Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 391:

  The polymerase chain reaction (PCR) is a powerful biological tool that allows the rapid amplification of any fragment of DNA without purification. In PCR, DNA primers are made to flank the specific DNA sequence to be amplified. These primers are then extended to the end of the DNA molecule with the use of a heat- resistant DNA polymerase. The newly synthesized DNA strand is then used as the template to undergo another round of replication.

  The 1st step in PCR is the melting of the target DNA into 2 single strands by heating the reaction mixture to approximately 94 oC, and then rapidly cooling the mixture to allow annealing of the DNA primers to their specific locations. Once the primer has annealed, the temperature is elevated to 72 oC to allow optimal activity of the DNA polymerase. The polymerase will continue to add nucleotides until the entire complimentary strand of the template is completed at which point the cycle is repeated (Figure 1)

  

  Figure 1

  One of the uses of PCR is sex determination, which requires amplification of intron 1 of the amelogenin gene. This gene found on the X-Y homologous chromosomes has a 184 base pair deletion on the Y homologue. Therefore, by amplifying intron 1 females can be distinguished from males by the fact that males will have 2 different sizes of the amplified DNA while females will only have 1 unique fragment size.

  The polymerase chain reaction most likely resembles which of the following cellular process?

  A. Transcription of DNA

  B. Protein synthesis

  C. DNA replication

  D. Translation

  Correct Answer: C

  As described in the passage, the polymerase chain reaction utilizes DNA primers that anneal to the DNA template. DNA polymerase then extends the DNA primers in an effort to replicate the DNA strand. These steps are identical to those of DNA replication in a cell. However, in a cell, the cycle occurs once per cellular division.

• Question 392:

  Several models have been developed for relating changes in dissociation constants to changes in the tertiary and quaternary structures of oligomeric proteins. One model suggests that the protein's subunits can exist in either of two distinct

  conformations, R and T. At equilibrium, there are few R conformation molecules: 10 000 T to 1 R and it is an important feature of the enzyme that this ratio does not change. The substrate is assumed to bind more tightly to the R form than to

  the T form, which means that binding of the substrate favors the transition from the T conformation to R.

  The conformational transitions of the individual subunits are assumed to be tightly linked, so that if one subunit flips from T to R the others must do the same. The binding of the first molecule of substrate thus promotes the binding of the

  second and if substrate is added continuously, all of the enzyme will be in the R form and act on the substrate. Because the concerted transition of all of the subunits from T to R or back, preserves the overall symmetry of the protein, this

  model is called the symmetry model. The model further predicts that allosteric activating enzymes make the R conformation even more reactive with the substrate while allosteric inhibitors react with the T conformation so that most of the

  enzyme is held back in the T shape.

  Experiment Evaluating Non-Symmetry Model Enzymes

  Experiments were performed with enzyme conformers that did not obey the symmetry model. The data is summarized in Figure 1.

  

  Figure 1: Equilibrium distribution of two conformers at different temperatures given the free energy of their interconversion. (modified from Mr.Holmium). The symmetry model would NOT account for an enzyme:

  A. with many different biologically active conformations.

  B. which engages in positive cooperativity.

  C. with a complex metal cofactor.

  D. which is a catalyst for anabolic reactions.

  Correct Answer: A

  The symmetry model describes an instance of something which may be described as positive cooperativity (paragraph 2). The model does not exclude the enzyme from having cofactors, and places no restriction on what the enzyme's function will be. However, the symmetry model does not account for the existence of any other conformations than the two described (paragraph 2).

• Question 393:

  Several models have been developed for relating changes in dissociation constants to changes in the tertiary and quaternary structures of oligomeric proteins. One model suggests that the protein's subunits can exist in either of two distinct conformations, R and T. At equilibrium, there are few R conformation molecules: 10 000 T to 1 R and it is an important feature of the enzyme that this ratio does not change. The substrate is assumed to bind more tightly to the R form than to the T form, which means that binding of the substrate favors the transition from the T conformation to R.

