MCAT-Section-2-Biological-Sciences – Section Two : Biological Sciences

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Question 1

All of the following are true regarding the function of neurons EXCEPT:

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Hyperpolarization at the end of an action potential is one mechanism by which neurons limit the rate at which action potentials may fire.
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The flow of sodium into the neuron depolarizes the membrane in the first phase of an action potential.
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The transmitting neuron secretes neurotransmitters into the synaptic cleft from its dendrites.
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An action potential is initiated when the axon hillock reaches the threshold potential.

Question 2

A certain chemical is found to inhibit the synthesis of all steroids. The synthesis of which of the following hormones would NOT be affected when a dose of this chemical is administered to a laboratory rat?

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Cortisol
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Aldosterone
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Epinephrine
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Testosterone

The adrenal cortex is another gland that secretes a family of steroid hormones called the corticosteroids. The two main types of corticosteroids are the mineralocorticoids, such as aldosterone, and the glucocorticoids, such as cortisol. This means that we can also rule out choices A and B, which leaves us with choice C, epinephrine. The adrenal medulla secretes the hormone epinephrine in response to any type of stress — good or bad. Epinephrine, along with norepinephrine, are compounds called catecholamines, and are synthesized from the amino acid tyrosine by chromaffin cells in the adrenal medulla. Hence, epinephrine synthesis would NOT be affected by this chemical, which makes choice C the correct answer.

Reference / correct answer:

Epinephrine

We’re told that a certain chemical inhibits the synthesis of steroids and we’re asked to decide which one of the hormones listed in the answer choices would not be affected if a dose of this chemical was administered to a laboratory rat. In other words, we need to know which of these hormones is NOT a steroid hormone. Steroids are a type of lipid with a carbon skeleton of four fused rings. Different types of steroids differ in the functional group that is attached to the carbon skeleton. Cholesterol is a very important steroid, and is in fact the precursor steroid molecule from which most other steroids, including steroid hormones, are synthesized. For example, the sex hormones — testosterone, estradiol, and progesterone — which are synthesized in the gonads, are steroid hormones modified from cholesterol. This means that we can rule out choice

D. The adrenal cortex is another gland that secretes a family of steroid hormones called the corticosteroids. The two main types of corticosteroids are the mineralocorticoids, such as aldosterone, and the glucocorticoids, such as cortisol. This means that we can also rule out choices A and B, which leaves us with choice C, epinephrine. The adrenal medulla secretes the hormone epinephrine in response to any type of stress — good or bad. Epinephrine, along with norepinephrine, are compounds called catecholamines, and are synthesized from the amino acid tyrosine by chromaffin cells in the adrenal medulla. Hence, epinephrine synthesis would NOT be affected by this chemical, which makes choice C the correct answer.

Question 3

Four major blood types exist in the human ABO blood system: types A, B, AB, and O; and there are three alleles that code for them. The A and B alleles are codominant, and the O allele is recessive. Blood types are derived from the presence of specific polysaccharide antigens that lie on the outer surface of the red blood cell membrane. The A allele codes for the production of the A antigen; the B allele codes for the production of the B antigen; the O allele does not code for any antigen. While there are many other antigens found on red blood cell membranes, the second most important antigen is the Rh antigen. Rh is an autosomally dominant trait coded for by 2 alleles. If this antigen is present, an individual is Rh+; if it is absent, an individual is Rh-. For example, a person with type AB blood with the Rh antigen is said to be AB+.

These antigens become most important when an individual comes into contact with foreign blood. Because of the presence of naturally occurring substances that closely mimic the A and B antigens, individuals who do not have these antigens on their red blood cells will form antibodies against them. This is inconsequential until situations such as blood transfusion, organ transplant, or pregnancy occur.

Erythroblastosis fetalis is a condition in which the red blood cells of an Rh+ fetus are attached by antibodies produced by its Rh- mother. Unlike ABO incompatibility, in which there are naturally occurring antibodies to foreign antigens, the Rh system requires prior sensitization to the Rh antigen before antibodies are produced. This sensitization usually occurs during the delivery of an Rh+ baby. So while the first baby will not be harmed, any further Rh+ fetuses are at risk.

The Coombs tests provide a method for determining whether a mother has mounted an immune response again her baby’s blood. The tests are based on whether or not agglutination occurs when Coombs reagent is added to a sample. Coombs reagent contains antibodies against the anti-Rh antibodies produced by the mother. The indirect Coombs test takes the mother’s serum, which contains her antibodies but no red blood cells, and mixes it with Rh+ red blood cells. Coombs reagent is then added. If agglutination occurs, the test is positive, and the mother must be producing anti-Rh antibodies. The direct Coombs test mixes the baby’s red blood cells with Coombs reagent. If agglutination occurs, the test is positive, and the baby’s red blood cells must have been attacked by its mother’s anti-Rh antibodies.

A couple decide to have a child. If the father’s genotype is AO and the mother has type B blood of unknown genotype, which of the following are possible blood types for their child?

I. A

II. B

III. A, B

IV. O

Select an option, then click Submit answer.

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I and II only
This means that statement I is also correct; but this doesn’t help you decide between choices B and D because they both contain statement I. In fact, you should have known that I was correct because it appears in both of these remaining choices. So what it comes down to is whether or not this child could have type O blood. Well, if the father donates an O allele, the child’s genotype will be OO and the phenotype could have type O. This means that statement IV is also correct; all four blood types are possibilities.
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I, II, and III only
Therefore, statements II and III are correct. Well, since III is correct, you can rule out choices A and C because they don’t contain it. Now let’s assume the mother’s genotype to be BO. This means that she can donate an O
allele to the child. In this case, if the father donates an A allele, the child’s genotype will be AO and the phenotype will be
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I, II, and IV only
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I, II, III, and IV
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A
II. B
III. A, B
IV. O

Reference / correct answer:

I, II, III, and IV

This is one of those questions requiring an understanding of simple genetics and the ABO system. If you didn’t already know it, you’re told in the passage that the A and B alleles are codominant to each other, and that the O allele is recessive. Codominance means that both the alleles are phenotypically expressed. So, when a person has both the A and the B alleles, a person is said to have type AB blood and expresses the properties ascribed to BOTH alleles — that is, their red blood cells have both the A and B antigens on their surface. During sexual reproduction, the mother and father each donate one allele to their offspring. In this case we know the father’s genotype is AO. This means that he can donate either an A allele or an O allele to his child. We don’t, HOWEVER, know the mother’s genotype; we only know that her phenotype is type B blood. Well, this means that her genotype is either BO or BB — we simply don’t know which one it actually is. Let’s first assume the mother to have the genotype BB and must therefore donate a B allele to the child. In this situation, if the father donates an A, the child’s genotype and phenotype will b ABk. If the father donates an O allele, the child’s genotype will be BO and the phenotype will be type

B. Therefore, statements II and III are correct. Well, since III is correct, you can rule out choices A and C because they don’t contain it. Now let’s assume the mother’s genotype to be BO. This means that she can donate an O

allele to the child. In this case, if the father donates an A allele, the child’s genotype will be AO and the phenotype will be

A. This means that statement I is also correct; but this doesn’t help you decide between choices B and D because they both contain statement I. In fact, you should have known that I was correct because it appears in both of these remaining choices. So what it comes down to is whether or not this child could have type O blood. Well, if the father donates an O allele, the child’s genotype will be OO and the phenotype could have type O. This means that statement IV is also correct; all four blood types are possibilities.

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