Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 381:

  Which of the following statements identifies a chemically based sensory system?

  I. Gustatory system

  II. Auditory system

  III.

  Olfactory system

  A.

  I only

  B.

  II only

  C.

  III only

  D.

  I and III only

  Correct Answer: D

  Statement I is correct. The gustatory system, or our sense of taste, is mediated by the chemical interaction between the compounds in the food we eat and our taste bud receptors. Statement II is incorrect. The auditory system is based on the

  mechanotransduction capabilities of inner ear cells, which take a mechanical stimulus (sound waves) and convert it into a neural stimulus. Statement III in correct. The olfactory system, or our sense of smell, is mediated by the chemical

  interaction between the compounds in the air and our olfactory receptors.

  Answer choice D is, therefore, the correct answer.

• Question 382:

  According to attachment theory, which of the following children is most likely to attach to a male psychologist, previously unknown to the child, in the course of a psychological study?

  A. A two-month old female infant raised in a safe, stable environment

  B. A five-month old male infant raised in a safe, stable environment

  C. An eight-month old male infant raised by a single caregiver who frequently neglect the child

  D. A thirteen-month old female infant raised by two caregivers who occasionally neglect the child

  Correct Answer: A

  Attachment theory describes a series of steps that infants will progress through as they grow. During the first three months of life, an infant will indiscriminately attach to any person and will respond equally to any caregiver. Thus (A) is an apt description of attaching to a previously unknown adult. Around 4 to 6 months' babies will begin to recognize certain caregivers but will still accept care from anyone. Thus in (B) the baby will probably accept care from the psychologist, but the infant in (A) is much more likely to attach to the psychologist. From 6 to 9 months a baby will exhibit a strong attachment preference for a single caregiver, although the pattern of that attachment will vary based on the relationship that has developed between the caregiver and the child. Despite the neglect, the child in (C) will still have a preference for a single caregiver. After 9 months, children slowly develop increasing independence and will slowly form multiple attachments. The child in (D) will, thus, begin to develop attachments to both caregivers, but not to the psychologist, who is a stranger.

• Question 383:

  The human kidney can be damaged from a number of causes resulting in a patient's inability to filter toxins (i.e. urea) from the body which could result in death. Complete kidney failure is usually first treated with dialysis which "cleans" the blood.

  

  Figure 1: Dialysis. The "dialyzer" is a glass container that has 3 main components: (1) blood percolating through to be cleaned; (2) a dividing membrane; and (3) the dialysate. The latter is liquid containing chemicals used to draw fluids and toxins out of the bloodstream and supply electrolytes and other chemicals to the bloodstream. (kidney.niddk.nih.gov)

  Considering the information provided, which of the following is NOT consistent with the process of dialysis?

  A. The membrane in the dialyzer separating the dialysate and the blood must be semipermeable.

  B. The concentration of toxins in the dialysate must increase during dialysis.

  C. The concentration of glucose and vital minerals in the dialysate must be similar to that of the patient's blood.

  D. Excessive minerals and toxins cross the membrane in the dialyzer by osmosis.

  Correct Answer: D

  On the Surface: Dialysis only refers to the transfer of the solute (diffusion), while the transfer of the solvent is called osmosis.

  Going Deeper: Diffusion is the movement of a substance down its concentration gradient (from a region of high concentration to one of lower concentration). This is the process that must be occurring in the dialyzer to remove toxins from the blood (because the dialysate, logically, would have no toxins so the toxins defuse out of the blood into the dialysate). Note that vital substances in the blood should be kept in the blood: options would include having a dialysate with the same concentration of those substances (so they don't diffuse out of the blood) or by adjusting the permeability of the membrane.

  Osmosis is the movement of a solvent across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration. In Biology and Biochemistry, that solvent is, of course, water. In this context, the easier way to imagine osmosis is that it is simply the diffusion of water across a semi-permeable membrane from a region of high water concentration (i.e. low solute concentration) to a region of low water concentration (i.e. high solute concentration). So, in summary, osmosis deals with how much water passes through the membrane, while dialysis deals with what type of solute (i.e., sodium, protein, glucose) passes through by simple diffusion.

