ACT science practice test 39

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.


A gene is composed of a series of exon and intron segments. Exons are the coding regions of a gene, the segments that contain the instructions for building a protein. A gene's exons are connected by noncoding regions, or introns.

To build a protein, the cell must first transcribe the gene into messenger RNA (mRNA). Then a process called RNA splicing removes the noncoding introns and connects all of the exons to produce an mRNA transcript that can be used to build the protein.

Tropomyosins are a family of proteins that help maintain the cytoskeleton structure in all cells and support the contraction of muscle cells. In the late 1980s, a group of scientists discovered that the alpha-tropomyosin (α-TM) gene can code for several different tropomyosin proteins within different tissues of the same organism.

Figure 6.1 shows the structure of the seven mRNA transcripts identified as the result of the scientific study. In each transcript, each box represents an exon. Each transcript was found to be a product of the same α-TM gene.

Figure 6.1

Source: http://www.bio.utexas.edu/research/tuckerlab/bright/phylo_3_10_04/index.html.

Scientists continue to study the α-TM gene as a model of alternative splicing, in which mRNA transcripts containing different combinations of exons can lead to the production of different proteins. Figure 6.2 shows the structure of the α-TM gene, which is composed of 12 exons connected by 11 introns.

Figure 6.2

Each exon in a gene codes for a specific series of amino acids in the corresponding protein. The complete α-TM gene codes for a protein composed of 284 total amino acids. Table 6.1 shows the series of amino acids coded by each of the 12 exons in the α-TM gene.

TABLE 6.1 Alpha-Tropomyosin Exon

Source: http://www.bio.utexas.edu/research/tuckerlab/bright/phylo_3_10_04/index.html.

1. To produce a tropomyosin protein, which of the following steps must occur first?

A. The introns are removed from the α-TM mRNA.
B. Exons are alternatively spliced to code a specific tropomyosin.
C. The α-TM gene is transcribed into mRNA.
D. Amino acids are arranged based on the α-TM mRNA sequence.

2. According to the passage, each mRNA transcript in Figure 6.1 is produced from:

A. the same gene.
B. multiple genes.
C. an independent gene.
D. the same tissue.

3. In Figure 6.1, what is the maximum number of exons present in an mRNA transcript?

A. 10
B. 7
C. 9
D. 11

4. Constitutive exons are present in all mRNA transcripts of a gene and are thought to be integral in the proteins' basic structure. Which of the following exons appears to be constitutive?

A. Exon 3
B. Exon 7
C. Exon 12
D. Exon 4

5. Alternatively spliced exons (ASEs) are those that only appear in certain mRNA transcripts. Which of the following cell types appears to have the least number of ASEs?

A. Myoblast
B. Brain
C. Nonmuscle/fibroblast
D. Smooth muscle

6. Which exons do not appear in any of the same mRNA transcripts?

A. Exons 10 and 12
B. Exons 7 and 11
C. Exons 2 and 3
D. Exons 3 and 11

7. Two types of muscle tissues-skeletal and cardiac-are both striated. Based on Figure 6.1, how do the α-TM mRNA transcripts of skeletal and cardiac muscle tissues differ?

A. One transcript contains a greater total number of exons.
B. The exons present in one transcript are absent in the other.
C. One contains Exon 2, while the other contains Exon 3.
D. Each transcript contains a different final exon.

8. The total number of exons in the α-TM gene is:

A. unknown.
B. 12
C. variable.
D. 11

9. Based on the data in Table 6.1, which α-TM exon codes for the longest sequence of amino acids?

A. Exon 8
B. Exon 4
C. Exon 11
D. Exon 6

10. Which α-TM mRNA transcript is missing amino acids 258-284?

A. Hepatoma
B. Myoblast
C. Smooth muscle
D. Brain

11. Based on Table 6.1, which mRNA transcript contains a repeated sequence of amino acids?

A. Striated muscle
B. Nonmuscle/fibroblast
C. Smooth muscle
D. Brain

12. A myoblast is an embryonic cell that can differentiate into a muscle cell. Based on Figure 6.1, which of the following happens to the α-TM mRNA transcript when a myoblast differentiates into a smooth muscle cell?

A. Exon 10 is added.
B. Exon 3 is replaced by Exon 2.
C. Exon 12 is replaced by Exon 11.
D. Exon 10 is removed.

13. Untranslated regions (UTRs) are sequences that exist at the beginning and end of every mRNA transcript. Instead of coding for amino acids, UTRs regulate the expression of the transcribed gene. In the α-TM mRNA, Exons 1 and 12 both contain UTRs. Based on the data in Table 6.1, which other exon contains a UTR?

A. Exon 5
B. Exon 8
C. Exon 11
D. Exon 3

14. The passage states that in addition to their function in all cells, tropomyosins also support contraction in muscle cells. It can be inferred that this extra function is related to which of the following sequences of amino acids?

A. Amino acids 39-80
B. Amino acids 81-125
C. Amino acids 258-284
D. Amino acids 1-38

15. A hepatoma is a tumor that forms within the liver. Based on Figure 6.1, it can be inferred that tumor formation may correlate to a loss of which exon?

A. Exon 2
B. Exon 10
C. Exon 11
D. Exon 7