Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 531:

  Synthetic dyes constitute a commercially significant area of organic chemistry. The color producing properties of these compounds are the result of highly delocalized electron systems giving rise to electronic transitions whose absorptions occur in the visible region. Most commercially useful dyes can be classified as one of three types -- anthraquinones, azo dyes, or triarylmethyl salts. Examples of each type are illustrated in Figure 1.

  

  Figure 1

  In order for a dye to be useful in the fabric industry, it must have sufficient affinity for the polymeric fibers of which the material is composed; the dye must not only impart a color to the fabric, but must also do so in a relatively permanent manner (color fastness). Proper design of synthetic polymers requires the placement of acidic or basic side chains along the polymer backbone such that binding sites are available for dying. Similarly, dyes must be produced not only with the appropriate color-producing structure, but also with an affinity for the fabric in question. The structural units of several common synthetic fibers are shown in Figure 2.

  

  Figure 2

  Nylon, Dacron, and many other synthetic fibers are produced via condensation reactions. Which of the following would be the best starting materials for the production of nylon 66?

  A. cis-2-butenoic acid and 1,6-hexanediamine

  B. butanedioic acid and 1,6-hexanediamine

  C. n-hexanoic acid and 2,5-hexanediamine

  D. hexanedioic acid and 1,6-hexanediamine

  Correct Answer: D

  Amides are produced by the condensation of amines with carboxylic acids:

  

  To produce a repeating chain, or polyamide, one would need to condense a diamine with a dicarboxylic acid. As indicated in figure 2, nylon 66 has a straight chain of six carbons between the two nitrogen atoms and six more carbon atoms, also unbranched, in the dicarbonyl portion of the monomer. As such, the amine starting material must therefore be 1,6-hexanediamine, while the carboxylate reactant is hexanedioic acid:

  Choice A is incorrect because cis-2-butenoic acid has only one carboxylate functionality and therefore would not condense on both ends, and it would produce a condensation product in which the butane double bond is still present. It also contains only four carbon atoms, rather than the required six. Choice B is incorrect since, while butanedioic acid can polymerize with the diamine, it would produce a polymer with only two methylene groups between the carbonyl carbons in each monomer. (This product would be called nylon 64.) Choice C is wrong on two counts: first, the hexanoic acid is a monocarboxylic acid, and thus will not combine with two amine molecules; second, any polyamide formed via condensation of an appropriate diacid and the suggested amine, 2,5- hexanediamine, would have two methyl group side chains.

• Question 532:

  Synthetic dyes constitute a commercially significant area of organic chemistry. The color producing properties of these compounds are the result of highly delocalized electron systems giving rise to electronic transitions whose absorptions occur in the visible region. Most commercially useful dyes can be classified as one of three types -- anthraquinones, azo dyes, or triarylmethyl salts. Examples of each type are illustrated in Figure 1.

  

  Figure 1

  In order for a dye to be useful in the fabric industry, it must have sufficient affinity for the polymeric fibers of which the material is composed; the dye must not only impart a color to the fabric, but must also do so in a relatively permanent manner (color fastness). Proper design of synthetic polymers requires the placement of acidic or basic side chains along the polymer backbone such that binding sites are available for dying. Similarly, dyes must be produced not only with the appropriate color-producing structure, but also with an affinity for the fabric in question. The structural units of several common synthetic fibers are shown in Figure 2.

  

  Figure 2

  Dacron belongs to which of the following general classifications?

  A. Polyamide

  B. Polyester

  C. Polyurethane

  D. Polypeptide

  Correct Answer: B

  Dacron, as shown in figure 2, is a polyester since it contains the COOR functionality in its monomer (subunit).

  Choice A is incorrect since polyamides would need to contain nitrogen in their monomers in order to contain an amide functional group, CONR2. Dacron does not. (However, nylon, also shown in figure 2, is a polyamide.) Choice C is wrong,

  like A, for lack of nitrogen, a necessary element to classify the monomeric compound as a urethane, ArNHCOOR.