  N.B. Notice the link between diffusion and osmosis and the Second Law of Thermodynamics (Physics). All spontaneous change leads to an increase in the entropy (randomness) of the universe. Of course, biological systems exist because processes which increase entropy can be coupled with processes which increase order, as long as the overall (net) entropy of the universe increases.

• Question 384:

  Apoptosis is the process of programmed cell death that can occur in multicellular organisms. The proteins involved in apoptosis are associated with pathways for cell cycle arrest and DNA repair. These processes are mostly regulated through the interplay of various proteins involved in feedback loops including some of the ones shown in Figure 1.

  

  Figure 1: Feedback loops forming a regulatory network affecting apoptosis, cell cycle arrest and DNA repair. (Bioformatics Institute) According to Figure 1, CDK2 activity would most reasonably increase due to all of the following EXCEPT:

  A. degradation of p21.

  B. high cyclin G concentrations.

  C. a mutation in the gene that produces PTEN.

  D. high p53 concentrations.

  Correct Answer: D

  Notice the key in the figure which will allow us to follow each arrow that stimulates the next protein and each symbol for negative feedback which means there will be some downregulation (amount/concentration goes down). [Notice a key step in the diagram: p21 inhibits CDK2]

  Degradation of p21 implies that the concentration of p21 in its active form goes down. The diagram shows that p21 has a negative influence on CDK2. In other words, when p21 is high, CDK2 goes low. But in our instance, p21 is low (degraded) so this allows CDK2 to rise unchecked.

  High cyclin G concentrations: From the bottom of Figure 1, we can see that high cyclin G leads to high mdm2 and low p53 (notice carefully, when we leave mdm2, there is only one place to go in the diagram because all the other symbols are pointing to mdm2 and only one symbol is pointing away). Note that we used the most direct route to get to CDK2 as the question used the words "most reasonably". Low p53 means low p21 which we established will lead to a rise in CDK2.

  A mutation in the gene that produces PTEN: The great majority of mutations will result in an ineffective gene product or none at all. Thus we have a decrease in PTEN which will lead to a rise in PIP3 (if you are unsure, think of what happens if PTEN goes up, then PIP3 must go down because of the negative feedback symbol), rise in AKT, rise in mdm2, decrease in p53 which we already established means an eventual rise in CDK2.

  High p53 concentrations: clearly we get the opposite of the above, meaning a decrease in CDK2. High p53 stimulates p21 which has a negative feedback on CDK2.

• Question 385:

  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.

  Both copies of intron 1 of the amelogenin gene on the 2 sex chromosomes can be referred to as:

  A. homologous chromosomes.

  B. recessive traits.

  C. alleles.

  D. spliced RNA.

  Correct Answer: C

  Alleles are two or more alternative forms of a gene that are found at the same place on homologous chromosomes. Since there is a 184 base pair deletion on the Y homologue and there is no deletion at the same position in the X homologue, intron 1 represents two different alleles of the same amelogenin gene.

• Question 386:

  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.

  What would PCR amplification of an individual's intron 1 of the amelogenin gene reveal if the individual were male?

  A. One type of intron 1 since the individual has one X chromosome and one Y chromosome.

  B. Two types of intron 1 since the individual has only one X chromosome.

  C. One type of intron 1 since the individual has only one X chromosome.

  D. Two types of intron 1 since the individual has one X chromosome and one Y chromosome.

  Correct Answer: D

  A male individual, which contains one X and one Y chromosome, should have 2 types of intron 1. The passage states that intron 1 on the Y chromosome has a deletion which renders it smaller in length than the corresponding allele on the X chromosome, thereby providing males with 2 different size fragments. Females which have 2 X chromosomes will then have only 1 type of intron 1 of uniform size.

• Question 387:

  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 use of PCR for sex determination relies on the fact that:

  A. the amelogenin gene is responsible for an autosomal recessive trait.

  B. the X and Y homologous chromosomes have different sizes of intron 1 of the amelogenin gene.

  C. females have an X and Y chromosome and males have two X chromosomes.

  D. intron 1 has a different nucleotide length than intron 2.

  Correct Answer: B

  According to the passage, sex determination can be determined by amplifying intron 1 of the amelogenin gene which is found on the sex chromosomes. Due to a deletion, intron 1 on the Y sex homologue is shorter than intron 1 on the X sex homolog. This difference in size can be used to distinguish between males and females because males have one X and one Y chromosome while females have two X chromosomes. Therefore, females will have only one uniform size of intron 1 which does not bear the deletion. In contrast, males will have 2 different sizes of intron 1 following its amplification. Hence, B. is the correct answer choice.

