Exam 3: Biology

Passage Obesity is a condition that affects over 30% of adults in the United States and is associated with comorbidities such as type 2 diabetes mellitus, heart disease, and cancer.  For individuals with clinically severe obesity, defined as having a body mass index above 40 kg/m², lifestyle interventions may produce insufficient weight reduction.  Weight loss surgery, such as Roux-en-Y gastric bypass (RYGB), has proved to be the most effective clinical intervention for facilitating massive and immediate weight loss in clinically obese patients.In RYGB, the stomach is first divided into a small gastric pouch and a distal gastric remnant, which remains attached to the duodenum.  Next, the jejunum is transected and its lower section is connected to the newly created gastric pouch to form the alimentary limb.  This connection is typically created with surgical staples.  The formation of the alimentary limb allows ingested food to bypass the region composed of the distal gastric remnant, the duodenum, and the upper jejunum; collectively termed the biliopancreatic limb.  Finally, the lumen of the upper jejunum is reconnected to the lumen of the lower jejunum to allow enzymes and compounds to flow freely from the biliopancreatic limb into the alimentary limb. Figure 1  RYGB surgically divides the digestive tract into the alimentary limb and the biliopancreatic limb, which then merge into the common limb.RYGB facilitates weight loss by reducing the functional volume of the stomach and altering intestinal anatomy to induce nutrient malabsorption.  However, the procedure is associated with several complications, such as steatorrhea, or fatty stools, due to disruption of lipid digestion.  Animal studies also suggest that RYGB promotes the growth of weight loss-inducing gut bacteria.  Researchers have found that transplanting gut microbiota of obese postoperative RYGB mice into nonoperated, obese mice lacking gut flora may reproduce the therapeutic effects of RYGB in transplant-recipient mice. Adapted from Bächler T, Le roux CW, Bueter M. How do patients' clinical phenotype and the physiological mechanisms of the operations impact the choice of bariatric procedure?. Clin Exp Gastroenterol. 2016;9:181-9. -In RYGB patients, ingested food will NOT pass through the:

Passage Trypanosoma brucei is a eukaryotic parasite that causes sleeping sickness in Sub-Saharan Africa.  The cell cycle of T. brucei differs from that of higher eukaryotes.  For example, the chromatin does not condense during mitosis, and the nuclear envelope remains intact.  In addition, the nucleus divides before cell division, and the cell divides along the longitudinal axis (Figure 1). Figure 1  Mitosis in T. bruceiA culture of T. brucei was arrested in S phase of the cell cycle by overnight incubation with the reversible inhibitor hydroxyurea.  The cells were then washed, and their progression through the cell cycle was monitored by flow cytometry and microscopy.  A subset of the culture was first exposed to the drug VX-680, which arrests cells upon completion of metaphase.  The results are shown in Figure 2. Figure 2  Percentages of a population of T. brucei cells in each phase of the cell cycle, measured at different time points in the (A) absence and (B) presence of VX-680 Adapted from Li Z, Umeyama T, Wang CC. The Aurora Kinase in Trypanosoma brucei plays distinctive roles in metaphase-anaphase transition and cytokinetic initiation. PLoS Pathog. 2009;5(9):e1000575. -T. brucei cells and higher eukaryotes are similar in that they both undergo:InterphaseProphaseCytokinesis

Passage In 1906, the Nobel Prize in Physiology or Medicine was awarded to two biologists for their groundbreaking work in the field of neuroanatomy.  Despite using the same technique to visualize neurons, the two biologists formed opposing theories regarding nervous system organization.Hypothesis 1:  Reticular TheoryBiologist 1 developed a neural staining technique that was carried out by fixing brain tissue in potassium dichromate (K₂Cr₂O₇) before treating it with silver nitrate (AgNO₃).  A silver chromate (Ag₂CrO₄) precipitate deposited within the neurons, making them appear black.  The silver impregnation method was revolutionary because of two features.  First, only a random subset of neurons within a given tissue sample (~1% of the total cell population) was subject to silver impregnation.  Second, the impregnated cells were stained in their entirety, allowing for visualization of the complete neuron.  Biologist 1 concluded that his findings supported the idea that the nervous system is a single reticulum of fused neural processes that transmit information via cytoplasmic continuity.Hypothesis 2:  Neuron DoctrineBiologist 2 modified the silver impregnation method by first subjecting the brain tissue to two rounds of staining ("double impregnation") rather than one.  Second, where Biologist 1 studied the brain tissue of older animals with myelinated nerve fibers, Biologist 2 employed the silver stain on brain tissue from younger animals, which made it easier for the silver to impregnate the neural processes due to relative lack of myelination.  Biologist 2 reasoned that the nervous system is not a single web of interconnected fibers but rather is made up of discrete neural units that communicate via close, but not continuous, contact.The neuron doctrine was later proven correct with the advent of electron microscopy, which allowed scientists to visualize the synapse (the point of communication between two distinct neurons).  Further investigation allowed researchers to ultimately discover that there are two types of synapses:  electrical and chemical. -The graphs below depict neural signaling across two different kinds of synapses.  One set corresponds to electrical signal transmission, and the other corresponds to chemical signal transmission. Based on this information, does the data in Set A depict action potential transmission across a chemical or electrical synapse?

