GRE Subject Test: Biology : Mutations and Variability

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Mutations And Variability

Which type of mutation creates a premature stop codon in the mRNA?

Possible Answers:

Frameshift mutation

Missense mutation

Nonsense mutation

Silent mutation

Correct answer:

Nonsense mutation

Explanation:

While a missense mutation involves substituting a base pair, resulting in a new amino acid, a nonsense mutation takes place when the new substituted codon is a stop codon. This causes the protein to stop being translated prematurely. Because of their impact on protein production, nonsense mutations very commonly prevent the formation of a functional protein.

Silent mutations result in no change in primary protein structure. Due to the degeneracy of the genetic code, a mutation can occur without changing the identity of the amino acid recruited during translation. A frameshift mutation results in a shift in the codon reading frame, severely altering the primary protein structure and often resulting in a truncated protein.

Example Question #1 : Understanding Types Of Mutation

A __________ mutation always results in the ribosome encountering a premature stop codon.

Possible Answers:

nonsense

missense

frameshift

deletion

Correct answer:

nonsense

Explanation:

Nonsense mutations are the name specifically given to mutations that cause the ribosome to encounter a premature stop codon and terminate translation early. A point mutation causes the transcription of a stop codon by changing the DNA transcript transcribe to the mRNA stop codons UAG, UAA, or UGA. Placement of this codon in the transcript will interrupt translation.

Missense mutations are a type of mutation that result in the inclusion of a different amino acid than the wild type protein. Frameshift mutations result in a change to the codon reading frame, and are typically caused by deletion or insertion mutations. Frameshift mutations have the most dramatic and detrimental effect on proteins. Deletion mutations result from removal of one or more base pairs.

Example Question #1 : Mutations And Variability

Sickle cell anemia is a disorder caused by the alteration of one amino acid in hemoglobin. Based on this, what kind of mutation causes sickle cell anemia? 

Possible Answers:

Frameshift mutation

Nonsense mutation

Silent mutation

Missense mutation

Correct answer:

Missense mutation

Explanation:

When only one amino acid is changed in a polypeptide, it is commonly caused by a point mutation, where one base pair has been changed. Silent, missense, and nonsense mutations can all be caused by a point mutation. Since the amino acid sequence has been changed, this is an example of a missense mutation. A silent mutation would not change the amino acid sequence, and a nonsense mutation would result in a premature stop codon during translation.

Example Question #2 : Mutations And Variability

What term best describes when one species exhibits two or more defined phenotypes within the same population?

Possible Answers:

Sympatry 

Allopatry 

Assortative mating

Polymorphism

Natural selection

Correct answer:

Polymorphism

Explanation:

The correct answer is polymorphism. A polymorphism refers to multiple phenoytpes (morphs) that exist within a population, generally as a result of multiple alleles for the same gene.

Sympatry and allopatry refer to mechanisms of speciation and natural selection favors a certain phenotype for its fitness or other survival advantages. Assortative mating describes a biased mating pattern based on either phenotype or behavior. 

Example Question #3 : Mutations And Variability

Which of the following is most accurate about single nucleotide polymorphisms (SNPs)?

Possible Answers:

SNPs occur in 1% or more of the population 

SNPs are more frequently found in AT-rich microsatellite regions

None of these

SNPs occur in only non-coding regions

SNPs occur in only coding regions

Correct answer:

SNPs occur in 1% or more of the population 

Explanation:

In order for a nucleotide substitution to be considered a SNP and not a random mutation, it must occur in 1% or more of the population. SNPs are more frequently found in non-coding regions. Typically, SNPs are much less commonly found in AT-rich microsatellites. 

Example Question #3 : Understanding Polymorphisms

What is the major difference between synonymous and non-synonymous substitutions?

Possible Answers:

Synonymous substitutions result in missense mutations, non-synonymous substitutions result in nonsense mutations

Synonymous substitutions do not result in an amino acid change in the protein, but non-synonymous substitutions do

Non-synonymous substitutions do not result in an amino acid change in the protein, but synonymous substitutions do

None of these

Non-synonymous substitutions result in missense mutations, synonymous substitutions result in nonsense mutations

Correct answer:

Synonymous substitutions do not result in an amino acid change in the protein, but non-synonymous substitutions do

Explanation:

If single nucleotide polymorphisms (SNPs) that occur in coding regions do not trigger an amino acid change in the protein, they are synonymous. A SNP can cause a missense mutation (an amino acid change in the protein) or a nonsense mutation (an amino acid change to a stop codon), both of these are nonsynonymous substitutions.  

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