Passage
Nerve cells must maintain the composition of their myelin sheaths to function properly. Galactocerebroside (GC) , shown in Figure 1, is a hydrolyzable lipid and an important component of the myelin sheath.
Figure 1 Structure of the myelin sheath component galactocerebrosideHomeostasis of GC is maintained by its dynamic synthesis and degradation, as shown in Figure 2. GC is synthesized by the enzyme ceramide galactosyltransferase (CGT) using UDP-galactose (UDP-Gal) and ceramide (Cer) as substrates. It is degraded by the enzyme galactocerebrosidase (GalC) , which hydrolyzes the glycosidic bond to release galactose (Gal) and ceramide. A deficiency in GalC activity leads to a buildup of GC, and prior research has connected this buildup with neurodegeneration.
Figure 2 Schematic of GCT and GalC activity in healthy and GalC deficient cellsOver 100 inactivating GalC mutations have been identified. Five of the most common mutations are listed in Table 1. The changes to both DNA and amino acid sequence are shown.Table 1 Five Common Galactocerebrosidase Mutations

-Based on Table 1, the D187V form of GalC resulted from what type of nucleotide change?

Question

Passage
Nerve cells must maintain the composition of their myelin sheaths to function properly. Galactocerebroside (GC) , shown in Figure 1, is a hydrolyzable lipid and an important component of the myelin sheath.

Figure 1 Structure of the myelin sheath component galactocerebrosideHomeostasis of GC is maintained by its dynamic synthesis and degradation, as shown in Figure 2. GC is synthesized by the enzyme ceramide galactosyltransferase (CGT) using UDP-galactose (UDP-Gal) and ceramide (Cer) as substrates. It is degraded by the enzyme galactocerebrosidase (GalC) , which hydrolyzes the glycosidic bond to release galactose (Gal) and ceramide. A deficiency in GalC activity leads to a buildup of GC, and prior research has connected this buildup with neurodegeneration.

Figure 2 Schematic of GCT and GalC activity in healthy and GalC deficient cellsOver 100 inactivating GalC mutations have been identified. Five of the most common mutations are listed in Table 1. The changes to both DNA and amino acid sequence are shown.Table 1 Five Common Galactocerebrosidase Mutations

-Based on Table 1, the D187V form of GalC resulted from what type of nucleotide change?

Quizplus · Accepted Answer

11ecba2d_0e62_bfe8_a3bf_b58c073d7c9d_MD0008_00 Nucleotides are classified based on the aromatic compounds from which their nitrogen bases are derived: pyrimidine or purine. The compound pyrimidine is composed of a single aromatic ring containing two nitrogen atoms, which is the basis for thymine (or uracil in RNA) and cytosine. The compound purine is composed of two aromatic rings. It is essentially pyrimidine fused to a five-member imidazole ring, which is the basis for adenine and guanine. The mnemonic "PURe As Gold" is helpful in remembering that purines are associated with A and G nucleotides; the remaining nucleotides are pyrimidines. According to Table 1, the D187V variant of GalC arises due to an A-to-T mutation in the DNA sequence. Adenine is a purine nucleotide, and thymine is a pyrimidine. Therefore, the D187V form of GalC results from replacement of a purine (A) by a pyrimidine (T). (Choice A) Adenine is a purine, not a pyrimidine. (Choice B) Replacement of a pyrimidine by a purine would be correct if the mutation were thymine to adenine instead of adenine to thymine. (Choice D) Thymine is a pyrimidine, not a purine. Educational objective: Nucleotides are classified as purines and pyrimidines based on the molecule from which they are derived. Pyrimidines contain a single aromatic ring (eg, cytosine, thymine, uracil). Purines contain two fused aromatic rings (eg, adenine, guanine).

Passage Nerve Cells Must Maintain the Composition of Their Myelin Sheaths