  Choice D is wrong since the monomer structure is not a peptide.

• Question 533:

  Synthetic dyes constitute a commercially significant area of organic chemistry. The color producing properties of these compounds are the result of highly delocalized electron systems giving rise to electronic transitions whose absorptions occur in the visible region. Most commercially useful dyes can be classified as one of three types -- anthraquinones, azo dyes, or triarylmethyl salts. Examples of each type are illustrated in Figure 1.

  

  Figure 1

  In order for a dye to be useful in the fabric industry, it must have sufficient affinity for the polymeric fibers of which the material is composed; the dye must not only impart a color to the fabric, but must also do so in a relatively permanent manner (color fastness). Proper design of synthetic polymers requires the placement of acidic or basic side chains along the polymer backbone such that binding sites are available for dying. Similarly, dyes must be produced not only with the appropriate color-producing structure, but also with an affinity for the fabric in question. The structural units of several common synthetic fibers are shown in Figure 2.

  

  Figure 2

  Cotton is a natural fiber composed of cellulose, a polymer of glucose. Which of the compounds shown in Figure 1 would adhere to a cotton fiber via hydrogen bonding?

  A. Alizarin only

  B. Malachite green only

  C. Alizarin and aniline yellow only

  D. Malachite green, alizarin, and aniline yellow

  Correct Answer: C

  Hydrogen bonding occurs between OH groups and NH groups in any combination. Since the glucose monomer in cellulose has OH groups, cellulose should be capable of hydrogen bonding to either alizarin, with its two OH groups, or to aniline yellow, with its aromatic amine functionality. Malachite green, on the other hand, has no acidic protons available for hydrogen bonding due to the methylation of the nitrogen atoms.

• Question 534:

  Nitric oxide, NO, has recently been found to have widespread physiological effects, acting as a major regulator in the nervous, cardiovascular, and immune systems. The production of NO in the body is regulated by specific NOS enzymes which exist in at least three different isoforms -- bNOS, eNOS, and macNOS. Each of these isoforms differ in location and function and serve to mediate different physiological responses to NO. Some physiological roles of NO have been demonstrated as follows:

  I. In the central nervous system, NO production is regulated by bNOS. Calcium ion concentrations of 200- 400 nM in the central nervous system activate bNOS to catalyze the formation of NO. NO exerts definite effects on brain function although its specific roles are not well established. bNOS inhibitors have been found to block the release of neurotransmitter from presynaptic neurons. Excess levels of NO are also thought to contribute to neurodegenerative disorders such as Alzheimer's disease.

  II. In the blood vessels, NO is produced by eNOS which is activated by Ca2+ concentrations of 200-400 nM. NO acts as the major endogenous vasodilator in blood vessels. It diffuses into smooth muscle cells and leads to muscle relaxation by stimulating cGMP formation through activation of guanylyl cyclase. In addition, NO regulates the vascular system by inhibiting platelet aggregation and adhesion.

  III. The role of NO in the immune system is regulated by macNOS through a pathway that is not Ca2+ dependent. Rather, exposure to cytokines, including interleukin-1 and interferon- , leads to synthesis of large amounts of NO by activation of macNOS in response to inflammatory stimuli. The NO produced plays a definitive role in the mediation of the activities of macrophages and neutrophils. NO also acts to inhibit the mechanism of viral replication.

  According to the passage, NO may counter viral infection by all of the following mechanisms EXCEPT:

  A. inducing white blood cells to engulf and destroy foreign agents.

  B. inhibiting the synthesis of viral RNA.

  C. denaturing viral protein coats in the interstitial fluid.

  D. blocking viral release from infected cells.

  Correct Answer: C

  The passage indicates that NO inhibits viral replication and mediates the activities of macrophages. It does not mention the denaturing of viral protein coats as an activity of NO. Choice A is incorrect because macrophages are white blood

  cells which engulf and destroy foreign agents and their activity is mediated by NO.

