Question 1

Assume a single-celled organism was discovered in water above 100°C.  On closer inspection, it is determined that this organism was previously classified under the Archaea domain.  Given this information, this organism most likely has:

Accepted Answer

11ecba2d_0fd5_d833_a3bf_a1e80717fd17_MD0008_00 Cells can exist in many forms and sizes; some live as independent (free-living) organisms and others reside as a group in a colony or comprise the body of a multicellular organism. Based on their shared characteristics, all living organisms can be classified into one of three domains: Bacteria, Archaea, or Eukarya. Eukaryotic organisms fall under the Eukarya domain, but prokaryotic organisms fall under either the Bacteria or the Archaea domain. In this scenario, the organism is a member of the Archaea domain. Organisms classified under Archaea share traits with bacteria. Like bacteria, archaea are unicellular (single-celled), have a circular chromosome, have no membrane-bound organelles or nucleus, and reproduce asexually via binary fission. However, unlike bacteria, archaea do not have the chemical peptidoglycan in their cell wall (Choice A). (Choice C) Telomeres are regions of repetitive nucleotide sequences at the ends of eukaryotic chromosomes that shorten with each round of cell division. Telomeres are present only on linear eukaryotic chromosomes and protect these ends from deteriorating. In contrast, both archaea and bacteria have circular chromosomes (lacking telomeres). (Choice D) The nucleus, along with membrane-bound organelles (eg, Golgi body, endoplasmic reticulum), is found only in eukaryotic cells. Archaea and bacteria are known as prokaryotes because they lack membrane-bound organelles and have no nucleus. Educational objective: Prokaryotes are unicellular organisms that can be further classified into the domains of Archaea or Bacteria. Organisms in these domains lack a nucleus and membrane-bound organelles but have circular chromosomes and are able to reproduce asexually through binary fission. However, bacteria (not archaea) have peptidoglycan in their cell wall as a distinguishing feature.

Question 2

Passage Domestic pigs have been used as valuable preclinical models for scientific research due to their numerous anatomical, developmental, and physiological similarities to humans. For example, oogenesis and embryonic development in pigs are remarkably similar to the human processes and have facilitated the study of impaired fertilization due to decreased oocyte competence (quality).Researchers have hypothesized that early termination of embryo development can arise from low mitochondrial DNA (mtDNA) copy number. Because mtDNA replication and reduction events have been extensively mapped in pigs, oocytes and sperm were isolated from their respective porcine gonads to test this hypothesis.Experiment 1To evaluate the developmental competence of porcine oocytes, researchers first measured the activity of glucose-6-phosphate dehydrogenase (G6PD), an enzyme that is downregulated as the oocyte matures. The oocytes were stained with Brilliant Cresyl Blue (BCB), a nontoxic blue dye that is converted to a colorless compound by G6PD. After 90 minutes of staining, oocytes were divided into two groups: one contained oocytes with colorless cytoplasm (BCBS1U1P1−S1S1P0) and the other contained oocytes with blue cytoplasm (BCBS1U1P1+S1S1P0). Gene expression profiles were compared to evaluate the number of oocytes in the BCBS1U1P1+S1S1P0 and BCBS1U1P1−S1S1P0 groups that had developed to metaphase II.Experiment 2BCBS1U1P1+S1S1P0 and BCBS1U1P1−S1S1P0 oocytes underwent in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), or supplementation with homogeneous mitochondria and ICSI (mICSI). During IVF, an isolated oocyte was placed in a well plate and incubated with sperm at 39 C for 4 hours. In ICSI, a single sperm was selected and injected directly into the oocyte.Table 1 Developmental Rates for BCBS1U1P1+S1S1P0 and BCBS1U1P1−S1S1P0 Oocytes Fertilized Subjected to IVF, ICSI, or mICSI Adapted from Cagnone GL, Tsai TS, Makanji Y, et al. Sci Rep. 2016;6:23229. -Which of the following statements best describes a BCB⁺ oocyte that is arrested in metaphase II?

Accepted Answer

11ecba2d_0fcb_29be_a3bf_ab7c99e58643_MD0008_00 Female gametogenesis (oogenesis) is a hormone-driven process that occurs in the female gonads (the ovaries) and involves mitosis and two meiotic divisions. Oogenesis begins in utero at approximately 4 weeks gestation. In the female embryo, oogonia (germ cells) are diploid stem cells that first multiply quickly via mitosis and become primary oocytes. The primary oocytes then commence the first meiotic division but become arrested at prophase I. From infancy to puberty, the ovaries are functionally inactive and the primary oocytes remain stalled in prophase I of meiosis. The chromosome pairs are arranged in a tetrad during this phase, and their proximity allows for genetic recombination. At puberty, ovulatory cycles begin, and the female is capable of reproduction. During each menstrual cycle, stimulation by follicle-stimulating hormone (FSH) followed by a surge in luteinizing hormone (LH) causes some primary oocytes to resume meiosis I. The last stage of meiosis I (telophase I) in a primary oocyte yields two unevenly divided haploid cells. One cell is the secondary oocyte, which has almost all the cytoplasm of the primary oocyte; the other is the first polar body (Choice B), which is smaller and does not develop into a mature oocyte. The secondary oocyte begins meiosis II, but it halts in metaphase II and the polar body degenerates. The secondary oocyte remains frozen in metaphase II until fertilization occurs, at which point it completes its second meiotic division (telophase II) into an ootid and second polar body. The zygote that results from fertilization is diploid due to genetic contribution from both the ovum and sperm nuclei. In the given scenario, a BCB⁺ porcine oocyte is similar to that of humans. The metaphase II arrest of the BCB⁺ oocyte is the typical stage at which an oocyte can be fertilized by a spermatozoon (Choice C), and this completes the second meiotic division. (Choice D) Nondisjunction, the failure of one or more pairs of sister chromatids to separate, can occur during anaphase of either meiosis I or meiosis II. Although homologous chromosomes have already segregated during anaphase I, nondisjunction of sister chromatids may still occur during anaphase II after fertilization. Educational objective: Primary oocytes are present in female embryos and remain arrested in prophase I of meiosis from birth to puberty. During puberty, normal menstrual cycle hormones stimulate some primary oocytes to resume differentiating into a secondary oocyte. However, prior to fertilization, secondary oocytes are arrested in metaphase II of meiosis.