  The conformational transitions of the individual subunits are assumed to be tightly linked, so that if one subunit flips from T to R the others must do the same. The binding of the first molecule of substrate thus promotes the binding of the second and if substrate is added continuously, all of the enzyme will be in the R form and act on the substrate. Because the concerted transition of all of the subunits from T to R or back, preserves the overall symmetry of the protein, this model is called the symmetry model. The model further predicts that allosteric activating enzymes make the R conformation even more reactive with the substrate while allosteric inhibitors react with the T conformation so that most of the

  enzyme is held back in the T shape.

  Experiment Evaluating Non-Symmetry Model Enzymes

  Experiments were performed with enzyme conformers that did not obey the symmetry model. The data is summarized in Figure 1.

  

  Figure 1: Equilibrium distribution of two conformers at different temperatures given the free energy of their interconversion. (modified from Mr.Holmium). Allosteric enzymes differ from other enzymes in that they:

  A. are not denatured at high temperatures.

  B. are regulated by compounds which are not their substrates and which do not bind to their active sites.

  C. they operate at an optimum pH of about 2.0.

  D. they are not specific to just one substrate.

  Correct Answer: B

  You must be familiar with how enzyme function is regulated to answer this question. An allosteric enzyme has a site other than the one for the substrate at which a molecule (not the substrate) that directs the function of the enzyme can bind.

  

  The above illustrates the allosteric regulation of an enzyme with a positive effector (on the left) and a negative effector (on the right).

• Question 394:

  Several models have been developed for relating changes in dissociation constants to changes in the tertiary and quaternary structures of oligomeric proteins. One model suggests that the protein's subunits can exist in either of two distinct conformations, R and T. At equilibrium, there are few R conformation molecules: 10 000 T to 1 R and it is an important feature of the enzyme that this ratio does not change. The substrate is assumed to bind more tightly to the R form than to the T form, which means that binding of the substrate favors the transition from the T conformation to R.

  The conformational transitions of the individual subunits are assumed to be tightly linked, so that if one subunit flips from T to R the others must do the same. The binding of the first molecule of substrate thus promotes the binding of the second and if substrate is added continuously, all of the enzyme will be in the R form and act on the substrate. Because the concerted transition of all of the subunits from T to R or back, preserves the overall symmetry of the protein, this model is called the symmetry model. The model further predicts that allosteric activating enzymes make the R conformation even more reactive with the substrate while allosteric inhibitors react with the T conformation so that most of the enzyme is held back in the T shape.

  Experiment Evaluating Non-Symmetry Model Enzymes

  Experiments were performed with enzyme conformers that did not obey the symmetry model. The data is summarized in Figure 1.

  

  Figure 1: Equilibrium distribution of two conformers at different temperatures given the free energy of their interconversion. (modified from Mr.Holmium).

  The substrate binds more tightly to R because:

  A. T has a higher affinity for the substrate than R.

  B. R has a higher affinity for the substrate than T.

  C. there are 10 000 times more T conformation molecules than R conformation molecules.

  D. the value of the equilibrium constant does not change.

  Correct Answer: B

  If a molecule has a high affinity for something, it is likely to be associated with it maximally. The substrate binds more tightly to the R conformation even though the R conformation is present in small amounts because R has a higher affinity for the substrate than T.

• Question 395:

  Several models have been developed for relating changes in dissociation constants to changes in the tertiary and quaternary structures of oligomeric proteins. One model suggests that the protein's subunits can exist in either of two distinct conformations, R and T. At equilibrium, there are few R conformation molecules: 10 000 T to 1 R and it is an important feature of the enzyme that this ratio does not change. The substrate is assumed to bind more tightly to the R form than to the T form, which means that binding of the substrate favors the transition from the T conformation to R.

  The conformational transitions of the individual subunits are assumed to be tightly linked, so that if one subunit flips from T to R the others must do the same. The binding of the first molecule of substrate thus promotes the binding of the second and if substrate is added continuously, all of the enzyme will be in the R form and act on the substrate. Because the concerted transition of all of the subunits from T to R or back, preserves the overall symmetry of the protein, this model is called the symmetry model. The model further predicts that allosteric activating enzymes make the R conformation even more reactive with the substrate while allosteric inhibitors react with the T conformation so that most of the enzyme is held back in the T shape.

