Question

Consider a three-base sequence within the coding region in the DNA template strand: 5'-...123...-3', in which 1, 2, and 3 refer to the relative positions of deoxyribonucleotides within a codon. What would be the effects of a point mutation that would change a purine for a pyrimidine at position 2?

1. (True/False) This mutation will always result in an altered amino acid sequence in the mutant protein compared to the original protein.

2. (True/False) The mutant amino acid, if changed, is more likely to be similar to the original amino acid than not. (Here, similar means that the two amino acids are both hydrophobic, or that both are basic, acidic, or polar. If an amino acid does not change between the mutant and the original, then count the mutant as similar.)

3. (True/False) Considering the above results, it is likely that more than half of the mutations would significantly affect the function of the protein in which it is located if the mutant amino acid is a key determinant of the protein's function.

Answer 1

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Base substitutions involving replacement of one purine for another or one pyrimidine for another (e.g., a mismatched A-A pair, instead of A-T) are known as transitions; the replacement of a purine by a pyrimidine, or vice versa, is called a transversion. Whereas Base substitutions involving replacement of one purine with another is called a transition. A substitution that produces the stop codon (AUG) is the most serious as it will end an amino acid chain prematurely. Other substitutions can have severe effects if the replacement of an amino acid radically changes the shape of the protein or the active site. Changes that have minimal effect on the protein configuration are less likely to dramatically effect the functioning of the protein.

1. (True/False) This mutation will always result in an altered amino acid sequence in the mutant protein compared to the original protein.

Answer: True

2. (True/False) The mutant amino acid, if changed, is more likely to be similar to the original amino acid than not. (Here, similar means that the two amino acids are both hydrophobic, or that both are basic, acidic, or polar. If an amino acid does not change between the mutant and the original, then count the mutant as similar.)

Answer: False.

3. (True/False) Considering the above results, it is likely that more than half of the mutations would significantly affect the function of the protein in which it is located if the mutant amino acid is a key determinant of the protein's function.

Answer: True

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