SCN3230-ABED_1: Module 7 Lesson 4 - 4

Lesson 4 — Changes in the Genetic Code

Self-Check

Answer the following questions; then, compare your answers to the suggested responses. Do you understand the types of mutations?

What feature of the genetic code helps to protect a cell from the effects of nucleotide substitution?
What is a frameshift mutation?
Why is a mutation caused by an insertion or a deletion more likely to have serious consequences for a cell than one caused by a substitution?
One mutation results in the replacement of a G nucleotide with a T nucleotide in the sense strand of a DNA molecule. Under what circumstances will this substitution produce each of the following mutations?
1. A silent mutation
2. A missense mutation
3. A nonsense mutation
Explain the difference between a germ line mutation and a somatic cell mutation. Which type of mutation contributes more to the variations among organisms?

Check your work.

Self-Check Answers

The redundancy of the genetic code protects a cell from the effects of substitution mutations. A change in the coding sequence of a gene does not always result in a change to the polypeptide product of a gene.
The insertion or deletion of one or two nucleotides results in a frameshift mutation. A frameshift mutation causes the entire reading frame of the gene to be altered. A shift in the reading frame usually results in a nonsense mutation.
A frameshift mutation is more likely to have serious consequences for a cell than a substitution mutation has because the frameshift mutation causes the entire reading frame of the gene after the insertion or deletion to be altered. A frameshift mutation occurs when a gene is altered by the insertion or deletion of one or two nucleotides. The resulting shift in the reading frame usually causes a nonsense mutation, a mutation that results in a nonfunctional protein. On the other hand, a substitution mutation of a single nucleotide may lead to a slightly altered but still functional polypeptide (a missense mutation). Missense mutations can be harmful, but generally they are less so than frameshift mutations are.
1. A silent mutation results if the substitution causes coding of the same amino acid.
2. A missense mutation results when the substitution causes coding of a different amino acid, resulting in a slightly altered protein.
3. A nonsense mutation results when the substitution produces a "stop" codon prematurely before the entire polypeptide is synthesized. This results in a polypeptide that is non-functional.
5. Somatic cell mutations occur in body cells and only affect the organism. Germ line mutations occur in gametes and may be passed onto future generations. Germ line mutations contribute more variation among organisms since they affect populations rather than just individuals.

Watch and Listen

Watch the following Biologix video titled Genes, Mutations and Viruses: Alterations in the Genetic Code because it is an excellent review of this lesson. It shows animations of mutations, discusses causes of mutations, and mentions sickle cell anemia specifically.

©Alberta Education. Genes, Mutations and Viruses: Alterations in the Genetic Code (0:00-28:52); Series 35 LearnAlberta.ca

Self-Check

Use the following information to answer the next two questions.

The following matches the normal coding sequence with the codons in the top row and the resulting amino acids in the bottom row.
mRNA codons	-GUU	- CAU	- UUG	- ACU	- CCC	- GAA	- GAA
amino acids	-val	- his	- leu	- thr	- pro	- glu	- glu

Identify the type of mutation that would occur if the first codon was changed as shown: GUU is changed to GUC.
1. Frameshift mutation: this would cause the entire reading frame of a protein to be altered.
2. Nonsense mutation: this would render the protein unable to code for a functional gene.
3. Silent mutation: this would have no effect on the cell's metabolism.
4. Missense mutation: this would result in an altered DNA molecule.

Identify the type of mutation that would occur if the third codon was changed as shown: UUG is changed to UAGG.
1. Frameshift mutation: this would cause the entire reading frame of the gene to be altered.
2. Nonsense mutation: this would render the gene unable to code for a functional polypeptide.
3. Silent mutation: this would have no effect on the cell's metabolism.
4. Missense mutation: this would result in an altered protein.

What type of point mutations occurred when the third codon was changed from UUG to UAGG in the question above?
1. Substitution
2. Insertion
3. Deletion

Identify the type of mutation that would occur if the second codon was changed from CAU to CAA.
1. Frameshift mutation: this would cause the entire reading frame of the gene to be altered.
2. Nonsense mutation: this would render the gene unable to code for a functional polypeptide.
3. Silent mutation: this would have no effect on the cell's metabolism.
4. Missense mutation: this would result in an altered protein.

What type of point mutation occurs when the second codon was changed from CAU to CAA in the question above?
1. Substitution
2. Insertion
3. Deletion

For a substance to be classified as a mutagen, it must cause
1. a change in DNA
2. enzymes to denature
3. protein production
4. mRNA to be produced

Distinguish between induced mutations and spontaneous mutations.

Distinguish between chemical mutagens and physical mutagens; then, give an example of each.

Check your work.

Self-Check Answers

c
a
b
d
a
a
A spontaneous mutation is caused by molecular interactions that occur naturally within cells. An induced mutation is caused by agents outside the cell, such as certain factors in the environment.
Physical mutagens cause changes in the structure of the DNA. High-energy radiation, such as that from x-rays and gamma rays, is the most damaging form of physical mutagen. A chemical mutagen is a molecule that can enter the nucleus of a cell and induce mutations by reacting chemically with the DNA. Examples of chemical mutagens include nitrites, gasoline fumes, and various compounds in cigarette smoke.