Genetic Analysis Exam Questions with Correct Answers
Capillary Electrophoresis (CE) - Answers Capillary electrophoresis (CE) is the primary method used for
separating and detecting short tandem repeat (STR) alleles in forensic DNA laboratories.
Primary Components for CE Instrument - Answers The basic primary components needed for CE
instruments are narrow glass capillaries, buffer vials, and two electrodes connected to a high voltage
power supply.
Other Components for CE Instrument - Answers A CE instrument also need a laser excitation source, a
fluorescence detector, an auto sampler to hold tubes, and a computer to control the sample injection
and detection.
How does separation within a capillary occur? - Answers Separation of DNA molecules occurs by running
the sample through a viscous polymer that serves as a sieving medium. The tangled polymer chains
serve as obstacles for the negatively charged DNA molecules, causing the larger fragments to move
slower through the capillary than the smaller, more agile fragments.
How do the DNA molecules move through the capillaries? - Answers The DNA navigates through the
tangled polymer molecules, separating based on the size of the molecule. The larger molecules moving
slower than the smaller more agile ones.
How is the DNA detected by the CE instrument? - Answers Detection of the sample is performed by
measuring the time from sample injection to sample detection with a laser placed near the end of the
capillary. The DNA fragments are excited by the laser as they pass the detection window, causing
fluorescent illumination. The smaller fragments will arrive first, followed by the larger fragments in order
of their migration speed, which correlates to the molecule's length, or number of base pairs. The relative
fluorescence intensity observed from the dyes as they pass through the detection window also allows
the instrument to detect and quantify the DNA molecules present.
Electrokinetic Injection - Answers Electrokinetic injection is the most common method to bring DNA into
a capillary. A voltage is applied to a liquid sample with one end of the capillary immersed in it. The
capillary also is enclosed with a platinum electrode (cathode) to apply the voltage. The flow of the
current generated by the voltage and resistance experienced pulls the negatively charged DNA into the
capillary. These injections are highly sensitive to the sample matrix, specifically the ionic strength of
small negatively charged ions (like chloride), which provide competition for the larger, slower DNA
molecules when entering the capillary. Electrokinetic injections produce narrow injection zones that
allow high-resolution DNA separations to occur in a relatively short separation distance.
Describe DNA separation in the capillary. - Answers The DNA is separated by a polymer in the capillary
once a voltage is applied and moves via an electrophoretic flow moving from negative cathode to
positive anode. The tangled molecules of the polymer act like pores, allowing the smaller DNA
fragments to migrate through the capillary faster than the larger fragments. This allows the DNA
fragments to separate by size over a relatively short distance. Fragments are detected when they cross
Capillary Electrophoresis (CE) - Answers Capillary electrophoresis (CE) is the primary method used for
separating and detecting short tandem repeat (STR) alleles in forensic DNA laboratories.
Primary Components for CE Instrument - Answers The basic primary components needed for CE
instruments are narrow glass capillaries, buffer vials, and two electrodes connected to a high voltage
power supply.
Other Components for CE Instrument - Answers A CE instrument also need a laser excitation source, a
fluorescence detector, an auto sampler to hold tubes, and a computer to control the sample injection
and detection.
How does separation within a capillary occur? - Answers Separation of DNA molecules occurs by running
the sample through a viscous polymer that serves as a sieving medium. The tangled polymer chains
serve as obstacles for the negatively charged DNA molecules, causing the larger fragments to move
slower through the capillary than the smaller, more agile fragments.
How do the DNA molecules move through the capillaries? - Answers The DNA navigates through the
tangled polymer molecules, separating based on the size of the molecule. The larger molecules moving
slower than the smaller more agile ones.
How is the DNA detected by the CE instrument? - Answers Detection of the sample is performed by
measuring the time from sample injection to sample detection with a laser placed near the end of the
capillary. The DNA fragments are excited by the laser as they pass the detection window, causing
fluorescent illumination. The smaller fragments will arrive first, followed by the larger fragments in order
of their migration speed, which correlates to the molecule's length, or number of base pairs. The relative
fluorescence intensity observed from the dyes as they pass through the detection window also allows
the instrument to detect and quantify the DNA molecules present.
Electrokinetic Injection - Answers Electrokinetic injection is the most common method to bring DNA into
a capillary. A voltage is applied to a liquid sample with one end of the capillary immersed in it. The
capillary also is enclosed with a platinum electrode (cathode) to apply the voltage. The flow of the
current generated by the voltage and resistance experienced pulls the negatively charged DNA into the
capillary. These injections are highly sensitive to the sample matrix, specifically the ionic strength of
small negatively charged ions (like chloride), which provide competition for the larger, slower DNA
molecules when entering the capillary. Electrokinetic injections produce narrow injection zones that
allow high-resolution DNA separations to occur in a relatively short separation distance.
Describe DNA separation in the capillary. - Answers The DNA is separated by a polymer in the capillary
once a voltage is applied and moves via an electrophoretic flow moving from negative cathode to
positive anode. The tangled molecules of the polymer act like pores, allowing the smaller DNA
fragments to migrate through the capillary faster than the larger fragments. This allows the DNA
fragments to separate by size over a relatively short distance. Fragments are detected when they cross
