GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with Mutation Types

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Example Questions

Example Question #1 : Help With Mutation Types

Which of the following is NOT a type of mutation that can occur in DNA?

Possible Answers:

Countersense

Nonsense

Silent

Missense

Correct answer:

Countersense

Explanation:

There are four principle types of mutation that can affect DNA. Most of these mutations result from point mutations affecting a single nucleotide residue, though nonsense mutations can be caused by insertions or deletions. Frame shift mutations are solely caused by insertions or deletions.

A silent mutation results from the degeneracy of the genetic code. In a silent mutation a single nucleotide is changed, but the overall translation product is unaffected. This can occur because multiple codons are capable of coding for the same amino acid.

A missense mutation results in the swapping of a single amino acid for another in the final translation product. If the amino acids are similar in character, missense mutations can still result in fully functional proteins. When the changed amino acid lacks characteristics of the original, it can result in protein misfolding and loss of function.

Nonsense mutations result in a premature stop codon, and early termination of the translation process. The final product is a shortened version of the protein, often lacking function.

Frame shift mutations cause a shift in the ribosomal reading frame. Every codon downstream of the mutation will be affected, and the protein will be completely altered. Frame shift mutations often result in premature stop codons.

Countersense is not a form of DNA mutation.

Example Question #2 : Help With Mutation Types

Which of the following could cause a frameshift mutation?

Possible Answers:

Deletions

Insertions or deletions

Mismatches

Insertions

Correct answer:

Insertions or deletions

Explanation:

Both insertions and deletions are capable of creating frameshift mutations. A frameshift mutation results in a shift in the reading frame of the gene, significantly altering translation. The cause of such a mutation is the insertion or deletion of any sequence of nucleotides within the gene that is not a multiple of three. The following examples detail different types of frameshift mutations.

Normal gene: ATT-CGT-AGG-TAC

Frameshift deletion examples: ATC-GTA-GGT-AC or ATT-CTA-C

Frameshift insertion examples: ATT-TCG-TAG-GTA-C or ACC-CGA-TAG-GTA-C

Mismatches would not cause a shift in the reading frame, like insertions or deletions.

Example Question #3 : Help With Mutation Types

A researcher has finally managed to design a plasmid that will allow him to begin expressing an important recombinant protein. After analyzing the sequencing data of the plasmid, however, he discovers that a mutation has occurred in the coding region of his protein. If he still wishes to express the wild type protein, does he need to construct a new plasmid?

Possible Answers:

It depends; a silent mutation would not affect the structure of the protein

It depends; a frameshift mutation would not affect the structure of the protein

No; one mutation should not affect the overall protein's function

Yes; any mutation is undesirable and will affect the protein's function

Correct answer:

It depends; a silent mutation would not affect the structure of the protein

Explanation:

This question is somewhat vague because we are given no other information other than the fact that there is a "mutation". Mutations can take many different forms and, therefore, we would need to know more information about the specific type of mutation before we can say whether or not the plasmid still contains a functional gene for the wild type protein. In particular, a silent mutation would result in the insertion of the exact same amino acid despite having a different codon (this is due to the redundancy of the genetic code). If a silent mutation has altered the sequence of the plasmid, it will not alter the structure or function of the protein and the plasmid will still be effective.

A frameshift mutation, however, would have disastrous effects on the protein as the translational reading frame would be shifted and the protein would most likely be truncated and nonfunctional.

Example Question #4 : Help With Mutation Types

Which of the following genotypes represents an individual with Turner syndrome? 

Possible Answers:

XXY

YO

XXX

XO

XYY

Correct answer:

XO

Explanation:

Individuals with Turners Syndrome have the XO genotype. They are phenotypically female but experience abnormalities. XXY is the genotype of an individual with Klinefelter syndrome. Individuals with the genotype XYY have what is known as XYY syndrome and are phenotypically normal, though they may be taller, and secrete excess testosterone. Triple-X syndrome is cause by an XXX genotype, and individuals are phenotypically normal. Lack of an X chromosome is lethal in humans. 

Example Question #4 : Help With Mutation Types

Which of the following terms best describes non-random association of alleles at different loci? 