Question 3

Which step(s) of the viral life cycle would most likely occur in lytic phages?Injection of viral genetic material into the bacterial hostIntegration of viral DNA into the bacterial genomeDegradation of the host genome

Accepted Answer

11ecba2d_0fd1_b978_a3bf_afae43108a3b_MD0008_00 All viruses are obligate intracellular parasites because they cannot replicate independently outside of a host cell. Bacteriophages are viruses that exclusively infect bacteria. The life cycle of a particular bacteriophage can be lytic or lysogenic, which will determine the method by which the phage uses the host's cellular machinery to create new progeny. The general steps involved during the lytic life cycle are described as follows: Attachment: The bacteriophage comes in contact with the bacterial cell wall and attaches to the host bacterium using its tail fibers. Viral genome entry: The phage uses its tail sheath to inject its genome into the cytoplasm of the bacterial host (Number I). Host genome degradation: Viral enzymes degrade the host genome into its nucleotide components to provide the building blocks for replication of the viral genome (Number III). Synthesis: Loss of the bacterial chromosome ends synthesis of host molecules. As a result, the host machinery (eg, ribosomes), now under the control of the viral genome, begins to synthesize the components needed for new viral progeny, which then assemble inside the host cell. Release: Many newly assembled viral progeny (virions) are released as the bacterium disintegrates (lysis) due to the action of lysozymes on the host cell wall. (Number II) In contrast, bacteriophages that have a lysogenic life cycle also attach to the bacterium but integrate their genome into the host DNA and are called prophages. This viral genome integration enables the host cell to survive and divide normally. While the host cell divides, the integrated viral DNA is replicated with the host DNA, and associated viral repressor proteins prevent the viral genome from being transcribed (latency). Induction can occur in response to environmental factors (eg, chemicals, radiation), causing the viral genome to excise itself from the host chromosome and enter the lytic life cycle. Educational objective: Bacteriophages use the host cell's machinery and resources to replicate their genome and synthesize viral proteins to form new virions. Phages with a lytic life cycle replicate rapidly and release progeny via lysis of the host cell. In contrast, phages with a lysogenic life cycle integrate their genome with the host genome and replicate as the cell divides.

Question 4

Which of the following helps maintain the resting membrane potential of a neuron?Passive transportAdenosine triphosphateMembrane selective permeability

Accepted Answer

11ecba2d_0fd1_4447_a3bf_6b2e59745d5f_MD0008_00 The unequal concentration of charged ions between the extracellular and intracellular fluid of all living cells results in an electrochemical gradient across the membrane that determines the membrane potential (voltage difference). The resting membrane potential of neurons is primarily due to the high concentration of potassium ions (K⁺) and the low concentration of sodium ions (Na⁺) inside the cell as compared to the outside. The following mechanisms contribute to maintenance of the resting membrane potential: Protein channels in the cell membrane enable certain ions to move down their concentration gradient across the membrane. For example, in resting neurons, potassium leak channels help maintain the membrane potential by enabling the passive transport (without using energy) of K⁺ out of the cell. Because the membrane is more permeable to K⁺ than to Na⁺ (ie, selective permeability), the resting membrane potential of neurons is approximately −70 mV, which is close to the negative equilibrium potential of K⁺ (Numbers I and III). Active transport pumps embedded in the outer membrane of neurons hydrolyze adenosine triphosphate (ATP) to provide energy to transport molecules against their concentration gradient. For example, sodium-potassium pumps (Na⁺K⁺ ATPase) transport 2 K⁺ into the cell for every 3 Na⁺ moved out of the cell. This is important for maintaining the unequal concentration of ions across the membrane; without active transport pumps, leakage of ions through the cell membrane would eventually result in equilibration and a membrane potential of 0 mV (Number II). Educational objective: The presence of protein channels in the cell membrane allows passive transport of certain ions down their electrochemical gradient. This selective membrane permeability is responsible for generating the resting membrane potential in nerve and muscle cells. Active transport pumps help maintain the concentration gradient and are critical for maintaining the resting membrane potential.

Question 5

Human sperm and oocytes are substantially different in all of the following aspects EXCEPT:

Accepted Answer

11ecba2d_0fd4_0370_a3bf_ebb4e2d2e164_MD0008_00 Ploidy is defined as the number of homologous chromosome sets in the nucleus of a cell. In the human body, with the exception of sperm and egg cells (reproductive cells), nucleated cells are diploid and known as somatic cells. Somatic cells have 46 individual chromosomes that appear as 23 pairs. Each somatic cell has two sets/copies (2N) of each chromosome, these include 22 pairs of homologous chromosomes (autosomes) and 1 pair of sex chromosomes. In contrast, mature egg and sperm are both haploid (1N) because they each contain 22 (unpaired) autosomes and 1 sex chromosome, resulting in 23 chromosomes. During fertilization, the egg and sperm form a zygote with a nucleus containing the full complement of 46 chromosomes. These 46 chromosomes are found in pairs, and in each pair one chromosome comes from the mother and one comes from the father. Accordingly, egg and sperm cells are similar in ploidy as they both are haploid cells and contribute the same number of chromosomes to a zygote. (Choice A) Sperm are the smallest cells in the human body and mostly contain a nucleus, an acrosome, mitochondria, and a negligible amount of cytoplasm. In contrast, the egg is the largest human cell and has organelles and a large amount of cytoplasm. (Choice B) During gestation, the germ cells of the embryo first undergo numerous mitotic divisions to proliferate, after which they are primed for meiosis, a type of cell division that results in the formation of gametes. However, the testes of newborn boys contain only immature germ cells (spermatogonia) that have not progressed beyond the mitotic stages. These immature spermatogonia begin to undergo meiosis at puberty (early teen years) when the gonads mature. In contrast, newborn girls have primary oocytes arrested at prophase of meiosis I and remain in this state until ovulatory cycles begin at puberty. (Choice D) In the adult male, millions of sperm cells are produced daily after the onset of puberty. However, females are born with their full complement of oocytes, and only a few mature during each menstrual cycle, which occurs approximately once every 28 days. Educational objective: Egg and sperm cells are haploid cells that contribute an equal number of chromosomes to a zygote during fertilization.

Question 6

A student adds 5 bacterial cells to a test tube containing fresh medium and incubates it for 3 hours at 37°C.  If the bacterial population shows an initial lag phase of 20 minutes followed by a doubling time of 40 minutes, what is the approximate number of bacterial cells present at the end of the incubation period?  (Note: Assume unrestricted growth during incubation.)

Accepted Answer

11ecba2d_0fd5_3bf2_a3bf_aded2839bed9_MD0008_00 Binary fission is the process by which unicellular organisms such as bacteria reproduce asexually. During this process, the parental bacterial cell doubles in size and then divides into two identical daughter cells. Subsequently, each daughter cell also divides into two, yielding four cells that then continue to divide in a similar manner. This type of growth is called exponential; the generation time (doubling time) is the time needed for a bacterial population to double, or the time needed for a single bacterium to divide into two. When a single cell divides by binary fission, the number of cells that result after a specified time is calculated by multiplying the original number of bacterial cells by 2ⁿ, where n equals the number of generations. The rate at which a bacterial population grows can be demonstrated by plotting a typical growth curve, which has a lag, log, stationary, and death phase. In the lag phase, bacteria become accustomed to the local conditions and synthesize the components (eg, enzymes, RNA) required for metabolism but do not divide. In the subsequent log phase, the bacterial population grows exponentially as bacteria are able to metabolize nutrients. According to the question, 5 bacterial cells were incubated for 3 hours (180 minutes); these exhibited a 20-minute lag phase and a 40-minute generation time. Assuming unrestricted growth during incubation, the bacterium could only have had a log phase and a lag phase. Therefore, the number of cells that result after incubation can be calculated as follows: Calculate the amount of time in which the bacterial population was in log phase: 180 min incubation time − 20 min lag phase = 160 min log phase Find the number of generations in the log phase period: 160 min log phase / 40 min generation time = 4 generations Multiply the original number of bacterial cells by 2ⁿ to get the final population size: 5 × 2⁴ = 80 cells Educational objective: Binary fission is the process by which a unicellular organism (eg, bacterium) divides into two identical daughter cells. The generation time refers to the time in which a bacterial population doubles. When a single cell divides by binary fission, the number of cells that result is calculated by multiplying the original number of bacterial cells by 2ⁿ, where n equals the number of generations.