  Experiment Evaluating Non-Symmetry Model Enzymes

  Experiments were performed with enzyme conformers that did not obey the symmetry model. The data is summarized in Figure 1.

  

  Figure 1: Equilibrium distribution of two conformers at different temperatures given the free energy of their interconversion. (modified from Mr.Holmium).

  The symmetry model describes a form of cooperative binding. Most enzymes do not engage in cooperative binding. The predicted shape of a graph representing reaction rate versus the addition of substrate to most enzymes would be expected to be:

  A. a hyperbola.

  B. a straight line with a positive slope.

  C. a straight line with a negative slope.

  D. sigmoidal.

  Correct Answer: A

  The amount of substrate-enzyme complex would increase steadily as more substrate is added until a point at which all enzymes are involved in a substrate-enzyme complex, and any more substrate added will have no effect (saturation kinetics). The graph would show a steadily slowing curve of positive slope which reaches a point at which it levels off into a horizontal line. This curve is called a hyperbola (see image below). A sigmoidal shape would be expected in cooperative binding (i.e. the symmetry model as described in the passage or hemoglobin). Note: This was a classic question in the old MCAT and, not surprisingly, the same concept comes up in the AAMC's new MCAT practice materials: the difference between the simple (rectangular) hyperbola and the sigmoidal curve suggesting cooperative binding (and also, the ability to recognize the shapes of these 2 curves independently). Also note that the myoglobin saturation curve is a hyperbola, but hemoglobin has a sigmoid shape due to the cooperative binding of oxygen molecules.

  And finally, note the positions of Vmax (= maximum velocity/reaction rate) and Km (substrate concentration at 1/2 Vmax) displaying Michaelis-Menten kinetics associated with the hyperbolic curve on the left, as opposed to the sigmoidal curve on the right (image from the GS BIO book or ebook, BCM 2.9):

  

• Question 396:

  The process of depolarization triggers the cardiac cycle. The electronics of the cycle can be monitored by an electrocardiogram (EKG). The cycle is divided into two major phases, both named for events in the ventricle: the period of ventricular contraction and blood ejection, systole, followed by the period of ventricular relaxation and blood filling, diastole.

  During the very first part of systole, the ventricles are contracting but all valves in the heart are closed thus no blood can be ejected. Once the rising pressure in the ventricles becomes great enough to open the aortic and pulmonary valves, the ventricular ejection or systole occurs. Blood is forced into the aorta and pulmonary trunk as the contracting ventricular muscle fibers shorten. The volume of blood ejected from a ventricle during systole is termed stroke volume.

  During the very first part of diastole, the ventricles begin to relax, and the aortic and pulmonary valves close. No blood is entering or leaving the ventricles since once again all the valves are closed. Once ventricular pressure falls below atrial pressure, the atrioventricular (AV) valves open. Atrial contraction occurs towards the end of diastole, after most of the ventricular filling has taken place. The ventricle receives blood throughout most of diastole, not just when the atrium contracts.

  Figure 1: Electronic and pressure changes in the heart and aorta during the cardiac cycle.

  

  The wall of the left ventricle is at least three times as thick as that of the right ventricle. This feature aids circulation by assuring that:

  A. blood entering the pulmonary artery is at a much higher pressure than blood entering the aorta.

  B. blood entering the aorta is at a much higher pressure than blood entering the pulmonary artery.

  C. the left ventricle has a higher blood capacity than the right ventricle at all times.

  D. the right ventricle has a higher blood capacity than the left ventricle at all times.

  Correct Answer: B

  It is commonly known that as a rule, the size of a muscle is proportional to its strength. The heart, which is a muscle, contains a chamber which must pump blood into the aorta to perfuse the grand majority of the body's tissues. Clearly, this chamber (= the left ventricle) must contain thicker muscle (= stronger) than a chamber that pumps blood only to the lungs (= the right ventricle through the pulmonary artery). The stronger chamber pumps blood with a greater force which means a higher pressure (recall from physics: P = F/A).