Possible Answers:

Crossing over

Independent assortment 

Linkage equilibrium

None of the other answers 

Linkage disequilibrium

Correct answer:

Linkage disequilibrium

Explanation:

The correct answer is linkage disequilibrium. This phenomenon occurs when two alleles do not segregate independently even though they are at different loci. Rather, they are statistically associated with one another above non-random conditions. This is important for understanding evolution of organisms from a common ancestor as well as for identification of disease-associated mutations or single nucleotide polymorphisms.  

Example Question #5 : Help With Mutation Types

Which of the following does not contribute to linkage disequilibrium?

Possible Answers:

Genetic drift

Natural selection

Rate of mutation 

Rate of recombination

Asexual reproduction

Correct answer:

Asexual reproduction

Explanation:

The correct answer is asexual reproduction. Linkage disequilibrium is the non-random association of alleles at different loci. Under typical genetic assumptions, alleles segregate independently. However, some alleles have statistical significant association. There are many causes for this "linkage", including natural selection of linked genes that may confer increased fitness, recombination events bringing distant alleles in close proximity, genetic drift favoring a subset of alleles over another due to random sampling, and rate of mutation (either insertions or deletions) that result in statistical association between alleles. 

Example Question #6 : Help With Mutation Types

When mutations occur after the embryo has been fertilized and result in two or more genetically distinct populations of cells is best described as which of the following? 

Possible Answers:

Aneuploidy 

None of the other answers 

Chimera 

Mosaicism 

Non-disjunction 

Correct answer:

Mosaicism 

Explanation:

The correct answer is mosaicism. Mosaicism within an organism occurs when mutations, such as non-disjunction, occur in a subset of cells to give rise to genetically distinct cell populations. Chimeras also have genetically distinct cell populations, however, they occur from the fusion of two fertilized embryos. Aneuploidy occurs when an organism has an incorrect, but consistent number of chromosomes. 

Example Question #7 : Help With Mutation Types

If two alleles, that is both copies of a given gene, are required for proper function of that gene and its product, a mutation in just one of the copies will result in a mutant phenotype. This is called (a) __________.

Possible Answers:

haplo-insufficiency 

hypomorphism

homozygosity 

heterozygosity

missense mutation 

Correct answer:

haplo-insufficiency 

Explanation:

Haplo-insufficent means that both copies of the gene must be fully functional to impart a wild-type phenotype. Mutating/inactivating one copy is enough to cause a mutant phenotype, which can result in a disordered state. Heterozygosity/homozygosity describe whether the alleles are the same or different, but do not necessarily describe the above case. A missense mutation is a type of mutation that changes the protein structure. A hypomorph is a type of mutation that results in reduced gene function, but does not necessarily describe the above case.  

Example Question #9 : Help With Mutation Types

Which of the following answers best describes a mutation where one allele is mutated in such a way that the resultant protein is structurally or functionally different, such that it interferes with the normal activity of the wild-type protein and disrupts that activity to form a partial or total mutant phenotype?

Possible Answers:

Missense mutation 

Dominant-negative mutation 

Frameshift mutation 

Point mutation

Nonsense mutation 

Correct answer:

Dominant-negative mutation 

Explanation:

While each of the mutations described above can lead to a dominant negative situation, the phrase dominant negative itself refers to the case where the mutant protein interacts with and interrupts the normal protein. 

Example Question #8 : Help With Mutation Types

Which of the following descriptions reflects a nonsense mutation?

Possible Answers:

A uracil is replaced by an adenine in the primary transcript, resulting in a severely shortened final protein product.

A mutation in a gene causes a valine to be replaced by a glutamic acid residue in the final protein product.

A transition mutation changes one nucleotide in a gene, but the final protein still has the same amino acid structure as it did prior to the mutation.

An adenine is improperly added into a gene, resulting in a completely different amino acid sequence downstream to the mutation.

Correct answer:

A uracil is replaced by an adenine in the primary transcript, resulting in a severely shortened final protein product.

Explanation:

Answering this question requires an understanding of what a nonsense mutation will entail for a protein. Nonsense mutations occur when a mutation in a gene causes an amino acid codon to be converted into a stop codon. This premature stop codon will cause the translation of the primary transcript to be prematurely terminated, resulting in a smaller than normal protein product. The uracil to adenine mutation that made a smaller protein reflects this type of mutation.

All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

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