Question 7

In a neuron, mitochondrial biogenesis is believed to occur primarily in the cell body, but mitochondria are often positioned at the presynaptic terminal, a distal site with high metabolic demand.  Given this information, which molecular mechanism is most likely responsible for mitochondrial transport from the cell body to the presynaptic terminal?

Accepted Answer

11ecba2d_0fd3_8e3f_a3bf_25ad4e9d7787_MD0008_00 A eukaryotic cell's cytoskeleton is an intracellular scaffolding (network) of fibers interspersed throughout the cytoplasm. The cytoskeleton is composed of three types of fibers: Microfilaments, intermediate filaments, and microtubules. Together, these fibers function to organize cellular components, support cellular motility (eg, cell movement, intracellular transport), and give the cell its shape. Microtubules are structural cytoplasmic filaments composed of tubulin subunits. These filaments have a number of critical functions, including serving as tracks for intracellular transport of organelles and vesicles. The movement of intracellular cargo along microtubules is mediated by two motor proteins: Kinesin: Moves intracellular cargo along microtubules in anterograde axonal transport (ie, away from the nucleus and toward distal sites). Dynein: Participates in retrograde axonal transport of intracellular cargo (ie, from distal sites toward the nucleus) (Choice D). According to the question, mitochondria are transported from the cell body toward the presynaptic terminal. Because mitochondria are being transferred from the nucleus and toward the synaptic terminal, this anterograde transport is most likely being performed by kinesin motors moving along the microtubules. (Choices B and C) Movement along actin microfilaments occur via myosin motors (not kinesin or dynein). The primary functions of myosin movement along microfilaments are cellular locomotion, cellular division, and muscle contractions. Educational objective: The intracellular scaffolding of a eukaryotic cell is composed of three families of protein filaments: Microfilaments, intermediate filaments, and microtubules. Intracellular transport of cargo (eg, organelles, vesicles) is mediated primarily by two microtubular motor proteins (kinesin and dynein). Kinesin mediates anterograde transport (ie, away from the nucleus) whereas dynein mediates retrograde transport (ie, toward the nucleus).

Question 8

Passage
A 6-day-old, genotypically normal (46,XX) female patient shows symptoms of hypoglycemia (low blood glucose) and hypotonia (decreased muscle tone).  Physical examination reveals abnormal development of the external genitalia, and laboratory studies show elevated urinary sodium content, increased serum levels of potassium, and abnormal serum concentration of several steroid hormones.  The patient is diagnosed with 21-hydroxylase deficiency, the most common type of congenital adrenal hyperplasia (CAH).CAH refers to a group of autosomal recessive diseases, each associated with defective production of one or more steroid hormones by the adrenal glands, two crescent-shaped glands located above each kidney in the abdominal cavity.  It has been reported that unequal crossing over involving chromosome 6 can result in CAH.  More than 90% of CAH diagnoses are associated with a deficiency of 21-hydroxylase, an enzyme encoded by the CYP21A2 gene on chromosome 6.Normally, 21-hydroxylase catalyzes chemical reactions involved in the synthesis of aldosterone and cortisol, the principal mineralocorticoid and glucocorticoid hormones, respectively.  As a result, patients with 21-hydroxylase deficiency exhibit diminished serum levels of aldosterone and cortisol.  However, synthesis and secretion of the sex hormone testosterone is excessive.  Glucocorticoid deficiency disrupts the negative feedback loop linking the brain to the adrenal gland, leading to hyperplasia (increased cell proliferation) of adrenal tissue.The patient's 21-hydroxylase deficiency is treated with regular administration of synthetic steroid hormones.  Her condition improves after receiving these medications.
-The neuroendocrine signaling cascade that induces adrenal hyperplasia involves coordinated activity among which organs?

Accepted Answer

11ecba2d_0fcc_b060_a3bf_7b5c6fc7043e_MD0008_00 The hypothalamus is a brain structure located inferior to the thalamus that regulates the synthesis and secretion of multiple classes of hormones. It serves as the interface between the nervous and endocrine systems by processing both cortical inputs from the brain and sensing the plasma concentration of numerous hormones. In response to these inputs, the hypothalamus controls body-wide endocrine function by influencing activity within the pituitary gland, an endocrine organ located inferior to the hypothalamus that is composed of a posterior and an anterior lobe. The posterior lobe of the pituitary gland contains the axonal processes of hypothalamic neurons. The hormones antidiuretic hormone (ADH) and oxytocin are synthesized by these specialized neurons and stored within their axon terminals, which form junctions with systemic blood vessels. As a result, the firing of hypothalamic neurons results in the release of ADH or oxytocin into the systemic circulation. The hypothalamus controls the anterior lobe of the pituitary gland by releasing signaling molecules into the hypophyseal portal system, a localized network of blood vessels that bridges the hypothalamus and anterior pituitary. These signaling molecules promote or inhibit the release of specific hormones into the systemic circulation by the anterior pituitary gland. For example, corticotrophin-releasing hormone (CRH) released by the hypothalamus stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH) into the systemic circulation. ACTH then acts on the adrenal glands to promote synthesis and secretion of glucocorticoids, completing the neuroendocrine pathway that regulates the serum level of cortisol and other glucocorticoids. (Choices A and D) The parathyroid glands secrete parathyroid hormone (PTH), which functions to regulate serum Ca²⁺ levels. The parathyroid gland does not directly regulate the synthesis and secretion of glucocorticoids. (Choice C) Although the liver is responsible for numerous functions (eg, carbohydrate and fat metabolism), it is not involved in neuroendocrine control of the adrenal glands. Educational objective: The hypothalamus is a brain structure that regulates numerous endocrine functions through its association with the pituitary gland. Together, the hypothalamus and pituitary gland regulate the synthesis and secretion of hormones that influence metabolism, reproduction, and other important functions.

Question 9

Passage
A 6-day-old, genotypically normal (46,XX) female patient shows symptoms of hypoglycemia (low blood glucose) and hypotonia (decreased muscle tone).  Physical examination reveals abnormal development of the external genitalia, and laboratory studies show elevated urinary sodium content, increased serum levels of potassium, and abnormal serum concentration of several steroid hormones.  The patient is diagnosed with 21-hydroxylase deficiency, the most common type of congenital adrenal hyperplasia (CAH).CAH refers to a group of autosomal recessive diseases, each associated with defective production of one or more steroid hormones by the adrenal glands, two crescent-shaped glands located above each kidney in the abdominal cavity.  It has been reported that unequal crossing over involving chromosome 6 can result in CAH.  More than 90% of CAH diagnoses are associated with a deficiency of 21-hydroxylase, an enzyme encoded by the CYP21A2 gene on chromosome 6.Normally, 21-hydroxylase catalyzes chemical reactions involved in the synthesis of aldosterone and cortisol, the principal mineralocorticoid and glucocorticoid hormones, respectively.  As a result, patients with 21-hydroxylase deficiency exhibit diminished serum levels of aldosterone and cortisol.  However, synthesis and secretion of the sex hormone testosterone is excessive.  Glucocorticoid deficiency disrupts the negative feedback loop linking the brain to the adrenal gland, leading to hyperplasia (increased cell proliferation) of adrenal tissue.The patient's 21-hydroxylase deficiency is treated with regular administration of synthetic steroid hormones.  Her condition improves after receiving these medications.
-Patients with CAH exhibit hypoglycemia due to numerous endocrine abnormalities.  In healthy individuals, hypoglycemia will trigger an increase in the serum level of:insulin.epinephrine.glucagon.

