The control of gene expression

1. Which mutation is most likely to cause a frameshift?

1. Substitution of one base
2. Deletion of one base
3. Deletion of three bases
4. Inversion

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B: Deletion of one base. Removing one base shifts the reading frame of all later codons.

2. Why might a substitution have no effect on the protein?

1. The code is universal
2. The code is degenerate, so the new codon may code for the same amino acid
3. Substitutions never reach mRNA
4. Proteins cannot change

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B: The code is degenerate, so the new codon may code for the same amino acid. Most amino acids have more than one codon.

3. What is an inversion mutation?

1. A base is added
2. A section of bases is reversed
3. A base is lost
4. A section is copied twice

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B: A section of bases is reversed. The group of bases is detached and reinserted the other way round.

4. Why might deleting three bases do less harm than deleting one?

1. It removes one amino acid without a frameshift
2. It never affects proteins
3. It adds a stop codon
4. It is always silent

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A: It removes one amino acid without a frameshift. The reading frame after the deletion stays the same.

5. Which is an example of a mutagenic agent?

1. Glucose
2. Ionising radiation
3. Insulin
4. Water

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B: Ionising radiation. Ionising radiation can damage DNA and increase the mutation rate.

6. Why might a large deletion mutation have a greater effect than a substitution?

1. It always creates a stop codon
2. It is more likely to cause a frameshift, altering many codons
3. It never affects the protein
4. It only affects introns

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B: It is more likely to cause a frameshift, altering many codons. Unless the deletion is a multiple of three bases, the reading frame shifts.

7. Which of these can act as a mutagen?

1. Water
2. Ionising radiation
3. Table salt
4. Glucose

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B: Ionising radiation. Radiation can directly damage DNA structure.

8. A 300-base coding sequence (100 codons) has base 100 deleted, in codon 34. About how many codons are altered?

1. 100
2. 3
3. 1
4. 67

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D: 67. The deletion causes a frameshift, so codon 34 to the end (about 67 codons) is read in a new frame. A substitution would change only one codon.

9. In sickle cell disease, a substitution changes a glutamic acid codon to valine in β-globin. Why does this alter haemoglobin?

1. A frameshift changes every later amino acid
2. Transcription of the β-globin gene is blocked
3. The haem group is deleted from the polypeptide
4. New R group alters tertiary structure and so function

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D: New R group alters tertiary structure and so function. The substitution changes one amino acid. Its different R group alters bonding, so the tertiary structure and function change. It is not a frameshift.

10. A substitution changes the codon UGG to UGA, a stop codon. What is the likely result?

1. No change because the code is degenerate
2. A shortened, probably non-functional polypeptide
3. A polypeptide one amino acid longer
4. A frameshift in all later codons

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B: A shortened, probably non-functional polypeptide. UGA ends translation early, giving a truncated polypeptide. Substitutions do not cause frameshifts, and a stop codon codes for no amino acid, so degeneracy does not apply.