ACT science practice test 6

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.


Accepted classification systems of life do not include viruses. Although viruses possess certain features of cellular organisms, including genetic material that codes for making new viral particles, they cannot replicate (make copies of) themselves without first infecting a living cell. Biologists agree that viruses originated from genetic material called nucleic acid, but it is difficult to prove any single theory regarding how this occurred. Three hypotheses of viral origin are presented here.

Coevolution Hypothesis

Some biologists argue that viruses evolved alongside other organisms over billions of years. They suggest that simple molecules of ribonucleic acid (RNA), a nucleotide that forms the genetic code for proteins, joined to form more complex sequences. These RNA sequences developed enzyme-like abilities including the ability to self-replicate and insert themselves into other nucleotide sequences. While some RNA sequences became incorporated into membrane-bound cells, others were packaged inside proteins as the first viral particles that could replicate after infecting cellular organisms (see Figure 1).

Figure 1

Cellular Origin Hypothesis

Some biologists claim that nucleotide sequences within prokaryotic (non-nucleated) and eukaryotic (nucleated) cellular organisms incorporated into a protein coating and escaped from the cell as a viral particle. Initially, DNA or RNA nucleotide sequences gained the code required for other cells to replicate them. Next, these sequences associated with proteins to form an outer capsid. Finally, the virion (viral particle) became capable of passing through the cell membrane and infecting other cells where it could be replicated. After the initial escape, viruses evolved independently from their initial host and ultimately could infect either prokaryotic or eukaryotic cells.

Regressive Evolution Hypothesis

An alternative explanation of viral origin is that viruses evolved from cellular organisms. Some cellular organisms, particularly certain bacteria, are obligate intracellular parasites because they must infect a host cell in order to reproduce. Regressive evolution suggests that some bacterial parasites gradually lost the structures required for survival outside of a cell. The result was a virus particle containing only nucleotides, a capsid (protein coating), and at times an outer membrane or envelope. This would account readily for viruses that contain complex deoxyribonucleic acid (DNA) similar to that found in bacteria and other cellular organisms (see Figure 2).

Figure 2

1. The development of which of the following is addressed in the passage by the Coevolution Hypothesis, but NOT by the Regressive Evolution Hypothesis?

F. Self-replication
G. Capsid
H. Deoxyribonucleic acid
J. Cell membrane transit

2. Supporters of all of the theories presented in the passage would agree with the conclusion that the first viruses:

A. evolved from bacteria.
B. could self-replicate outside a cell.
C. were enclosed within a membrane.
D. contained nucleic acid.

3. The Coevolution Hypothesis does NOT provide an explanation for the earliest virus particles possessing:

F. protein.
G. enzyme-like activity.
H. nucleotides.
J. DNA.

4. If the Cellular Origin Hypothesis is correct, which of the following conclusions can be made about modern T4 DNA viruses, which infect Escherichia coli bacteria, and modern PP7 RNA viruses, which infect Pseudomonas aeruginosa bacteria?

A. T4 and PP7 are more closely related to each other than to bacteria genetically.
B. T4 and PP7 are only distantly related genetically through a cellular organism.
C. T4 and PP7 both evolved from prokaryotic organisms.
D. T4 and PP7 both evolved from eukaryotic organisms.

5. The discovery of which of the following living organisms would provide the most support for the Regressive Evolution Hypothesis?

F. Extracellular parasites with DNA resembling a known virus
G. Extracellular parasites with unique RNA nucleotide sequences
H. Intracellular parasites with DNA resembling a known virus
J. Intracellular parasites with unique RNA nucleotide sequences

6. Supporters of all the theories presented would agree with which of the following conclusions about the origin of viruses?

A. Viral capsids contain a protein structure similar to the cell walls of modern bacteria.
B. The first viruses did not originate before the first cellular organisms.
C. RNA viruses are more advanced than DNA viruses.
D. The first virus contained DNA and was surrounded by an envelope similar to a cell membrane.

