Practice Test: Reading Comprehension - 5 | 100 RCs for GMAT PDF Download

Directions: Read the given passage carefully and answer the questions as follows:

Lysosomal storage diseases form a category of genetic disorders resulting from defective enzymes that normally function to degrade unneeded molecules in cells. These enzymes do their work in the lysosome, a small compartment in a cell analogous to a garbage disposal.

The lysosome contains between thirty and forty different degradative enzymes. When any one of the lysosomal enzymes is defective, the molecules requiring that specific enzyme for their degradation will accumulate and cause that individual’s lysosomes to swell enormously. The physiological effects of such swelling on the individual include motor and mental deterioration, often to the point of premature death. But each disease resulting from one specific defective lysosomal enzyme has its own characteristic pathology. The age of onset, rate of progression, and severity of the clinical symptoms observed in patients with the same defective lysosomal enzyme are highly variable. For many years, this variability in patients with the same defective enzyme puzzled scientists. Only recently have researchers begun to answer the riddle, thanks to a genetic analysis of a lysosomal storage disorder known as Tay-Sachs disease.
As in most lysosomal storage diseases, patients suffering from Tay-Sachs disease show both mental and motor deterioration and variability in age of onset, progression, and severity. Physicians have categorized the patients into three groups: infantile, juvenile, and adult, based on onset of the disease. The infantile group begins to show neurodegeneration as early as six months of age. The disease advances rapidly and children rarely live beyond 3 years old. The first symptoms of the disease appear in juvenile cases between 2 and 5 years of age, with death usually occurring around age 15. Those with the adult form generally live out a normal lifespan, suffering from milder symptoms than do those with the infantile and juvenile forms. Researchers hoped that the categorization would yield insight into the cause of the variability of symptoms among Tay-Sachs patients, but this turned out not to be the case.
In Tay-Sachs disease, undegraded materials accumulate mainly in the lysosomes in the brains of patients, but the kinds of molecules left undegraded and the specific identity of the defective lysosomal enzyme responsible for the malfunction were not discovered until the 1950s and 60s, respectively. The main storage molecule was found to be a lipid-like material known as GM2 ganglioside. The defective enzyme was later identified as hexosaminidase.

In 1985, the gene coding for the normal hexosaminidase enzyme was cloned and its DNA sequence determined.
Shortly thereafter, the DNA sequences of genes encoding hexosaminidase from many Tay-Sachs patients were studied. It soon became apparent that not one or two but many different types of mutations in the hexosaminidase gene could result in Tay-Sachs disease. Some of the mutations prevented the synthesis of any hexosaminidase, preventing all such enzyme activity in the cell. Patients with this type of mutation all had the infantile form of Tay-Sachs disease. Other mutations were found in certain regions of the gene coding for areas of the enzyme known to be critical for its catalytic activity. Such mutations would allow for only extremely crippled hexosaminidase activity.
Most of the patients with these mutations clustered in the juvenile category. Adult Tay-Sachs patients presented mutations in the regions of the hexosaminidase gene that were less important for the enzyme’s activity than were those affected in juvenile patients. Scientists quickly hypothesized that the variation in age of onset and severity of Tay-Sachs disease correlated with the amount of residual enzymatic activity allowed by the genetic mutation.
Though more research is needed to demonstrate similarity with other lysosomal storage diseases, the work done on Tay-Sachs disease has already offered a promising glimpse into the underlying mechanisms of these disorders.