ACT Science Practice Test 20

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.

Over the past several decades, scientists have seen a rapid decline in honeybee populations worldwide. In an effort to boost population sizes, the European Union recently instituted a temporary two-year ban on neonicotinoids, a class of pesticides thought to be harmful to honeybees.

Two scientists present their viewpoints regarding the value of instituting a similar ban in the United States.

Scientist 1

A short-term ban on the class of pesticides called neonicotinoids is a viable option that should seriously be considered by the United States. Studies have found neonicotinoid concentrations in pollen and nectar that can be lethal to pollinators. Although research has not identified a direct link between neonicotinoids and a reduction in honeybee populations, recent studies suggest that these pesticides may increase honeybees' susceptibility to parasites and diseases. The health of honeybee populations directly affects the agriculture industry and the overall ecosystem. Twenty-three percent of crops grown in the United States are pollinated by honeybees. Some crops, such as almonds, apples, onions, and carrots, are pollinated almost exclusively by honeybees. The reproductive rates of these crops vary directly with the availability of honeybees. Many of the plants that make up the base of the food web in the natural ecosystem also rely on these pollinators. Because the honeybee's role as pollinator is so pervasive, any measures that have the potential to support the health of honeybee populations should be taken.

Scientist 2

Honeybees are important pollinators for both natural ecosystems and the agriculture industry, and the health of their populations should be monitored closely. Instituting a ban on neonicotinoids, however, is unnecessary. Based on current research, the benefits of neonicotinoid use to the agriculture industry outweigh the threat to honeybee health. Though the exact causes are difficult to identify, researchers attribute the decrease in honeybee populations in recent years to weather, environmental stress, disease, and varroa mites. Environmental stressors include nectar and water that is scarce or of poor quality and exposure to pesticides, although researchers have found the latter to have the weakest correlation to honeybee loss of all stressors. Therefore, a ban on neonicotinoids will not be an effective approach for improving the health of honeybee populations. A more effective method should address varroa mites and disease, the greatest known threats to honeybee health.