All GRE Subject Test: Biology Resources
Example Question #1 : Mutations And Variability
Which type of mutation creates a premature stop codon in the mRNA?
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 #2 : Mutations And Variability
A __________ mutation always results in the ribosome encountering a premature stop codon.
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 #3 : 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?
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.