ACT science practice test 31

Directions: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.

You are NOT permitted to use a calculator on this test.

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


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

1. In a standard gel electrophoresis procedure, the first well is generally reserved for:

A. a DNA ladder.
B. the buffer solution.
C. smaller DNA fragments.
D. the DNA sample of greatest interest.

2. Applying an electric current to the electrophoresis apparatus causes the DNA fragments to travel:

A. toward the wells.
B. toward the cathode.
C. away from the anode.
D. away from the cathode.

3. According to the passage, the role of a DNA ladder is to:

A. propel the DNA fragments through the agarose gel.
B. provide an approximation of a DNA fragment's size.
C. identify the base sequence in a DNA fragment.
D. determine the total number of bases in a DNA fragment.

4. Which of the following would be a disadvantage of running a gel electrophoresis at a voltage of 6 V/cm?

A. Poor resolution of large DNA fragments
B. A short total run time for the procedure
C. A total run time of more than one day
D. Brittleness of the agarose gel

5. According to Table 3.5, as the concentration of agarose increases, the range of DNA fragment sizes that can be resolved:

A. expands.
B. is constant.
C. shrinks.
D. expands, then shrinks.

6. Which combination of factors would provide the best results for DNA fragments of 0.5-0.7 kb?

A. 0.7% agarose and 0.5 V/cm
B. 1.0% agarose and 0.5 V/cm
C. 1.2% agarose and 5 V/cm
D. 1.5% agarose and 5 V/cm

7. When observing electrophoresis results, the largest DNA fragments will appear:

A. closest to the cathode.
B. closest to the anode.
C. as the largest bands.
D. as the smallest bands.

8. Based on the information in the passage, Allele 2 traveled through the agarose gel faster than:

A. Allele 1 but slower than Allele 3.
B. neither Allele 1 nor Allele 3.
C. Allele 3 but at the same rate as Allele 1.
D. both Alleles 1 and 3.

9. According to the passage, a single darker band, as seen in Lane E, most likely indicates an:

A. error during the electrophoresis process.
B. error when collecting the DNA sample.
C. individual missing an allele due to mutation.
D. individual with two copies of the same allele.

10. What is the approximate size of Allele 1?

A. 3.0 kb
B. 1.0 kb
C. 8.0 kb
D. 0.5 kb

11. What is the most common allele combination represented in the DNA samples shown in Figure 3.3?

A. Two copies of Allele 2
B. Allele 1 and Allele 2
C. Two copies of Allele 1
D. Allele 1 and Allele 3

12. Which of the following provides the best explanation for the result shown in Lane J?

A. Individual J carries Alleles 1 and 3.
B. Individual J carries Alleles 2 and 3.
C. Lane J contains the DNA ladder.
D. Sample J contains DNA from two individuals.

13. Which allele combination is not represented in the DNA samples shown in Figure 3.3?

A. Allele 1 and Allele 3
B. Allele 1 and Allele 2
C. Two copies of Allele 3
D. Allele 2 and Allele 3

14. Which agarose concentration was most likely used in the electrophoresis in Figure 3.3?

A. 1.50%
B. 2.00%
C. 1.00%
D. 1.20%

15. Which combination of factors would cause the slowest migration of DNA fragments?

A. 0.5% agarose and 7 V/cm
B. 2.0% agarose and 7 V/cm
C. 2.0% agarose and 0.25 V/cm
D. 0.5% agarose and 0.25 V/cm