• Question 397:

  The process of depolarization triggers the cardiac cycle. The electronics of the cycle can be monitored by an electrocardiogram (EKG). The cycle is divided into two major phases, both named for events in the ventricle: the period of ventricular contraction and blood ejection, systole, followed by the period of ventricular relaxation and blood filling, diastole.

  During the very first part of systole, the ventricles are contracting but all valves in the heart are closed thus no blood can be ejected. Once the rising pressure in the ventricles becomes great enough to open the aortic and pulmonary valves, the ventricular ejection or systole occurs. Blood is forced into the aorta and pulmonary trunk as the contracting ventricular muscle fibers shorten. The volume of blood ejected from a ventricle during systole is termed stroke volume.

  During the very first part of diastole, the ventricles begin to relax, and the aortic and pulmonary valves close. No blood is entering or leaving the ventricles since once again all the valves are closed. Once ventricular pressure falls below atrial pressure, the atrioventricular (AV) valves open. Atrial contraction occurs towards the end of diastole, after most of the ventricular filling has taken place. The ventricle receives blood throughout most of diastole, not just when the atrium contracts.

  Figure 1: Electronic and pressure changes in the heart and aorta during the cardiac cycle.

  

  According to Fig. 1, the opening of the aortic and pulmonary valves is NOT associated with:

  A. ventricular systole.

  B. a rise and fall in aortic pressure.

  C. a drop and rise in left ventricular volume.

  D. the third phase of the cardiac cycle.

  Correct Answer: C

  Close analysis of Figure 1 (compare to previous questions especially Q2) reveals that during the period that the aortic and pulmonary valves are OPEN, the curve for left ventricular volume drops (= decreases) but does not rise (= increase).

• Question 398:

  Several models have been developed for relating changes in dissociation constants to changes in the tertiary and quaternary structures of oligomeric proteins. One model suggests that the protein's subunits can exist in either of two distinct conformations, R and T. At equilibrium, there are few R conformation molecules: 10 000 T to 1 R and it is an important feature of the enzyme that this ratio does not change. The substrate is assumed to bind more tightly to the R form than to the T form, which means that binding of the substrate favors the transition from the T conformation to R.

  The conformational transitions of the individual subunits are assumed to be tightly linked, so that if one subunit flips from T to R the others must do the same. The binding of the first molecule of substrate thus promotes the binding of the second and if substrate is added continuously, all of the enzyme will be in the R form and act on the substrate. Because the concerted transition of all of the subunits from T to R or back, preserves the overall symmetry of the protein, this model is called the symmetry model. The model further predicts that allosteric activating enzymes make the R conformation even more reactive with the substrate while allosteric inhibitors react with the T conformation so that most of the enzyme is held back in the T shape.

  Experiment Evaluating Non-Symmetry Model Enzymes

  Experiments were performed with enzyme conformers that did not obey the symmetry model. The data is summarized in Figure 1.

  

  Figure 1: Equilibrium distribution of two conformers at different temperatures given the free energy of their interconversion. (modified from Mr.Holmium). What assumption is made about the T and R conformations and the substrate?

  A. In the absence of any substrate, the T conformation predominates.

  B. In the absence of any substrate, the R conformation predominates.

  C. In the absence of any substrate, the T and R conformations are in equilibrium.

  D. In the absence of any substrate, the enzyme exists in another conformation, S.

  Correct Answer: A

  Explanation: Paragraph 1. Information concerning the relative amounts of T and R conformations present before substrate is added is given in the passage.

• Question 399:

  The process of depolarization triggers the cardiac cycle. The electronics of the cycle can be monitored by an electrocardiogram (EKG). The cycle is divided into two major phases, both named for events in the ventricle: the period of ventricular contraction and blood ejection, systole, followed by the period of ventricular relaxation and blood filling, diastole.