Accepted Answer

11ecba2d_0fcd_4ca1_a3bf_657b0c895cae_MD0008_00 Homeostasis is the process by which an organism maintains a relatively stable internal environment in response to external or internal changes. For example, blood pressure, blood pH, and body temperature are all maintained within specific ranges that are optimal for normal bodily function. Failure to achieve homeostasis of one or more parameters can lead to disease and, potentially, death. The serum level of glucose is also subject to homeostatic control primarily through the actions of hormones. When blood glucose levels are low (eg, during periods of fasting), the hormone glucagon is secreted from the alpha cells of the pancreas, and the autonomic nervous system promotes the release of epinephrine and norepinephrine by the adrenal medulla. Glucagon is a peptide hormone that promotes gluceoneogenesis (the synthesis of glucose from other molecules) and glycogenolysis (the breakdown of glycogen into glucose). Epinephrine and norepinephrine also promote glycogenolysis. Therefore, the overall effect of glucagon, epinephrine, and norepinephrine is to promote the synthesis of glucose and its release into the systemic circulation, thereby correcting the low serum level of glucose. According to the passage, symptoms of congential adrenal hyperplasia (CAH) include hypoglycemia (low serum glucose). In healthy individuals, low serum glucose can be expected to increase the release of glucagon, epinephrine, and norepinephrine because these hormones would act to restore serum level of glucose to the normal range (Numbers II and III). (Number I) In response to high serum glucose concentration, the peptide hormone insulin is secreted from the beta cells of the pancreas. Insulin promotes the uptake of glucose by fat, liver, and skeletal muscle cells. Insulin also promotes glycogenesis, the synthesis of the glucose storage molecule glycogen. Overall, insulin decreases blood glucose concentration by promoting the cellular uptake and storage of glucose. Educational objective: Glucose is a key energy source for many tissues in the human body. The serum level of glucose is tightly regulated through the actions of the hormones insulin, glucagon, epinephrine, norepinephrine, and the glucocorticoids.

Question 10

During a parasitic infection, which of the following would most likely be responsible for phagocytosis of organisms and the release of histamine, respectively?

Accepted Answer

11ecba2d_0fd6_2654_a3bf_ebab2092aed9_MD0008_00 A number of cells and proteins are directly involved in protecting the body against foreign pathogens such as parasites. These cells and proteins, along with the skin and mucous membranes, constitute the innate immune system. The innate immune system activates relatively quickly (minutes to hours) and is said to work in a nonspecific manner because of its ability to target a variety of pathogens for destruction. The cells that make up the innate immune system include macrophages, dendritic cells, natural killer cells, and basophils. The primary function of macrophages, which can either travel the body or remain fixed in a specific tissue, is to degrade pathogens and dead body cells. The degradative function of macrophages is performed via phagocytosis, the process of engulfing solid particles (eg, parasites) designated for destruction into phagocytic vesicles called phagosomes. Within the cell, phagosomes typically fuse with lysosomes to form phagolysosomes, which enable the degradation of the engulfed particles using specialized enzymes that work at low pH. In contrast, the primary function of basophils, a type of granulocyte, is to release chemical mediators such as histamine that enhance the immune response. Released histamine causes vasodilation and permeability of vessels, which stimulates an inflammatory response by increasing blood flow and the delivery of immune cells (eg, neutrophils and macrophages) at the site of infection. Therefore, during a parasitic infection, macrophages function to phagocytose organisms and basophils release histamine. (Choice A) The answer choices here are reversed. Although it has been noted that basophils can perform phagocytosis and macrophages can release histamine, these processes are uncommon. As a result, the reverse (macrophages, basophils) is the most likely answer choice. (Choices B and D) Natural killer cells are cellular components of the innate immune system. Natural killer cells release toxins to destroy body cells, such as neoplastic and virus-infected cells, that have reduced expression of major histocompatibility complex molecules. T-cells are cellular components of the adaptive immune system. T-cells are important in mediating a number of immune responses, including those against viruses. These types of cells are not known to directly phagocytose bacteria. Educational objective: Macrophages are one of the cells responsible for phagocytosis, the process of engulfing solid particles (eg, bacteria) designated for destruction into phagocytic vesicles called phagosomes. In contrast, basophils function to release chemical mediators such as histamine that stimulate an inflammatory response.

Question 11

Which of the following nucleic acid complexes would undergo correction by the DNA mismatch repair system?

Accepted Answer

11ecba2d_0fd2_7ccd_a3bf_21cf450829ce_MD0008_00 During DNA replication, DNA polymerase may mistakenly incorporate an incorrect nucleotide into the newly synthesized strand, creating a base pair mismatch between the daughter and the parent strands (ie, G-T and A-C mispairing instead of the proper G-C and A-T pairing). The presence of the mismatched base activates the DNA mismatch repair (MMR) machinery, in which a nuclease enzyme excises the erroneous nucleotide, as well as several nucleotides flanking either side of the incorrectly matched base, from the daughter strand. DNA polymerase then incorporates the appropriate nucleotides, and DNA ligase catalyzes the formation of new phosphodiester bonds to seal the gap between the strands. In this scenario, the correct DNA sequence would need to contain a pair of mismatched bases. The DNA complex in Choice C has a mismatch at the twelfth base pair that, once corrected, would produce an accurate complementary DNA strand. (Choices A and D) The MMR machinery corrects only replication errors within DNA strands. These complexes depict transcription, the production of an RNA transcript from a DNA template. The mRNA synthesized from the DNA template can be identified by the presence of A-U instead of A-T base pairs. The complex in Choice A shows a normal (error-free) mRNA strand transcribed from a DNA strand. In contrast, the complex in Choice D shows that RNA polymerase made an error during transcription at the fifth base of the mRNA strand (ie, C should be U). Regardless, the MMR would not be activated for either complex because it is not involved in mRNA synthesis or repair of transcription errors. (Choice B) There is no mismatched base in these DNA strands; therefore, the MMR machinery would not be activated. Educational objective: Base pair mismatches occur when DNA polymerase incorporates an incorrect nucleotide into the newly synthesized strand. DNA mismatch repair is the process by which base pair mismatches between the daughter and the parent strands are corrected during DNA replication.

