Review Questions for Exam 2 Flashcards
12-1. Viruses cause many human infections such as hepatitis, encephalitis, and the “common cold.” Which one of the following is a common trait of all these viruses?
D. In order to reproduce, they must use the infected cells DNA, RNA, and protein-synthesizing machinery.
-Viruses consist of either a DNA or RNA genome but not both. All the viruses that cause the common cold are RNA viruses. When viruses infected bacteria (prokaryotes), they are called bacteriophages or phages. Plasmids are not viruses and are not infectious. Plasmids are small, circular DNA molecules that can replicate autonomously, whereas viruses cannot and must use the host cell’s DNA, RNA, and protein-synthesizing machinery.
12-2. Many drugs used to treat cancer inhibit DNA replication, whereas some will inhibit the pathways required to synthesize proteins from certain genes. Which one of the following accurately describes a step leading from DNA replication to the synthesis of a protein?
B. Transcription of a gene generates a single-stranded RNA that is complementary to both strands of the DNA.
-rRNA and tRNA are part of the apparatus from protein synthesis, but the sequences of rRNA and tRNA do not encode proteins. mRNA carries the genetic information that is converted into the amino acid sequence of a protein, but that is used in the process of translation. Transcription of a gene from DNA generates RNA that is complementary to only one strand of DNA.
12-3. Gout is cause by the deposition of irate crystals in the joints and kidney. Purines are metabolized to uric acid, whereas pyrimidines, when metabolized to uric acid, where as pyridines, when metabolized, do not generate uric acid. Which one of the following should be restricted in the diet of a patient with gout?
B. Guanine
-Adenine and guanine are purines, which form rate during their metabolism. Cytosine, thymine, and uracil are pyrimidines that follow different metabolic pathways and do not form uric acid. Deoxyribose is a component of DNA, but it is a sugar and not a purine base, and it is not converted to uric acid.
12-4. A patient has a microcytic, hypochromic anemia. In order to ascertain the cause of the anemia, the patient’s hemoglobin was isolated, and it has determined that there was much more Beta-chain present than alpha-chain, indicating an alpha-thalassemia. To determine the genetic basis of the thalassemia, nucleic acids were isolated from the blood of the patient. During the procedure of isolating the nucleic acids, both heat and alkali treatment were required. The alkali and heat were included owing to which ONE of the following?
D. Alkali separates the strands of the DNA and degrades RNA to nucleotides.
-Both alkali and heat cause the two strands of DNA to separate. Alkali does not break the phophodiester bonds of DNA but does cleave the phosphodiester bonds of RNA, degrading RNA to nucleotides. RNA is single stranded, not double stranded. In the analysis of DNA, many techniques call for it separation from RNA (the alkali treatment) and its denaturation (separation of the double strands).
12-5. Targeting certain structural features of eukaryotic mRNA can result in an inhibition of protein synthesis. Which one of the following describes a unique aspect of eukaryotic mRNA?
E. The poly(A) tail is found at the 3’-end of the mRNA.
-The leader sequence beings with an (N^7)-methylguanosine cap art the 5’-end of the mRNA. The coding region of the mRNA then follows. The 3’-end of the mRNA contains a polyadenine tail, which aids in the stability fo the mRNA. Only the tail of the mRNA is added after transcription of the mRNA.
12-6. Some chemotherapeutic drugs alter the ability of DNA polymerase to faithfully replicate. For the DNA sequence 5’-ATCGATCGATCGATCG-3’, which one of the following represents the sequence and polarity of the complementary strand?
D. 5’-CGAUACGUAGCACUAGCUUAACGCA-3’
-The complementary strand must run in the opposite direction, so the 5’-end must base-pair with the G at the 3’-end of the given strand. Therefore, the 5’-end of the complementary strand must be C. G would then base-pair to C, A to T, and T to A. Answer B is incorrect because the bases do not base-pair with each other with the sequences indicated, C is incorrect because U is not found in DNA, and E is incorrect because it hast he wrong polarity (the 5’-T in answer E would not base-pair with the 3’G in the given sequence).