  During the very first part of systole, the ventricles are contracting but all valves in the heart are closed thus no blood can be ejected. Once the rising pressure in the ventricles becomes great enough to open the aortic and pulmonary valves, the ventricular ejection or systole occurs. Blood is forced into the aorta and pulmonary trunk as the contracting ventricular muscle fibers shorten. The volume of blood ejected from a ventricle during systole is termed stroke volume.

  During the very first part of diastole, the ventricles begin to relax, and the aortic and pulmonary valves close. No blood is entering or leaving the ventricles since once again all the valves are closed. Once ventricular pressure falls below atrial pressure, the atrioventricular (AV) valves open. Atrial contraction occurs towards the end of diastole, after most of the ventricular filling has taken place. The ventricle receives blood throughout most of diastole, not just when the atrium contracts.

  Figure 1: Electronic and pressure changes in the heart and aorta during the cardiac cycle.

  

  Would the walls of the atria or ventricles expected to be thicker?

  A. Atria, because blood ejection due to atrial contraction is high.

  B. Atria, because blood ejection due to atrial contraction is low.

  C. Ventricles, because ventricular stroke volume is high.

  D. Ventricles, because ventricular stroke volume is low.

  Correct Answer: C

  Thicker walls in a particular part of the heart would indicate that it is more muscular, and therefore is more efficient or forceful during its contraction. This immediately rules out answer choices B and D, as they suggest that the thicker walled chamber would be less efficient or forceful. It should be known from the biology review (BIO 7.2) that the ventricles are more muscular (thicker-walled), but in the event that this is not initially known, information regarding the function of both the atria and the ventricles from the passage may help lead to this conclusion. First, systole, the period of contraction, refers to the period of contraction of the ventricles not the atria (paragraph 1, line 5). This would indicate that the contraction of the ventricles might be more relevant in some way. Second, during diastole, atrial contraction occurs after most of the ventricle is already filled (paragraph 3, sentence 4) and serves to push the small amount of blood necessary to complete the filling into the ventricles. Since the atrial contraction does not need to move a large amount of blood (the ventricle is already mostly full), it does not need to be as muscular.

• Question 400:

  The process of depolarization triggers the cardiac cycle. The electronics of the cycle can be monitored by an electrocardiogram (EKG). The cycle is divided into two major phases, both named for events in the ventricle: the period of ventricular contraction and blood ejection, systole, followed by the period of ventricular relaxation and blood filling, diastole.

  During the very first part of systole, the ventricles are contracting but all valves in the heart are closed thus no blood can be ejected. Once the rising pressure in the ventricles becomes great enough to open the aortic and pulmonary valves, the ventricular ejection or systole occurs. Blood is forced into the aorta and pulmonary trunk as the contracting ventricular muscle fibers shorten. The volume of blood ejected from a ventricle during systole is termed stroke volume.

  During the very first part of diastole, the ventricles begin to relax, and the aortic and pulmonary valves close. No blood is entering or leaving the ventricles since once again all the valves are closed. Once ventricular pressure falls below atrial pressure, the atrioventricular (AV) valves open. Atrial contraction occurs towards the end of diastole, after most of the ventricular filling has taken place. The ventricle receives blood throughout most of diastole, not just when the atrium contracts.

  Figure 1: Electronic and pressure changes in the heart and aorta during the cardiac cycle.

  

  The graph below shows the effects on stroke volume of stimulating the sympathetic nerves to the heart.

  

  According to the graph, the net result of sympathetic stimulation on stroke volume is to:

  A. approximately double stroke volume at any given end diastolic volume.

  B. decrease stroke volume at any given end diastolic volume.

  C. increase stroke volume at any given end diastolic volume.

  D. leave stroke volume relatively unchanged.

  Correct Answer: C

  This question is easily answered by reading the graph. Compared to the curve labeled 'control', the curve labeled 'sympathetic stimulation' is always at a higher stroke volume for a given end-diastolic volume. However, by looking at stroke volume values for both curves at any end-diastolic volume value (i.e. 200), it is clear that the stroke volume for sympathetic stimulation is always less than twice the control (i.e. 160 vs. 100), thus eliminating answer choice A. Thus, we are left with answer choice C.