Question 12

Stenosis is the abnormal narrowing of a tubular structure in the body, such as a blood vessel.  Stenosis of the afferent arterioles in the kidney would be expected to:

Accepted Answer

11ecba2d_0fd0_a806_a3bf_cbee5608d7ee_MD0008_00 The kidneys are composed of functional units called nephrons. In nephrons, the glomerular capillaries (which are collectively known as the glomerulus), Bowman's capsule, the renal tubule, and the collecting duct work together to filter blood and concentrate the waste products as urine. Blood is filtered in the glomerulus by high hydrostatic (blood) pressure, which forces excess fluid and waste products (filtrate) across the porous endothelium and into Bowman's space. Proteins and cells are too large to pass through the pores. The volume of fluid filtered through the kidney per unit time is known as the glomerular filtration rate (GFR). Circulating blood enters the glomerulus through the afferent arteriole and exits through the efferent arteriole. The degree of vascular constriction of these two arterioles controls the rate of blood flow into and out of the glomerulus, thereby affecting the GFR through changes in glomerular hydrostatic pressure. According to the question, stenosis is an abnormal narrowing of a blood vessel, a condition that mimics the physiological effect of vasoconstriction. As a result, stenosis of the afferent arteriole would limit the amount of blood reaching the glomerulus, leading to decreased glomerular hydrostatic pressure and, consequently, a decreased GFR. (Choices B and C) The tendency of the body to oppose flow is known as resistance. Stenosis, or narrowing, of the afferent arteriole would cause an increase in resistance because of the reduced diameter of the afferent arteriole. When resistance within the afferent arteriole is increased, blood flow to the glomerulus is expected to decrease. (Choice D) Urine production is influenced by the GFR, the rate of reabsorption (movement of substances from the filtrate back into the bloodstream), and the rate of secretion (movement of substances from the bloodstream to the filtrate). When GFR decreases, as it does due to stenosis of the afferent arteriole, less filtrate is produced, and urine output decreases. Educational objective: The volume of fluid filtered through the kidney per unit time is known as the glomerular filtration rate (GFR), which can be modulated by controlling blood flow through the glomerulus. Higher hydrostatic (blood) pressure in the glomerulus increases GFR, and lower hydrostatic pressure decreases GFR.

Question 13

Which of the following would most likely cause an embryo to implant in a location other than the uterine lining?

Accepted Answer

11ecba2d_0fd4_9fb1_a3bf_171d9eae0a82_MD0008_00 Oogenesis is the process by which females produce sex cells (gametes) called eggs. All of a woman's developing eggs (oocytes) are produced during fetal development. At birth, their maturation is arrested in prophase I; these arrested eggs are known as primary oocytes. At puberty, the menstrual cycle begins, and each month a single primary oocyte develops into a secondary oocyte by continuing meiosis up to metaphase II. During the follicular phase of the cycle, the follicle (oocyte plus supporting granulosa cells) matures until it bulges at the outer wall of the ovary. Maturation and growth of the follicle is driven by the release of follicle-stimulating hormone (FSH) from the anterior pituitary. A subsequent release of luteinizing hormone (LH) stimulates follicle rupture and the release of the secondary oocyte near the opening of the fallopian tube (ovulation). Fertilization (joining of male and female gametes into a zygote) becomes possible after the released secondary oocyte has been drawn into the fallopian tube. It is normal for fertilization to occur in the fallopian tube prior to implantation within the uterus. Typically, small hair-like structures within the fallopian tube called fallopian cilia help propel the fertilized oocyte toward the uterus for implantation. However, without an adequate number of cilia in the fallopian tube, improper implantation of the fertilized egg can occur outside the uterus. (Choices B and D) A surge in LH and follicular rupture are necessary steps for ovulation to occur. LH provides the necessary stimulus for follicular rupture and the release of a secondary oocyte into the fallopian tube. Because these steps must happen before fertilization can occur, they would not be the cause of embryonic attachment outside the uterus. (Choice C) Gastrulation is a post-implantation process by which a developing embryo progresses from the blastula stage to the tri-layered gastrula stage. Because this process occurs post-implantation, abnormal gastrulation would not cause implantation outside the uterus. Educational objective: After fertilization, fallopian cilia help propel the fertilized oocyte toward the uterus for implantation. An inadequate number of cilia in the fallopian tube can cause implantation of the fertilized egg outside the uterus.

Question 14

Passage
A 6-day-old, genotypically normal (46,XX) female patient shows symptoms of hypoglycemia (low blood glucose) and hypotonia (decreased muscle tone).  Physical examination reveals abnormal development of the external genitalia, and laboratory studies show elevated urinary sodium content, increased serum levels of potassium, and abnormal serum concentration of several steroid hormones.  The patient is diagnosed with 21-hydroxylase deficiency, the most common type of congenital adrenal hyperplasia (CAH).CAH refers to a group of autosomal recessive diseases, each associated with defective production of one or more steroid hormones by the adrenal glands, two crescent-shaped glands located above each kidney in the abdominal cavity.  It has been reported that unequal crossing over involving chromosome 6 can result in CAH.  More than 90% of CAH diagnoses are associated with a deficiency of 21-hydroxylase, an enzyme encoded by the CYP21A2 gene on chromosome 6.Normally, 21-hydroxylase catalyzes chemical reactions involved in the synthesis of aldosterone and cortisol, the principal mineralocorticoid and glucocorticoid hormones, respectively.  As a result, patients with 21-hydroxylase deficiency exhibit diminished serum levels of aldosterone and cortisol.  However, synthesis and secretion of the sex hormone testosterone is excessive.  Glucocorticoid deficiency disrupts the negative feedback loop linking the brain to the adrenal gland, leading to hyperplasia (increased cell proliferation) of adrenal tissue.The patient's 21-hydroxylase deficiency is treated with regular administration of synthetic steroid hormones.  Her condition improves after receiving these medications.
-Glucocorticoid deficiency is known to impair healing of superficial wounds, memory formation, and other physiologic phenomena.  This deficiency most likely leads to these wide-ranging effects because:

Accepted Answer

11ecba2d_0fcb_ed0f_a3bf_5fa8edc32837_MD0008_00 Hormones are chemical messengers that influence physiologic function through endocrine signaling, a form of long-distance cell-to-cell communication. In endocrine signaling, hormones are secreted into the bloodstream by specialized cells and transported throughout the body via the circulatory system. Once the hormone reaches its target tissue, it alters cellular function in one of the following ways: Water-soluble (hydrophilic) hormones generally bind a cell surface receptor and initiate a second messenger cascade, leading to changes in protein activity (eg, enzymes, membrane channels). Water-insoluble (hydrophobic) hormones generally diffuse across the lipid bilayer of the plasma membrane and bind to a cytosolic receptor to ultimately regulate gene expression in the nucleus. Hormones affect the function of diverse and distant tissues in the body. This is because hormone receptors can be expressed in a variety of cell types located throughout the body, and any cell that expresses the correct receptor is capable of responding to the receptor's ligand (hormone). For example, glucocorticoids released into the bloodstream from the adrenal cortex influence the energy metabolism of skeletal muscle cells, liver cells, and fat cells, which all express glucocorticoid receptors. However, in glucocorticoid deficiency, an insufficient amount of glucocorticoid hormones in the blood leads to diminished binding of these hormones to their receptors. This condition would compromise the normal function of the various cells that express glucocorticoid receptors. (Choice A) Although glucocorticoid signaling increases serum glucose levels by promoting gluconeogenesis, the widespread effects of glucocorticoid deficiency are better explained by the diminished activation of body-wide glucocorticoid receptors due to the decreased overall synthesis and secretion of glucocorticoids. (Choice C) Glucocorticoids do not affect thyroid hormone secretion by the thyroid gland. (Choice D) Tissue damage caused by adrenal gland enlargement would have no direct effect on activities such as memory formation and the healing of superficial wounds because these functions occur outside the abdominal cavity. Educational objective: Hormones are a diverse group of molecules that participate in endocrine signaling, a form of long-distance cell-to-cell communication. Any cell type that expresses a hormone receptor will respond to serum levels of the hormone.

