ACT Science Practice Test 16

Agarose gel electrophoresis is a technique in which an electric field is used to separate fragments of DNA by size. Figure 3.2 illustrates a common setup of an electrophoresis apparatus. A square of agarose gel is prepared and placed in a tray of buffer solution. DNA in solution is loaded into small slits (wells) in the top of the gel. A solution of DNA fragments of known length, called a DNA ladder, is loaded in the first well. DNA samples to be studied are loaded in the remaining wells, and an electric current is applied to the apparatus. Since DNA is negatively charged, the DNA molecules in the wells travel toward the opposite, positive end of the gel. Smaller DNA fragments are able to move through the gel more easily and thus move faster than longer fragments. This causes the fragments to separate according to size as the procedure runs. Comparison to the DNA ladder provides an estimate of the separated fragments' sizes.

Figure 3.2

In addition to fragment size, several factors can affect the rate of migration of DNA fragments through the agarose gel. Table 3.4 provides a summary of the effects of agarose gel concentration and voltage of the electric current.

TABLE 3.4 Factors Affecting Fragment Migration

Table 3.5 identifies the agarose gel concentration needed for optimum resolution of DNA fragments within various size ranges.

TABLE 3.5 Agarose Concentrations

Source: http://www.idtdna.com/pages/decoded/decoded-articles/pipet-tips/decoded/2011/06/17/running-agarose-and-polyacrylamide-gels.

One application of the gel electrophoresis technique is to identify the alleles an individual carries for a particular gene. Although there may be multiple possible alleles (versions) for a specific gene, each individual carries exactly two copies. When subjected to electrophoresis, each allele separates out into a distinct band, allowing that individual's pair of alleles to be identified. A single darker band indicates two copies of the same allele.

Figure 3.3 shows electrophoresis results for a gene with three possible alleles. Allele 2 is known to contain extra bases as compared to Allele 1. Allele 3 is known to be missing bases as compared to Allele 1. DNA samples from 16 different individuals are loaded in Lanes A through P. The sizes of the known fragments in the DNA ladder are listed along the left.

Figure 3.3

A student wanted to test human reaction time to different stimuli to determine the conditions that cause the fastest reaction. The student conducted three experiments to test reaction time.

Experiment 1

The student used a computer program to record the time between the sounding of a tone and the student pressing the spacebar on the keyboard. This process was repeated 10 times per trial. The program then averaged the 10 response times to produce an average for the trial. The student conducted three trials using a tone length of 200 milliseconds (ms) and three trials with a tone length of 400 ms. Results are shown in Table 3.6.

TABLE 3.6 Experiment 1

Experiment 2

The student then used the same computer program to record the time between the sounding of a tone or the appearance of an image on the screen and the student pressing the spacebar. This process was repeated 10 times per trial, with the computer again averaging the 10 response times for each trial. The student conducted three trials using the tone as the stimulus and three trials using the image. Each stimulus lasted for a duration of 400 ms. Results are shown in Table 3.7.

TABLE 3.7 Experiment 2

Experiment 3

The student repeated the previous experiment but alternated the stimulus (tone versus image) with each trial. Results are shown in Table 3.8.

TABLE 3.8 Experiment 3