  Choice B is incorrect because NO inhibits viral replication and may do so by inhibiting the synthesis of viral RNA.

  Choice D is incorrect because NO may inhibit viral replication by inhibiting viral release from infected cells.

• Question 535:

  Synthetic dyes constitute a commercially significant area of organic chemistry. The color producing properties of these compounds are the result of highly delocalized electron systems giving rise to electronic transitions whose absorptions occur in the visible region. Most commercially useful dyes can be classified as one of three types -- anthraquinones, azo dyes, or triarylmethyl salts. Examples of each type are illustrated in Figure 1.

  

  Figure 1

  In order for a dye to be useful in the fabric industry, it must have sufficient affinity for the polymeric fibers of which the material is composed; the dye must not only impart a color to the fabric, but must also do so in a relatively permanent manner (color fastness). Proper design of synthetic polymers requires the placement of acidic or basic side chains along the polymer backbone such that binding sites are available for dying. Similarly, dyes must be produced not only with the appropriate color-producing structure, but also with an affinity for the fabric in question. The structural units of several common synthetic fibers are shown in Figure 2.

  

  Figure 2

  Certain natural protein fibers such as silk or wool can be treated with aqueous base, then with solutions containing cationic dyes such as malachite green to produce color fast yarns. The most likely explanation for the affinity of malachite green for silk or wool via this process is that:

  A. many of the R groups on the amino acids of which these fibers are composed contain COOH groups.

  B. very few of the R groups on the amino acids of which these fibers are composed contain OH groups.

  C. the aqueous base hydrolyzes some of the peptide linkages in these fibers.

  D. the aqueous base neutralizes the cationic dye.

  Correct Answer: A

  Since malachite green, as depicted in figure 1, is cationic, it follows that it will have an affinity for anionic binding sites. Such anionic binding sites, alluded to in the passage, would be produced from acidic side chains upon treatment with

  aqueous base, and acidic side chains in natural proteins would most likely contain the COOH functionality.

  Choice B is incorrect since acidic side chains on natural amino acids often contain OH groups. Choice C is wrong since treatment with base is part of the process of producing color-fast yarns; this treatment makes the color stick, it does not

  break down the yarn into smaller polypeptide chains as choice C suggests.

  Choice D is incorrect because the base reacts with the fiber before the dye is added, and not with the dye.

• Question 536:

  Nitric oxide, NO, has recently been found to have widespread physiological effects, acting as a major regulator in the nervous, cardiovascular, and immune systems. The production of NO in the body is regulated by specific NOS enzymes which exist in at least three different isoforms -- bNOS, eNOS, and macNOS. Each of these isoforms differ in location and function and serve to mediate different physiological responses to NO. Some physiological roles of NO have been demonstrated as follows:

  I. In the central nervous system, NO production is regulated by bNOS. Calcium ion concentrations of 200- 400 nM in the central nervous system activate bNOS to catalyze the formation of NO. NO exerts definite effects on brain function although its specific roles are not well established. bNOS inhibitors have been found to block the release of neurotransmitter from presynaptic neurons. Excess levels of NO are also thought to contribute to neurodegenerative disorders such as Alzheimer's disease.

  II. In the blood vessels, NO is produced by eNOS which is activated by Ca2+ concentrations of 200-400 nM. NO acts as the major endogenous vasodilator in blood vessels. It diffuses into smooth muscle cells and leads to muscle relaxation by stimulating cGMP formation through activation of guanylyl cyclase. In addition, NO regulates the vascular system by inhibiting platelet aggregation and adhesion.

