A 34-year-old woman comes to the office after her sister was diagnosed with breast cancer. She has no nipple discharge, breast lumps, or discomfort. Breast examination is normal. Her sister was found to have a multiple base pair insertion affecting exon 11 of the BRCA1 gene that leads to a frameshift mutation. A screening test to evaluate for a similar insertion mutation in the patient's BRCA1 gene is performed. The test uses polymerase chain reaction (PCR) to amplify the target exon and gel electrophoresis to assess the size of the exon compared to the wild-type allele. Which of the following must be known in order to perform the amplification part of this analysis?

Question

A 34-year-old woman comes to the office after her sister was diagnosed with breast cancer.  She has no nipple discharge, breast lumps, or discomfort.  Breast examination is normal.  Her sister was found to have a multiple base pair insertion affecting exon 11 of the BRCA1 gene that leads to a frameshift mutation.  A screening test to evaluate for a similar insertion mutation in the patient's BRCA1 gene is performed.  The test uses polymerase chain reaction (PCR) to amplify the target exon and gel electrophoresis to assess the size of the exon compared to the wild-type allele.  Which of the following must be known in order to perform the amplification part of this analysis?

Quizplus · Accepted Answer

11ec160b_36e2_631a_9249_353c6451fac3_MD0001_00 Polymerase chain reaction (PCR) is used to amplify small fragments of DNA (eg, genes, exons, noncoding regions) by repeated replication. Several elements are required for PCR. The first is a source DNA template that includes the target region to be amplified (eg, BRCA1 exon 11) as well as flanking sequences adjacent to the target region. The oligonucleotide sequence of these flanking regions must be known in order to make the primers necessary to start PCR. However, the exact nucleotide sequence of the target region does not need to be known (Choice D). A thermostable DNA polymerase (not denatured at high temperatures) is then used to replicate the DNA template from a pool of supplied deoxynucleotide triphosphates using the following 3 steps: These steps are repeated to obtain millions of copies of the target DNA segment in a short period of time (exponential amplification). (Choice A) Restriction sites are cleaved by specific endonucleases and can be used to identify polymorphisms within variant alleles on gel electrophoresis (restriction fragment length polymorphism analysis). However, knowing the restriction enzyme susceptibility sites within the target region is not necessary for PCR amplification. (Choice B) In PCR, only DNA segments are amplified and studied; proteins are not involved. Proteins can be analyzed using Western blots, but even then, exact amino acid sequences do not need to be known. (Choice C) Complementary DNA (cDNA) is produced by the reverse transcriptase enzyme using mRNA as a template in reverse transcriptase PCR; it is not used in regular PCR. cDNA can be used to express eukaryotic proteins in prokaryotes by transferring the cDNA into the genome of a recipient organism. Educational objective: Polymerase chain reaction is used to amplify small fragments of DNA (eg, genes, exons, noncoding regions) by repeated replication. It requires primers that are complementary to the regions of DNA flanking the segment of interest. Thermostable DNA polymerase, deoxynucleotide triphosphates, and a source DNA template strand are also necessary.

A 34-Year-Old Woman Comes to the Office After Her Sister