12-7. Certain drugs inhibit bacterial RNA synthesis. If the DNA strand 5’GCTATGCATCGTGATC GAATTGCGT-3’ serves as a template for the synthesis of RNA, which one of the following choices gives the sequence and polarity of the newly synthesized RNA?
C. 5’-ACGCAAUUCGAUCACGAUGCAUAGC-3’
-The RNA strand must be complementary to the DNA strand, and A in DNA base-pairs with DNA base-pairs with U in RNA, where as T in DNA base-pairs with A in RNA, G in DNA base-pairs with C in RNA, and C in DNA base-pairs with G in RNA. Answer A and E are incorrect because they contain T, which is found in DNA, not RNA. Answer B is incorrect because the base-pairing rules are broken when the strands are aligned in antiparallel fashion. Answers D is incorrect (if one were to switch the 5’- and 3’-ends, the answer would be correct).
12-8. Understanding the structure of DNA, and the process of replication, enabled various drugs to be developed that interfered with DNA replication. In DNA, the bond between the deoxyribose sugar and the phosphate is best described by which one of the following?
D. A covalent bond
-The phosphate is in an ester bond between two deoxyribose groups, generating the phosphodiester bond in the DNA backbone (these are covalent bonds). None of the other types of bonds is correct.
12-9. Certain drugs can intercalate between DNA bases and alter the backbone of the DNA. The backbone of a DNA strand is composed of which of the following?
A. Phosphates and sugars
-The DNA backbone is composed of the phosphates and deoxyribose in phosphodiester link-ages. The bases are internal to the backbone, base-paired to bases in the complementary strand, and form stacking interactions with in the double helix.
12-10. Analysis of one strand of a double-stranded piece of dNA displayed 20 mol %A, 25 mol %T, 30 mole %G, and 25 mole %C. Which of the following accurately represents the compositions of the complementary strand?
A. A is 25 mol%, T is 20 mol %, G is 25 mol %, and C is 30 mol %.
-The base pairs in double-stranded DNA require that [A]=[T], and [C]=[G]. Therefore, if the concentration of A in one strand is 20 mol %, the concentration of T in the complementary strand must also be 20 mol %. For the example given, then, [A] would be 25 mol %, [T] would be 20 mol %, [G] would be 25 mol %, and [C] would be 30 mol %.
39-1. Similarities between CPSI and CPSII include which one of following? A. Carbon source B. Intracellular location C. Nitrogen source D. Regulation by N-acetylglutamate E. Regulation by UMP
A. Carbon source
-Both CPSI and CPSII use carbon dioxide as the carbon source in the production of carbamoyl phosphate. CPSI is located in the mitochondria, whereas CPSII is in the cytoplasm (thus, B is incorrect). CPSI can fix ammonia; CPSII requires glutamine as the nitrogen source (thus, C is incorrect). N-Acetylglutamate activates CPSI; CPSII but has no effect on CPSI (thus, E is also incorrect).
39-2. Gout can result from a reduction in activity of which one of the following enzymes?
B. HGPRT
- A lack of HGPRT activity (Lesch-Nyhan syndrome) leads to an accumulation of PRPP levels, which induces increased purine synthesis, leading to excessive levels of purines in the cells. The degradation of the extra purines leads to uric acid production and gout. A loss of either PRPP synthetase activity or glutamine phosphoribosylamidotransferase activity would lead to reduced purine synthesis and hypouricemia (thus, A and D are incorrect). A lack of glucose 6-phosphate dehydrogenase would hinger ribose 5-phosphate production and thus would not lead to excessive purine synthesis. A lack of purine nucleoside phosphorylase would hinder the salvage pathway, leading to an accumulation of nucleosides. Purine nucleoside phosphorylase activity is required to synthesize uric acid, so in the absence of this enzyme, less uric acid would be produced (thus, E is incorrect).
39-3. A 20-year-old boy is exhibiting developmental delay and has started to bite his lips and fingers. Orange-colored “sand” is found in his diapers. This child has an inability to metabolize which one of the following molecules?