Question 15

A subset of aggressive cancers has a relatively high growth rate, leading to the formation of large tumors.  An effective drug against fast-growing tumors would most likely NOT target which stage of the cell cycle?

Accepted Answer

11ecba2d_0fd2_f1fe_a3bf_a3ae4b57eedf_MD0008_00 The mechanism by which cells duplicate their content and divide is controlled by "checkpoints" (restriction points) within the cell cycle. Many cancers arise from the accumulation of multiple genetic defects (eg, inherited or from mutations) that allow uncontrollable cell division without the normal rate of apoptosis (programmed cell death). According to the question, a subset of aggressive cancers can cause the growth of large tumors, masses of abnormal cells. Because the cells within these tumors are actively dividing, it can be assumed that the majority of the cells are in interphase and not arrested in G₀. Therefore, to treat these aggressive cancers, clinicians may use anticancer drugs that try to destroy actively dividing cells (phases G₁ to M) and not those in G₀. (Choice B) Cell growth, protein synthesis, and DNA damage repair occur in preparation for mitosis during the G₂ phase of actively dividing cells. Cancer cells divide at higher rates than normal cells and cycle through G₂ frequently. Therefore, anticancer drugs that target G₂ can decrease the number of cell divisions. As a result, survivability may be improved because cancer cells halted at G₂ are less likely to disrupt physiological function and metastasize (spread). (Choices C and D) Control of cellular division occurs at checkpoints within the cell cycle. At the G₁/S phase transition, the cell commits to undergoing a division cycle whereas the G₂/M phase transition acts as a quality-control step in which the cell cycle is stopped and cellular components are checked for abnormalities. Checkpoints are regulated primarily by cyclins and cyclin-dependent kinases. Cells with abnormalities that cannot be repaired undergo apoptosis; those that pass the checkpoints continue to divide. Anticancer drugs often target these checkpoints due to the regulatory roles on the cell cycle. Educational objective: Most cells in the human body are arrested in G₀. However, cellular transition into G₁ prepares a cell for division and DNA synthesis (S phase). In the G₂ phase, DNA is checked for errors and the cell ensures that sufficient organelles and cytoplasm are available for cell division. Subsequently, the cell divides in the M phase via mitosis and cytokinesis. Compounds that inhibit cell division typically target the cell cycle in phases G₁ to M.

Question 16

Passage
A 6-day-old, genotypically normal (46,XX) female patient shows symptoms of hypoglycemia (low blood glucose) and hypotonia (decreased muscle tone).  Physical examination reveals abnormal development of the external genitalia, and laboratory studies show elevated urinary sodium content, increased serum levels of potassium, and abnormal serum concentration of several steroid hormones.  The patient is diagnosed with 21-hydroxylase deficiency, the most common type of congenital adrenal hyperplasia (CAH).CAH refers to a group of autosomal recessive diseases, each associated with defective production of one or more steroid hormones by the adrenal glands, two crescent-shaped glands located above each kidney in the abdominal cavity.  It has been reported that unequal crossing over involving chromosome 6 can result in CAH.  More than 90% of CAH diagnoses are associated with a deficiency of 21-hydroxylase, an enzyme encoded by the CYP21A2 gene on chromosome 6.Normally, 21-hydroxylase catalyzes chemical reactions involved in the synthesis of aldosterone and cortisol, the principal mineralocorticoid and glucocorticoid hormones, respectively.  As a result, patients with 21-hydroxylase deficiency exhibit diminished serum levels of aldosterone and cortisol.  However, synthesis and secretion of the sex hormone testosterone is excessive.  Glucocorticoid deficiency disrupts the negative feedback loop linking the brain to the adrenal gland, leading to hyperplasia (increased cell proliferation) of adrenal tissue.The patient's 21-hydroxylase deficiency is treated with regular administration of synthetic steroid hormones.  Her condition improves after receiving these medications.
-A 10-day-old male was diagnosed with 21-hydroxylase deficiency despite having normal physical examination results.  Which of the following best explains why physical examination results can be normal in males with this condition?

Accepted Answer

11ecba2d_0fce_8523_a3bf_7d69093e1238_MD0008_00 The hormones testosterone, estrogen, and progesterone are referred to as sex hormones because of their role in sexual development and reproduction. Although testosterone can be detected in the serum of females, it is typically considered a male sex hormone because of its role in the development of male traits and behaviors. Testosterone production in males is related to the presence of the sex-determining region Y (SRY) gene on the Y chromosome, a sex chromosome that exists alongside one X chromosome in male cells. The SRY gene controls male sex determination and promotes development of the testes, the male sex organs where testosterone is produced. Testosterone within the fetal bloodstream then promotes the development of other male sex organs. Because female cells have two X chromosomes and no Y chromosome, female fetuses are exposed to less testosterone, causing development of female sex organs. According to the passage, 21-hydroxylase deficiency is associated with defective glucocorticoid synthesis and excessive testosterone synthesis. In female patients, excess testosterone would lead to abnormal development of the female external genitalia that can be detected on physical examination. Because testosterone is a normal contributor of male sexual development, excess testosterone would not significantly alter development of the external sex organs in males, and a physical examination would yield no abnormal results. (Choice A) Although the testes and adrenal glands both secrete steroid hormones (hormones derived from cholesterol), the testes cannot synthesize glucocorticoids. As a result, testicular endocrine activity cannot correct for the glucocorticoid deficiency seen in 21-hydroxylase deficiency. (Choice B) The Y chromosome is a sex chromosome that contains very few genes, most of which are related to male development. According to the passage, the gene for 21-hydroxylase is located on chromosome 6, an autosome. The Y chromosome would not contain extra copies of autosomal genes such as those mutated in individuals with 21-hydroxylase deficiency. (Choice D) Glucocorticoids (eg, cortisol) do not play a role in the sexual development of males or females, and therefore cannot explain the genital abnormalities seen in patients with 21-hydroxylase deficiency. Educational objective: Testosterone is a sex hormone that facilitates the development of male sex organs and secondary sexual characteristics.