7. Which of the following questions is raised by the Coevolution Hypothesis, but is NOT answered in the passage?

F. Why were some RNA sequences packaged into protein structures and others incorporated into cell structures?
G. Why did obligate intracellular parasites lose their ability to survive outside of cells?
H. How could two different types of cellular organisms account for the origin of viruses?
J. How did virions develop the ability to pass through the cell membrane out of the cell?

Wind causes topsoil deflation, a type of erosion that is affected by plant and organic cover as well as water content of the soil. Scientists performed 2 experiments using equal-sized fields containing the same volume of soil. The soil samples were primarily a mixture of sand and silt, but differed in the percentage of clay they contained. Soil X was composed of 5% clay and soil Y was composed of 40% clay. Large fans were used to simulate wind. Topsoil deflation was measured in kilograms per hectare (kg/ha) following 10 hours of wind.

Experiment 1

A mixture of compost and straw was used to represent plant and organic cover. The percentage of soil covered with the mixture was considered to approximate an equivalent percentage of natural vegetative cover. One field remained uncovered, and the other fields were covered with different percentages of compost and straw. The topsoil deflation from each field was recorded in Table 1.

Experiment 2

Rainfall was simulated using a sprinkler system. Sprinklers were turned on for either 4 hours or 8 hours for fields of each kind of soil. Two additional fields composed of each type of soil were left unwatered. Afterward, soil samples were taken from all of the fields to determine their water content percentage, which was recorded in Table 2. Wind was applied as in Experiment 1 and topsoil deflation for all fields was recorded in Table 3.

8. According to the results of Experiments 1 and 2, topsoil deflation will be minimized by:

A. decreased organic cover, increased amount of rainfall, and the use of either Soil X or Y as topsoil.
B. decreased organic cover, decreased amount of rainfall, and the use of Soil Y as topsoil.
C. increased organic cover, increased amount of rainfall, and the use of Soil Y as topsoil.
D. increased organic cover, increased amount of rainfall, and the use of Soil X as topsoil.

9. If Experiment 1 were repeated using a soil containing 10% clay with 0% organic cover, which of the following would be the most likely topsoil deflation amount?

F. 110,200 kg/ha
G. 99,800 kg/ha
H. 70,700 kg/ha
J. 60,200 kg/ha

10. To further investigate the effect of water content on erosion from topsoil deflation, the scientists should repeat Experiment:

A. 1, using a different type of topsoil.
B. 1, using plastic covers over the fields.
C. 2, using no sprinklers.
D. 2, using fields exposed to various amounts of rainfall.

11. What assumption in experimental design is most important to consider when applying the findings of Experiment 1 to a practical situation?

F. The quantity of topsoil deflation is independent of the percentage of clay present in the soil.
G. The presence of straw on the soil does not accurately simulate vegetation and organic cover.
H. Air movement from fans provides an accurate simulation of the wind responsible for topsoil deflation.
J. Compost is more effective than water content in the prevention of topsoil erosion.

12. In Experiment 2, the water content in the two soil types was similar after 4 hours of sprinkling, yet the topsoil deflation was significantly different. Which of the following statements provides the best explanation for these findings?

A. Topsoil erosion is independent of the water content found in the soil.
B. Fields are susceptible to topsoil deflation only when water completely evaporates from the topsoil.
C. Soil with a lower percentage of clay is more prone to erosion from topsoil deflation than one with a higher percentage of clay.
D. Water is trapped in the topsoil by wind and this increases the rate of topsoil deflation.

13. If Experiment 2 were repeated with soil containing 10% clay, which of the following values would be expected for water content and topsoil deflation in a field following 8 hours of water sprinkling?

F. water content of 17%; topsoil deflation of 13,400 kg/ha
G. water content of 21%; topsoil deflation of 9,700 kg/ha
H. water content of 15%; topsoil deflation of 10,900 kg/ha
J. water content of 14%; topsoil deflation of 101,000 kg/ha