets after the gap
    • The gap removes a whole codon and leaves later triplets intact
    • Translation starts a new chain at the base just after the gap

  2. One DNA base is replaced by another within a protein-coding sequence. Which description identifies the mutation and a possible coding effect?

    • A deletion; the missing base changes later triplet boundaries
    • A substitution; the changed codon can have a different meaning
    • An insertion; the added base changes later triplet boundaries

  3. Proofreading during DNA copying and later mismatch repair can correct copying errors before they persist as mutations.

    Circle one:   True   False

  4. A DNA substitution changes an internal mRNA codon from CAA to UAA, before the usual stop. CAA encodes glutamine; UAA means stop. What happens during translation?

    • The amino acid chain ends early
    • Glutamine is still added at this position
    • An extra glutamine is added at this position

  5. A DNA substitution changes an mRNA codon from CAA to CAG. Both encode glutamine. What happens at this position in the amino acid sequence?

    • The later codon boundaries shift by one base
    • Translation ends at this position in the chain
    • Glutamine stays at this position

  6. Mutation A changes an active-site amino acid and tests show reduced substrate binding. Mutation B changes an amino acid elsewhere, with unchanged folding and activity in the same tests. Which comparison matches the results?

    • A has unchanged binding; B has reduced enzyme activity
    • A has lost all binding; B has unchanged enzyme activity
    • A has reduced binding; B has unchanged enzyme activity

  7. A DNA substitution replaces one base without changing the base count. The enzyme has one amino acid replacement and its usual chain length, but no longer binds its substrate. Which possible mechanism fits these observations?

    • Different folding changes the shape of the active site
    • An early stop removes the enzyme's active-site region
    • A one-base deletion shifts the later reading frame

  8. A DNA substitution changes one amino acid in a protein, yet the organism's observed trait stays the same. Which explanation fits both observations?

    • The changed codon still encodes the original amino acid
    • The amino acid replacement leaves the protein able to do its job
    • Repair restored the original DNA before the altered protein formed
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Small DNA changes, big protein effects W1-mt_dTOQH3vGSz-s1

  1. Later bases regroup into different triplets after the gap · A one-base deletion shifts later codon boundaries. The regrouped triplets can encode different amino acids or an early stop.
  2. A substitution; the changed codon can have a different meaning · Replacing a base is a substitution. The changed mRNA codon may encode another amino acid or a stop, or keep its original amino acid.
  3. True · Proofreading acts during copying. Mismatch repair can correct errors left afterward. Both reduce the number of copying errors that persist.
  4. The amino acid chain ends early · The new UAA codon ends translation before the usual endpoint. The shortened protein may lose function, but that is not guaranteed.
  5. Glutamine stays at this position · CAA and CAG encode the same amino acid. This is a silent substitution because the amino acid sequence stays unchanged.
  6. A has reduced binding; B has unchanged enzyme activity · A shows reduced binding, which does not mean binding has stopped completely. B shows unchanged folding and activity in the stated tests.
  7. Different folding changes the shape of the active site · An amino acid replacement can alter folding or active-site shape without shortening the chain. An early stop would shorten it; a one-base deletion would contradict the stated substitution.
  8. The amino acid replacement leaves the protein able to do its job · An amino acid replacement can leave the protein able to do its job. This can explain why the trait stays the same even though the mutation and amino acid replacement occurred.
Worksheet · LightMySky