Question 17

Passage Domestic pigs have been used as valuable preclinical models for scientific research due to their numerous anatomical, developmental, and physiological similarities to humans. For example, oogenesis and embryonic development in pigs are remarkably similar to the human processes and have facilitated the study of impaired fertilization due to decreased oocyte competence (quality).Researchers have hypothesized that early termination of embryo development can arise from low mitochondrial DNA (mtDNA) copy number. Because mtDNA replication and reduction events have been extensively mapped in pigs, oocytes and sperm were isolated from their respective porcine gonads to test this hypothesis.Experiment 1To evaluate the developmental competence of porcine oocytes, researchers first measured the activity of glucose-6-phosphate dehydrogenase (G6PD), an enzyme that is downregulated as the oocyte matures. The oocytes were stained with Brilliant Cresyl Blue (BCB), a nontoxic blue dye that is converted to a colorless compound by G6PD. After 90 minutes of staining, oocytes were divided into two groups: one contained oocytes with colorless cytoplasm (BCBS1U1P1−S1S1P0) and the other contained oocytes with blue cytoplasm (BCBS1U1P1+S1S1P0). Gene expression profiles were compared to evaluate the number of oocytes in the BCBS1U1P1+S1S1P0 and BCBS1U1P1−S1S1P0 groups that had developed to metaphase II.Experiment 2BCBS1U1P1+S1S1P0 and BCBS1U1P1−S1S1P0 oocytes underwent in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), or supplementation with homogeneous mitochondria and ICSI (mICSI). During IVF, an isolated oocyte was placed in a well plate and incubated with sperm at 39 C for 4 hours. In ICSI, a single sperm was selected and injected directly into the oocyte.Table 1 Developmental Rates for BCBS1U1P1+S1S1P0 and BCBS1U1P1−S1S1P0 Oocytes Fertilized Subjected to IVF, ICSI, or mICSI Adapted from Cagnone GL, Tsai TS, Makanji Y, et al. Sci Rep. 2016;6:23229. -The results of Experiment 1 indicate that the oocytes stained blue are:

Accepted Answer

11ecba2d_0fca_b48d_a3bf_6bff6320073e_MD0008_00 In Experiment 1, oocytes selected from porcine ovaries were initially stained with Brilliant Cresyl Blue (BCB) dye to measure the activity of the enzyme G6PD. Based on the passage, G6PD converts BCB to a colorless compound and is downregulated as the oocyte matures. Staining with BCB provides a viable mechanism of separating immature noncompetent oocytes from mature competent ones, which will likely be selected as these are more likely to result in viable progeny on fertilization. Consequently, mature (competent) oocytes would have a blue cytoplasm because G6PD activity would be low and the enzyme would not be sufficiently active to convert BCB into a clear compound. In contrast, an immature (noncompetent) oocyte would show high levels of G6PD activity, and when stained with BCB, would show a colorless cytoplasm because G6PD would convert BCB into a clear compound. (Choices A and C) An oocyte with a clear (not blue) cytoplasm shows high G6PD activity, which results in conversion of BCB to a clear compound, indicating an immature and noncompetent oocyte. (Choice D) Oocytes with blue cytoplasm are most likely competent (mature) with low G6PD activity. Educational objective: Oocytes can be competent or noncompetent based on their maturation status. Oocytes that are competent are mature and most likely result in viable progeny after being fertilized by sperm.

Question 18

Passage
A 6-day-old, genotypically normal (46,XX) female patient shows symptoms of hypoglycemia (low blood glucose) and hypotonia (decreased muscle tone).  Physical examination reveals abnormal development of the external genitalia, and laboratory studies show elevated urinary sodium content, increased serum levels of potassium, and abnormal serum concentration of several steroid hormones.  The patient is diagnosed with 21-hydroxylase deficiency, the most common type of congenital adrenal hyperplasia (CAH).CAH refers to a group of autosomal recessive diseases, each associated with defective production of one or more steroid hormones by the adrenal glands, two crescent-shaped glands located above each kidney in the abdominal cavity.  It has been reported that unequal crossing over involving chromosome 6 can result in CAH.  More than 90% of CAH diagnoses are associated with a deficiency of 21-hydroxylase, an enzyme encoded by the CYP21A2 gene on chromosome 6.Normally, 21-hydroxylase catalyzes chemical reactions involved in the synthesis of aldosterone and cortisol, the principal mineralocorticoid and glucocorticoid hormones, respectively.  As a result, patients with 21-hydroxylase deficiency exhibit diminished serum levels of aldosterone and cortisol.  However, synthesis and secretion of the sex hormone testosterone is excessive.  Glucocorticoid deficiency disrupts the negative feedback loop linking the brain to the adrenal gland, leading to hyperplasia (increased cell proliferation) of adrenal tissue.The patient's 21-hydroxylase deficiency is treated with regular administration of synthetic steroid hormones.  Her condition improves after receiving these medications.
-Given that the fusion of male and female gametes results in fertilization, which statement best explains how unequal crossing over during gametogenesis can lead to 21-hydroxylase deficiency in the fertilized embryo?  One of the parental gametes:

Accepted Answer

11ecba2d_0fcf_4874_a3bf_7517a3d90972_MD0008_00 In eukaryotes, reproductive cells (gametes) are produced via meiosis, during which a parent cell (2n) divides to produce four genetically distinct daughter cells containing half the original number of chromosomes (n). Crossing over which occurs during prophase I of meiosis, is the process by which homologous chromosomes swap genetic material through the breakage and exchange of corresponding DNA segments between non-sister chromatids. As a result, crossover events give rise to daughter cells with chromosomes containing allele combinations that differ from those in the parent cell, leading to genetic recombination and increased genetic diversity. When homologous chromosomes fail to align properly, unequal crossing over occurs. Unequal crossing over is the exchange of unequal segments of DNA between non-sister chromatids, yielding one chromosome that contains extra copies of one or more genes (ie, gene duplication) and another chromosome that lacks those genes entirely (ie, gene deletion). According to the passage, 21-hydroxylase deficiency occurs due to insufficient activity of 21-hydroxylase, the enzyme encoded by the CYP21A2 gene on chromosome 6. If unequal crossing over involving chromosome 6 occurs, it may result in a daughter cell that contains a chromosome lacking CYP21A2, which would contribute to the development of 21-hydroxylase deficiency in offspring. (Choices A and B) Nondisjunction is defined as the failure of homologous chromosomes to separate in anaphase of meiosis I or the failure of sister chromatids to separate in anaphase of meiosis II. Consequently, nondisjunction produces gametes that either lack a copy of a chromosome or contain an extra copy of a chromosome. Unequal crossing over of genetic material between non-sister chromatids gives rise to gametes that lack or contain an extra copy of individual genes, not entire chromosomes. (Choice C) 21-hydroxylase deficiency occurs due to insufficient 21-hydroxylase activity, which can be caused by reduced CYP21A2 expression or by loss of function of the 21-hydroxylase enzyme itself. Although it can result from unequal crossing over, duplication of CYP21A2 would cause enhanced 21-hydroxylase activity (not deficiency) due to greater expression of the gene. Educational objective: Crossing over is a form of genetic recombination involving the exchange of genetic material between non-sister chromatids of homologous chromosomes. Unequal crossing over can result in offspring that lack important genes.

Question 19

The genetic code is said to be degenerate because there are 64 different codons, but translation produces only 20 unique amino acids.  The degeneracy of the genetic code is due to which mechanism?