  III. The role of NO in the immune system is regulated by macNOS through a pathway that is not Ca2+ dependent. Rather, exposure to cytokines, including interleukin-1 and interferon- , leads to synthesis of large amounts of NO by activation of macNOS in response to inflammatory stimuli. The NO produced plays a definitive role in the mediation of the activities of macrophages and neutrophils. NO also acts to inhibit the mechanism of viral replication.

  A patient has accidentally ingested a toxin which acts as an eNOS inhibitor. According to the passage, the effects of this toxin would most likely include:

  A.

  B. decreased blood pressure.

  C. increased immune response.

  D. decreased urine production.

  E. increased blood pressure.

  Correct Answer: D

  eNOS produces NO in endothelial tissue, leading to vasodilation. This would tend to decrease blood pressure. An eNOS inhibitor would thus block production of NO, decreasing vasodilation and thereby increasing blood pressure.

  Choice A is incorrect because an eNOS inhibitor would produce an increase in blood pressure. Choice B is incorrect because a macNOS regulates the production of NO in the immune system. Also, inhibiting NO production would not

  increase immune response which is positively linked to NO concentration.

  Choice C is incorrect because increased blood pressure would tend to increase urine production.

• Question 537:

  Nitric oxide, NO, has recently been found to have widespread physiological effects, acting as a major regulator in the nervous, cardiovascular, and immune systems. The production of NO in the body is regulated by specific NOS enzymes which exist in at least three different isoforms -- bNOS, eNOS, and macNOS. Each of these isoforms differ in location and function and serve to mediate different physiological responses to NO. Some physiological roles of NO have been demonstrated as follows:

  I. In the central nervous system, NO production is regulated by bNOS. Calcium ion concentrations of 200- 400 nM in the central nervous system activate bNOS to catalyze the formation of NO. NO exerts definite effects on brain function although its specific roles are not well established. bNOS inhibitors have been found to block the release of neurotransmitter from presynaptic neurons. Excess levels of NO are also thought to contribute to neurodegenerative disorders such as Alzheimer's disease.

  II. In the blood vessels, NO is produced by eNOS which is activated by Ca2+ concentrations of 200-400 nM. NO acts as the major endogenous vasodilator in blood vessels. It diffuses into smooth muscle cells and leads to muscle relaxation by stimulating cGMP formation through activation of guanylyl cyclase. In addition, NO regulates the vascular system by inhibiting platelet aggregation and adhesion.

  III. The role of NO in the immune system is regulated by macNOS through a pathway that is not Ca2+ dependent. Rather, exposure to cytokines, including interleukin-1 and interferon- , leads to synthesis of large amounts of NO by activation of macNOS in response to inflammatory stimuli. The NO produced plays a definitive role in the mediation of the activities of macrophages and neutrophils. NO also acts to inhibit the mechanism of viral replication.

  A patient with hypertension (high blood pressure) is treated with nitroglycerin, which spontaneously breaks down to give NO. This treatment will be:

  A. effective because NO will activate eNOS.

  B. effective because NO will lead to vasodilation.

  C. ineffective because eNOS will not be activated.

  D. ineffective because NO will build up, leading to neurotoxicity.

  Correct Answer: B

  The passage states that in blood vessels, NO acts as a major vasodilator. Thus, NO could be used to alleviate high blood pressure. Nitroglycerin, as stated in the question stem, breaks down spontaneously (i.e., without requiring catalyzation

  by an NOS enzyme). Thus, by administering nitroglycerin, NO will be generated (without eNOS) in the patient's body and its presence will lead to vasodilation, causing a drop in blood pressure and relieving the hypertension.

  Choice A is incorrect because NO will not activate eNOS. eNOS is activated by Ca2+ and functions normally to produce NO.

  Choice C is incorrect because NO will be spontaneously generated from the nitroglycerin without requiring catalyzation by eNOS. Thus, although eNOS will not be activated, the NO generated from the administered nitroglycerin will dilate the

  blood vessels and relieve hypertension. Choice D is incorrect because NO is very unstable and will not tend to build up. It is also not likely that NO would be capable of crossing the blood/brain barrier and affecting the nervous system.

