9.11.3 Genetic Mutation: Insertion and Deletion

Question 1

note

Answer 1

• Frameshift mutations, such as insertions and deletions, can occur when DNA strands within a chromosome break and join back together.
• In an insertion, one or more nucleotide pairs are inserted into the DNA molecules. In a deletion, one or more nucleotide pairs are removed from the DNA molecule.
• If the number of nucleotides inserted or deleted is not a multiple of three, a frame shift mutation will occur. The result is an improper grouping of codons, which leads to a different protein product. If the insertion or deletion is a multiple of three, the reading frame may be recovered.
• Remember: DNA codes for mRNA, which codes for proteins. A frameshift mutation affects the sequences of codons. The resulting protein is nonfunctional.
- An insertion is a mutation in which a nucleotide is added into a DNA sequence, resulting in a shift of the reading frame.
- An example of the consequences of an insertion can be seen in the sentence to the left. Each word in the sentence has three letters just as each codon in a DNA strand consists of three base pairs. If an O is inserted between the H and the E, the reading frame will shift. Now, if every three letters are read as a word, the sentence has no meaning.
- Similarly, if a base pair is inserted into a coding DNA
sequence, the resulting strand will have different codons from that point forward, resulting in a nonfunctional protein.
- A deletion is a mutation in which a nucleotide is taken out of a DNA sequence, also resulting in a frameshift.
- Just as in the insertion example above, a deletion shifts the reading frame, causing the sentence to be meaningless. If a base pair is removed from of a coding DNA sequence, then the resulting strand will code for a nonfunctional protein because of the shifting of the reading frame.
- In the example on the left, the nucleotide thymine is inserted into the DNA strand, resulting in an additional
thymine/adenine nucleotide pair in the strand. Before the insertion, the bottom strand had a codon sequence of “UUU, ACG, CAA….” After the insertion, the codon sequence changes to “UUU, AAC, GCA…”
- The dictionary of the genetic code shows that the second codon, “CGC”, codes for the amino acid arginine. After the insertion, the second codon now codes for threonine. Furthermore, all subsequent codons have changed and now code for the wrong amino acid. The insertion mutation is deleterious, resulting in a nonfunctioning protein.

Question 2

Is a mutation in which three base pairs are inserted always more severe than a mutation in which one base pair is inserted? Why or why not?

Answer 2

• No, three base pairs are an entire codon and the reading frame may be reestablished more easily with a change of three base pairs than with a change of one base pair.

Question 3

True or false?

Nonsense and frameshift mutations may both result in nonviable protein products.

Answer 3

• true

Question 4

True or false?

A frameshift in a gene specifying for an enzyme usually results in a loss of enzyme activity.

Answer 4

• true

Question 5

True or false?

A frameshift mutation is also known as a missense mutation.

Answer 5

• false

Question 6

Which of the following is a cause of frameshift mutations?

Answer 6

• DNA strands within a chromosome breaking and joining back together.

Question 7

A __________ mutation results when one or two nucleotide pairs are inserted into the DNA molecule.

Answer 7

• frameshift