• Question 388:

  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.

  Why is a heat resistant DNA polymerase required for successive replication in the polymerase chain reaction, rather than simply a human DNA polymerase?

  A. The high temperatures required to melt the DNA double strand may denature a normal human cellular DNA polymerase.

  B. The high temperatures required to melt the DNA would cause human DNA polymerase to remain bound to the DNA strand.

  C. Heat resistant DNA polymerase increases the rate of the polymerase chain reaction at high temperatures whereas human DNA polymerase lowers the rate.

  D. Heat resistant DNA polymerase recognizes RNA primers whereas human DNA polymerase does not.

  Correct Answer: A

  Since mammalian cells function at 37 oC, this is also the optimal temperature for enzyme activity. The temperatures used in PCR, which are well above 70 oC, would easily denature human DNA polymerase which is why a heat resistant DNA polymerase is required. Using human DNA polymerase would require new enzyme after each cycle, and therefore would not be very efficient.

• Question 389:

  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.

  Which of the following statements could be used to correctly describe the overall polymerase chain reaction?

  A. It is an anabolic reaction that breaks down new DNA strands.

  B. It is an anabolic reaction that synthesizes new DNA strands.

  C. It is a catabolic reaction that breaks down new DNA strands.

  D. It is a catabolic reaction that synthesizes new DNA strands.

  Correct Answer: B

  This question requires knowledge of the definition of anabolism and catabolism. A catabolic reaction involves the breakdown of macromolecules, whereas an anabolic reaction involves the synthesis of macromolecules from individual building blocks. PCR entails the synthesis (amplification) of a new DNA strand using a DNA template and free nucleotides, therefore, it is an anabolic reaction that synthesizes new DNA strands.

• Question 390:

  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). All of the following statements are consistent with Figure 1 EXCEPT:

  A. the products must have less free energy than the reactants in the exergonic reactions at the various temperatures.

  B. the equation for the equilibrium constant K used to construct the graph is derived from G = -RT ln K

  C. the 3 different temperature curves intersect at a point where the reaction is at equilibrium.

  D. higher temperatures favor relatively more of the more stable conformer.

  Correct Answer: D

  Conformational isomers exist in a dynamic equilibrium, where the relative free energies of isomers determine the population of each isomer and the energy barrier of rotation determines the rate of interconversion between isomers.

  Answer choice A: The graphs shows that for exergonic reactions (this means G < 0), the equilibrium constant K is always greater than 1 (meaning that the forward reaction is favored which implies that the products are more stable). Recall

  that the equilibrium constant K is defined as the product of the product concentrations divided by the product of reactant concentrations.

  Answer choice B: divide both sides by -RT then raise both sides to the power of e. Remember from your rules of logarithms than e to the power of ln x = x. Also recall that ln is log to the base e.

  Answer choice C: From the graph, at a free energy difference of 0 kcal/mol (= equilibrium), this also gives an equilibrium constant of 1.

  Answer choice D: A negative difference in free energy means that a conformer interconverts to a thermodynamically more stable conformation, thus the equilibrium constant will always be greater than 1. However, notice that K decreases with

  increasing temperature meaning that there is more, relatively, of the less stable conformer (in other words, energy from the higher temperature is able to override the energy barrier to conversion to the less stable conformer). Thus the statement in answer choice D is incorrect which makes answer choice D the correct answer!

  Going a bit further: notice that a positive difference in free energy means the conformer already is the more stable one, so the interconversion is an unfavorable equilibrium (K < 1). Even for highly unfavorable changes (large positive G), the

  equilibrium constant between two conformers can be increased by increasing temperature, meaning the amount of the less stable conformer present at equilibrium does increase slightly (see graph).

  Note that: An endergonic process is accompanied by or requires the absorption of energy, the products being of greater free energy than the reactants. An exergonic process is the opposite and thus accompanied by the release of energy.

  Incidentally, the enzyme subunits described in the passage exist in one of two conformations which stands for 'tensed' (T) or 'relaxed' (R), and as described relaxed subunits bind substrate more readily than those in the tense state. At any

  rate, T/R was not to be considered in this question.