• Question 538:

  Nitric oxide, NO, has recently been found to have widespread physiological effects, acting as a major regulator in the nervous, cardiovascular, and immune systems. The production of NO in the body is regulated by specific NOS enzymes which exist in at least three different isoforms -- bNOS, eNOS, and macNOS. Each of these isoforms differ in location and function and serve to mediate different physiological responses to NO. Some physiological roles of NO have been demonstrated as follows:

  I. In the central nervous system, NO production is regulated by bNOS. Calcium ion concentrations of 200- 400 nM in the central nervous system activate bNOS to catalyze the formation of NO. NO exerts definite effects on brain function although its specific roles are not well established. bNOS inhibitors have been found to block the release of neurotransmitter from presynaptic neurons. Excess levels of NO are also thought to contribute to neurodegenerative disorders such as Alzheimer's disease.

  II. In the blood vessels, NO is produced by eNOS which is activated by Ca2+ concentrations of 200-400 nM. NO acts as the major endogenous vasodilator in blood vessels. It diffuses into smooth muscle cells and leads to muscle relaxation by stimulating cGMP formation through activation of guanylyl cyclase. In addition, NO regulates the vascular system by inhibiting platelet aggregation and adhesion.

  III. The role of NO in the immune system is regulated by macNOS through a pathway that is not Ca2+ dependent. Rather, exposure to cytokines, including interleukin-1 and interferon- , leads to synthesis of large amounts of NO by activation of macNOS in response to inflammatory stimuli. The NO produced plays a definitive role in the mediation of the activities of macrophages and neutrophils. NO also acts to inhibit the mechanism of viral replication.

  A "knock out" mouse with a mutant bNOS protein was generated by recombination techniques. The mutant protein was identical to the wild-type protein except for the identity of amino acid 675; the mutated bNOS has Tryptophan instead of Cysteine at position 675. Which of the following is responsible for the mutant protein?

  A. Frame shift mutation

  B. Single base-pair deletion

  C. Point mutation

  D. Nonsense mutation

  Correct Answer: C

  A point mutation is the substitution of a single nucleotide. Point mutations can result in missense mutations in which a single amino acid is substituted for another in the translated protein, or in nonsense mutations. The mutation described in the question is a missense mutation. Choice A is incorrect because a frame shift mutation changes the reading frame of the mRNA and would change the amino acid sequence of the entire protein from that point on. Choice B is incorrect because deletion of a single base pair would be a frame shift mutation leading to changes in the amino acid sequence from that point on. Choice D is incorrect because a nonsense mutation is a point mutation that introduces a stop codon into the mRNA. This leads to translation of a shorter protein.

• Question 539:

  Early experimentation on the single-celled organism Acetabularia led to important discoveries about the role of the nucleus in regulating cell function. Acetabularia is an enormous single cell with three distinct regions: a cap, a root-like rhizoid,

  and a stalk which connects the two. The following experiments were conducted to study the development of the cell:

  Experiment 1

  The stalk of an Acetabularia was cut, fragmenting the cell. The fragment which included the cap died shortly afterwards while the fragment containing the rhizoid regenerated to form a complete Acetabularia.

  Experiment 2

  The nucleus from Acetabularia mediterranea, which has a flat cap, was transplanted into Acetabularia crenulata, which has a tufted cap, following removal of the Acetabularia crenulata nucleus. The Acetabularia crenulata cap eventually

  assumed the flat shape.

  Experiment 3

  The nucleus of Acetabularia mediterranea was removed from the young cell before it first formed a cap. A normal cap formed several weeks later. The cell proved to be inviable and died shortly thereafter.

  Experiment 4

  A young Acetabularia was fractioned into a number of portions before it first formed a cap. Several weeks later, both the portion containing the nucleus and the portion containing the apical tip of the stalk formed caps. The other portions did

  not form caps.

