Question 44 - Reading Practice Test for the ACT

The following passages consist of excerpts taken from two Biology & Nature articles posted online at eScienceNews.com.

Passage 1
Fructose alters hundreds of brain genes, which can lead to a wide range of diseases

A range of diseases. . .are linked to changes to genes in the brain. A new study by UCLA life scientists has found that hundreds of those genes can be damaged by fructose, a sugar that’s common in the Western diet, in a way that could lead to those diseases. However, the researchers discovered good news as well: An omega-3 fatty acid known as docosahexaenoic acid, or DHA, seems to reverse the harmful changes produced by fructose.

“DHA changes not just one or two genes; it seems to push the entire gene pattern back to normal, which is remarkable,” said Xia Yang, a senior author of the study and a UCLA assistant professor of integrative biology and physiology. “And we can see why it has such a powerful effect. . .”

DHA strengthens synapses in the brain and enhances learning and memory. . .

To test the effects of fructose and DHA, the researchers trained rats to escape from a maze. . .

The animals that had been given only the fructose navigated the maze about half as fast than the rats that drank only water. . .The rats that had been given fructose and DHA, however, showed very similar results to those that only drank water. .

The rats receiving a high-fructose diet had much higher blood glucose, triglycerides and insulin levels than the other two groups. Those results are significant because in humans, elevated glucose, triglycerides and insulin are linked to obesity, diabetes and many other diseases.

The research team sequenced more than 20,000 genes in the rats’ brains, and identified more than 700 genes in the hypothalamus. . . and more than 200 genes in the hippocampus. . . that were altered by the fructose. The altered genes they identified, the vast majority of which are comparable to genes in humans, are among those that interact to regulate metabolism, cell communication and inflammation. Among the conditions that can be caused by alterations to those genes are Parkinson’s disease, depression, bipolar disorder, and other brain diseases. . .

The scientists found that fructose removes or adds a biochemical group to cytosine, one of the four nucleotides that make up DNA. (The others are adenine, thymine and guanine.) This type of modification plays a critical role in turning genes “on” or “off.”

Passage 2
Too much “noise” can affect brain development

. . .University of California, Irvine biologists have determined that uncontrolled fluctuations (known as “noise) in the concentration of the vitamin A derivative Retinoic acid (RA) can lead to disruptions in brain organization during development. Identifying how a cell responds to a signal made by another cell, despite the level of noise present, may improve our understanding of developmental disorders.

During development, RA is an important secreted molecule that aids in the proper organization of the brain. The cellular response to RA depends upon its concentration, which is determined by its production, movement through tissue and interactions with many proteins within the cell. During normal development, cells can filter the “noise” in RA levels and establish appropriate brain organization. Schilling and study lead author Julian Sosnik wanted to measure the fluctuations in RA and determine how cells respond to the proper amount despite the presence of constant noise.

To accomplish this, they used fluorescence lifetime imaging to exploit the auto-fluorescent nature of RA and measure its distribution across the developing zebrafish embryo. The team found that RA forms a gradient in the embryo, with a lower concentration at the head. They also observed that a large amount of noise exists within the RA gradient.

They identified one protein within developing cells that interacts with RA to help reduce the noise. When this protein was altered, cells could no longer control the level of noise within the RA gradient, which led to disruptions in brain organization.

With this, the researchers concluded that noise reduction within cells is critical for the proper response to the RA gradient and normal organization of the brain. . .