E. Hypoxanthine
-The child has Lesch-Nyhan syndrome, a deficiency of HGPRT activity. As such, hypoxanthine can not be converted to IMP, and guanine cannot be converted to GMP. One consequence of this is elevated uric acid levels, owing to an accumulation of hypoxanthine and guanine. Pyrimidine metabolism is not altered in Leshch-Nyhan patients, so thymine and uracil metabolism is normal. Adenine is converted to AMP by APRT, so a loss of HGPRT activity does not alter adenine metabolism.
39-4. Allopurinol can be used to treat gout because of its ability to inhibit which one of the following reactions?
B. Xanthine to uric acid
-Allopurinol inhibits the conversation of both hypoxanthine to xanthine and xanthine to uric acid. This occurs because both of those reactions are catalyzed by xanthine oxidase, the target of allopurinol. Answer A is incorrect because AMP is not converted directly to XMP (AMP, when degraded is deaminated to form IMP, which loses its phosphate to become ionsine, which undergoes phosphorolysis to generate hypoxanthine and ribose 1-phosphate). Answer C is incorrect because the inosine-to-hypoxanthine conversion, catalyzed by nucleoside phosphorylase, is not inhibited by allopurinol. Answer D is incorrect because the conversion of hypoxanthine to xanthine occurs at the free-base level, not at the nucleotide level. Answer E is incorrect because GMP is converted first to guanine and ribose 1-phosphate, and the guanine is then converted to xanthine by guanase.
39-5. The regulation of ribonucleotide reductase is quite complex. Assuming that an enzyme deficiency leads to highly elevated levels dGTP, what effect would you predict on the reduction of ribonucleotides to deoxyribonucleotides under these conditions?
B. The formation of dADP will be favored.
-If dGTP were to accumulate in cells, the dGTP would bind to the substrate specificity site of ribonucleotide reductase and direct the synthesis of dADP. This would lead to elevations of dATP levels, which would inhibit the activity of ribonucleotide reductase. The inhibition of ribonucleotide reductase leads to a cessation of cell proliferation, as the supply of deoxyribonucleotide for DNA synthesis becomes limiting. Answer A is incorrect because ATP would need to bind to the substrate specificity site to direct the synthesis of dCDP. That would not occur under these conditions of elevated dGTP levels. Answer C is incorrect because the enzyme works only on diphosphate; AMP would never be a substrate for this enzyme. Answer D is incorrect because the thioredoxin is always regenerated and does not become rate-limiting for the reductase reaction. Answer E is incorrect because dGTP does not bind tot eh activity site of the reducatase; only ATP (activator) or dATP (inhibitor) is capable of binding to the activity site.
39-6. A patient with von Gierke’s disease displays the symptoms of gout. This most likely occurs owing to the overproduction of which one of the following molecules?
A. PRPP
-A patient with von Gierke’s disease is lacking glucose 6-phosphatase activity, and glucose 6-phosphate cannot be converted to free glucose. Under conditions of gluconeogenesis, and glycogenolysis, in the live glucose 6-phosphate will accumulate. The high glucose 6-phosphate levels will drive the heose monophosphate (HMP) shunt pathways to produce additional ribose 5-phosphate, which will stimulate pRPP synthetase to produce more PRPP. The elevated PRPP leads to increased activity of aminophosphoribosyltransferase, producing more AMP and GMP. As the AMP and GMP are not required by the cell, they are degraded and produce uric acid, which accumulates, precipitates, and leads to the symptoms of a gout attack. Elevated levels of NADPH may occur owing to an increase in the oxidative steps of the HMP shunt, but this will not lead to an increase in uric acid production. Glucose levels will not increase because of the defect in glucose 6-phosphatase. UDP-glucose levels may increase, but if they do, glycogen synthesis will be stimulated and this will not affect uric acid production. Aspartate levels may increase (owing to increased protein turnover to provide substrates for gluconeogenesis) in patients with von Gierke’s disease, but higher aspirate will not lead to uric acid production (because aspartate will not simulate purine ring synthesis).