  The mRNA sequence shown below is transcribed from which of the following DNA fragments?

  z5'-UGUAAUC-3'

  mRNA

  A. 5'-GAUUACA-3'

  B. 5'-ACATTAG-3'

  C. 5'-CTAATGT-3'

  D. 5'-GATTACA-3'

  Correct Answer: D

  mRNA is transcribed through complementary base pairing from the template strand of DNA. The sequence of mRNA runs antiparallel to the strand of DNA from which it was synthesized. The base pairs in DNA bind to base pairs in RNA in the following way -- G-C, C-G, A-U, T-A. Note that RNA contains uracil in place of thymine. The complementary strand of DNA to the given mRNA is: The DNA strand above must be rewritten in the 5' 3' direction to fit the answer choices.

  

  

  

  Choice A is incorrect because there is no uracil in DNA.

  Choice B is incorrect because the sequence is backwards. Note that the strands must be antiparallel. Choice C is incorrect because the base pairs are not complementary.

• Question 540:

  Nitric oxide, NO, has recently been found to have widespread physiological effects, acting as a major regulator in the nervous, cardiovascular, and immune systems. The production of NO in the body is regulated by specific NOS enzymes which exist in at least three different isoforms -- bNOS, eNOS, and macNOS. Each of these isoforms differ in location and function and serve to mediate different physiological responses to NO. Some physiological roles of NO have been demonstrated as follows:

  I. In the central nervous system, NO production is regulated by bNOS. Calcium ion concentrations of 200- 400 nM in the central nervous system activate bNOS to catalyze the formation of NO. NO exerts definite effects on brain function although its specific roles are not well established. bNOS inhibitors have been found to block the release of neurotransmitter from presynaptic neurons. Excess levels of NO are also thought to contribute to neurodegenerative disorders such as Alzheimer's disease.

  II. In the blood vessels, NO is produced by eNOS which is activated by Ca2+ concentrations of 200-400 nM. NO acts as the major endogenous vasodilator in blood vessels. It diffuses into smooth muscle cells and leads to muscle relaxation by stimulating cGMP formation through activation of guanylyl cyclase. In addition, NO regulates the vascular system by inhibiting platelet aggregation and adhesion.

  III. The role of NO in the immune system is regulated by macNOS through a pathway that is not Ca2+ dependent. Rather, exposure to cytokines, including interleukin-1 and interferon- , leads to synthesis of large amounts of NO by activation of macNOS in response to inflammatory stimuli. The NO produced plays a definitive role in the mediation of the activities of macrophages and neutrophils. NO also acts to inhibit the mechanism of viral replication.

  Based on information in the passage, which of the following is a possible effect of NO in the brain?

  A. Inhibit the fusing of neurotransmitter vesicles at the presynaptic membrane

  B. Promote neurotransmitter release into the synaptic cleft

  C. Enhance vasoconstriction to maintain blood pressure

  D. Decrease neurotransmitter synthesis by the presynaptic neuron

  Correct Answer: B

  The passage states that bNOS inhibitors have been found to block the release of neurotransmitter. Since bNOS inhibitors would decrease the production of NO, this implies that NO functions to promote the release of neurotransmitter. The passage does not describe the mechanism by which NO promotes neurotransmitter release, so any mechanism that would increase such release is a possibility. Choice A is incorrect because NO promotes the release of neurotransmitter. Inhibiting the fusing of vesicles at the presynaptic membrane would inhibit the release of neurotransmitter. Choice C is incorrect because the passage states that NO acts as a vasodilator, not as a vasoconstrictor. Besides, vasoconstriction to maintain blood pressure is not necessarily related to brain function. Choice D is incorrect because NO promotes the release of neurotransmitter. While the passage does not imply a role for NO in neurotransmitter synthesis, decreasing neurotransmitter synthesis would most likely tend to decrease the amount of neurotransmitter released.