9.11.2 Genetic Mutation: Different Forms of Point Mutations

Question 1

A base-pair substitution can lead to one of several types of mutations:

Answer 1

· A silent mutation occurs when the base-pair substitution has no phenotypic effect because of redundancy in the genetic code.
· A missense mutation occurs when the base-pair substitution causes one amino acid to be substituted for another.
· A nonsense mutation occurs when the base-pair substitution forms a stop codon, resulting in a prematurely shortened protein that is typically nonfunctional.

Question 2

Phenylketonuria

Answer 2

• a disease that can be caused by a nonsense mutation. Phenylketonurics lack a functional form of the
  enzyme phenylalanine hydroxylase, which results in the excess buildup of phenylpyruvic acid (a byproduct of phenylalanine metabolism).

Question 3

note

Answer 3

• A silent mutationoccurs when a base-pair substitution results in a codon that codes for the same amino acid as the original codon. Silent mutations are possible because of the redundancy of the genetic code. Several codons that differ by only one base pair can code for the same amino acid.
• In the example to the left, the base thymine (T) is replaced with guanine (G). This replacement changes the mRNA codon from CCU to CCC. Both codons code for the amino acid serine.
• A missense mutation occurs when a base pair substitution alters a codon so that it codes for a different amino acid.
• In the example to the left, thymine substitutes for the base adenine (A). This substitution changes the mRNA codon from CAU to CAA. The original codon coded for histidine, but the new codon codes for glycine.
• Nonsense mutations occur when a base-pair substitution forms a stop codon. The resulting polypeptide is shortened because translation will not proceed through the stop codon.
• In the example to the left, the guanine in the DNA is changed to thymine. The resulting mRNA codon is UGA, a stop codon.

Question 4

A point mutation leads to the substitution of the codon “CCC” instead of “CCU”. The mutation has no effect on the polypeptide chain. What type of mutation is involved?

Answer 4

• a silent mutation

Question 5

Which of the following terms does not describe a type of mutation caused by a base-pair substitution?

Answer 5

• Translocation

Question 6

Which of the following statements is the best definition of a missense mutation?

Answer 6

• A base-pair substitution that causes one amino acid to be substituted for another

Question 7

Which type of mutation would cause the synthesis of a full-length protein with a different structure from the normal protein?

Answer 7

• Missense mutation

Question 8

What type of mutation causes sickle cell anemia?

Answer 8

• a missense mutation

Question 9

Which type of mutation causes phenylketonuria?

Answer 9

• a nonsense mutation

Question 10

Which of the following is an example of a silent mutation?

Answer 10

• A base-pair substitution that does not cause a change in the amino acid sequence