Accepted Answer

11ecba2d_0fd0_0bc5_a3bf_f3d29d538273_MD0008_00 The genetic code explains how four kinds of DNA nucleotides (A, T, G, and C) can specify the 20 amino acids necessary for protein synthesis. Sequences of three nucleotides in DNA or RNA are known as codons. Genes contain codons that are sequentially transcribed into mRNA codons that code for amino acids during translation. Mathematically, there can only be 64 (or 4³) possible arrangements of the four DNA nucleotides into codons. However, the genetic code is often considered degenerate or redundant because although there are 64 codons, only 20 amino acids are used for protein synthesis. In translation, each mRNA codon (except the stop codons) base pairs with the complementary anticodon of a tRNA molecule that is "charged" with a corresponding amino acid. The anticodon of a single tRNA molecule can base pair with different mRNA codons that code for the same amino acid. This redundancy of the genetic code can be explained in part by the wobble hypothesis. This hypothesis states that the first two nucleotides on the mRNA codon require traditional (Watson-Crick) base pairing with their complementary nucleotides on the tRNA anticodon, but the third nucleotide on the codon may undergo less stringent ("wobble") base pairing in a non-Watson-Crick manner. Codon degeneracy is therefore explained by nontraditional base pairing at the third position of the codon and anticodon. (Choice A) Alternative splicing cannot explain degeneracy because it refers to the post-transcriptional selection of exons for translation. Depending on the sets of exons selected, the same gene can encode different proteins. (Choice B) tRNA charging refers to the covalent bonding (attachment) of an amino acid to the 3′-OH end of the tRNA molecule. Errors during tRNA charging do not explain genetic code degeneracy but can result in the wrong amino acid being added to the growing polypeptide chain (missense mutation). (Choice C) The genetic code is redundant but not ambiguous. A single tRNA molecule can bind multiple different codons; however, a single tRNA can be charged with only one amino acid. Therefore, there is no ambiguity between a tRNA molecule and the amino acid it is meant to carry. Educational objective: The genetic code is considered "degenerate" because more than one codon can code for the same amino acid. This occurs because the third position of the mRNA codon and tRNA anticodon can undergo nontraditional base pairing, allowing a single tRNA molecule to bind different codons.

Question 20

Passage
A 6-day-old, genotypically normal (46,XX) female patient shows symptoms of hypoglycemia (low blood glucose) and hypotonia (decreased muscle tone).  Physical examination reveals abnormal development of the external genitalia, and laboratory studies show elevated urinary sodium content, increased serum levels of potassium, and abnormal serum concentration of several steroid hormones.  The patient is diagnosed with 21-hydroxylase deficiency, the most common type of congenital adrenal hyperplasia (CAH).CAH refers to a group of autosomal recessive diseases, each associated with defective production of one or more steroid hormones by the adrenal glands, two crescent-shaped glands located above each kidney in the abdominal cavity.  It has been reported that unequal crossing over involving chromosome 6 can result in CAH.  More than 90% of CAH diagnoses are associated with a deficiency of 21-hydroxylase, an enzyme encoded by the CYP21A2 gene on chromosome 6.Normally, 21-hydroxylase catalyzes chemical reactions involved in the synthesis of aldosterone and cortisol, the principal mineralocorticoid and glucocorticoid hormones, respectively.  As a result, patients with 21-hydroxylase deficiency exhibit diminished serum levels of aldosterone and cortisol.  However, synthesis and secretion of the sex hormone testosterone is excessive.  Glucocorticoid deficiency disrupts the negative feedback loop linking the brain to the adrenal gland, leading to hyperplasia (increased cell proliferation) of adrenal tissue.The patient's 21-hydroxylase deficiency is treated with regular administration of synthetic steroid hormones.  Her condition improves after receiving these medications.
-What changes in blood volume and serum sodium content would be expected as a result of using synthetic hormones to replenish aldosterone in CAH patients?

Accepted Answer

11ecba2d_0fce_0ff2_a3bf_894a759a39a9_MD0008_00 Located in the posterior abdomen on the left and right sides of the body, the kidneys are two bean-shaped organs that participate in excretory, metabolic, and homeostatic functions. The homeostatic functions of the kidneys occur primarily through the filtration of blood and the selective secretion or reabsorption of filtered contents. Filtration of blood by the kidneys yields a fluid (called the filtrate) that undergoes significant changes in volume and composition prior to excretion as urine. Nutrients such as glucose and amino acids are reabsorbed into the body, and toxins are secreted. The majority of filtered sodium (Na⁺) and water (H₂O) is reabsorbed. However, the reabsorption of sodium and water is subject to some degree of adjustment based on physiological conditions. For example, the hormone aldosterone, which is secreted by the adrenal cortex in response to high serum potassium (K⁺) and low blood pressure (via the renin-angiotensin system), acts on the kidney to promote sodium reabsorption and potassium secretion. However, more sodium is reabsorbed than potassium is secreted. Consequently, an ion gradient is generated between the filtrate and the interstitial fluid in the adjacent interstitial space, causing water to be reabsorbed into the interstitial fluid (and eventually the bloodstream) via osmosis. As a result, blood volume and therefore blood pressure increase. According to the passage, patients with congenital adrenal hyperplasia (CAH) exhibit deficient synthesis and secretion of aldosterone. After administering synthetic aldosterone, increased sodium and water reabsorption by the kidneys will result in increased blood volume and increased serum sodium content. (Choices B and D) Aldosterone promotes the reabsorption of sodium by the kidneys, leading to increased serum sodium content. (Choices C and D) Aldosterone promotes the reabsorption of water by the kidneys (a side-effect of the reabsorption of sodium), leading to increased blood volume. Educational objective: Aldosterone is secreted by the adrenal cortex in response to elevated plasma potassium and decreased blood pressure. Aldosterone acts on the kidneys to increase the reabsorption of sodium, which in turn leads to increased reabsorption of water and, ultimately, increased blood volume and pressure.

Exam 3: Biology

Assume a single-celled organism was discovered in water above 100°C. On closer inspection, it is determined that this organism was previously classified under the Archaea domain. Given this information, this organism most likely has:

Which step(s) of the viral life cycle would most likely occur in lytic phages?Injection of viral genetic material into the bacterial hostIntegration of viral DNA into the bacterial genomeDegradation of the host genome

Which of the following helps maintain the resting membrane potential of a neuron?Passive transportAdenosine triphosphateMembrane selective permeability

Human sperm and oocytes are substantially different in all of the following aspects EXCEPT:

During a parasitic infection, which of the following would most likely be responsible for phagocytosis of organisms and the release of histamine, respectively?

Which of the following nucleic acid complexes would undergo correction by the DNA mismatch repair system?

Stenosis is the abnormal narrowing of a tubular structure in the body, such as a blood vessel. Stenosis of the afferent arterioles in the kidney would be expected to:

Which of the following would most likely cause an embryo to implant in a location other than the uterine lining?

A subset of aggressive cancers has a relatively high growth rate, leading to the formation of large tumors. An effective drug against fast-growing tumors would most likely NOT target which stage of the cell cycle?

The genetic code is said to be degenerate because there are 64 different codons, but translation produces only 20 unique amino acids. The degeneracy of the genetic code is due to which mechanism?