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?

Passage The adrenal glands are divided into two anatomically distinct regions, the adrenal medulla and the adrenal cortex.  The adrenal medulla is a collection of irregularly shaped cells located in the interior portion of each adrenal gland.  In response to changing serum levels of corticosteroid hormones, medullary cells synthesize and secrete the catecholamines norepinephrine and epinephrine.  Norepinephrine and epinephrine are water-soluble compounds, and their actual secretion into the bloodstream is stimulated by acetylcholine (ACh)-mediated signaling in the sympathetic component of the autonomic nervous system.  Figure 1 shows the enzyme-mediated synthesis of norepinephrine and epinephrine. Figure 1The adrenal cortex is the outer portion of the adrenal gland and is composed of cells that synthesize and secrete lipophilic hormones derived from cholesterol.  Hormones produced by the adrenal cortex include the principal glucocorticoid hormone (cortisol) and the mineralocorticoid hormone (aldosterone).The hypothalamus and pituitary gland regulate and control the endocrine functions of the adrenal cortex (Figure 2).  Secretion of corticotrophin-releasing hormone (CRH) from the hypothalamus promotes the synthesis and release of adrenocorticotrophic hormone (ACTH) by the pituitary gland.  ACTH acts on adrenal cortex cells to promote the growth of cortical tissue and to increase the synthesis and secretion of glucocorticoids.  In addition to its body-wide functions as the principal stress hormone, cortisol also regulates adrenal activity by inhibiting CRH release from the hypothalamus and ACTH release from the pituitary. Figure 2 -Administration of a drug causes increased secretion of norepinephrine and epinephrine by the adrenal medulla.  This most probably occurs because the drug:

Passage Obstructive respiratory illnesses are characterized by an increased difficulty exhaling much of the air in the lungs.  Asthma and chronic obstructive pulmonary disease (COPD) are two of the most common obstructive respiratory illnesses.  Asthma is due to allergen hypersensitivity and can be identified by temporary airway inflammation, which causes the bronchioles to narrow.  The decreased airway diameter increases the resistance to airflow.  Subsequently, air becomes "trapped" in the lungs, restricting the amount of air that can be moved in each breath.COPD refers to a spectrum of disorders and can be a result of emphysema, the irreversible destruction of pulmonary tissue.  The breakdown of elastic proteins in the lungs results in a loss of pulmonary resiliency and chronic lung hyperinflation.  Because the walls between individual alveoli are broken down, the alveolar sacs that remain are larger.Lung diseases can be diagnosed and monitored using spirometry.  A spirometer measures the flow rate and volume of inhaled and exhaled air.  In one pulmonary test, a patient is asked to exhale forcibly and completely into a spirometer after maximum inhalation.  This test is done to measure the patient's forced expiratory volume in one second (FEV1) and forced vital capacity (FVC), the total amount of air exhaled in a single breath. Figure 1  Forced expiratory spirogram of a healthy individual Adapted from T. Barreiro, "An Approach to Interpreting Spirometry." American Family Physician. ©2004 AAFP. -Which of the following FEV1 spirograms would be expected in a patient with asthma?

Passage Adeno-associated virus 2 (AAV2) is a nonpathogenic, nonenveloped DNA virus that requires helper viruses such as adenovirus (AV) or herpes simplex virus (HSV) to replicate efficiently.  Because it can transduce human cells, including quiescent (nondividing) cells, without causing disease, AAV2 is attractive as a gene therapy vector.  In clinical trials, a modified AAV2 virus has successfully treated the retinal disease Leber's congenital amaurosis as well as the blood clotting disorder hemophilia B.  AAV2 transduces cells via a multistep process, shown in Figure 1. Figure 1  Schematic showing the mechanism of transduction by AAV2Studies indicate that unlike AV and HSV, AAV2 inhibits pathways other than the endosomal pathway, such as trafficking to the Golgi, increasing the percentage of invading virus particles that arrive at the nucleus. Adapted from Xiao PJ, Samulski RJ. Cytoplasmic trafficking, endosomal escape, and perinuclear accumulation of adeno-associated virus type 2 particles are facilitated by microtubule network. J Virol. 2012;86(19):10462-73. -Prior to reaching their final destination in the plasma membrane, the AAV2-binding proteins on the cell surface passed through all of the following cellular compartments EXCEPT:

Passage Parathyroid hormone (PTH) is an important regulator of calcium and phosphate homeostasis.  Specifically, PTH functions to increase plasma calcium concentration and reduce plasma phosphate levels.  PTH secretion is closely regulated by changes in plasma calcium concentration, which are detected by the human calcium-sensing receptor (hCaSR), a G protein-coupled receptor located on chief cells of the parathyroid gland.  hCaSR activation ultimately inhibits PTH release by inducing a second messenger cascade in which intracellular calcium levels rise due to the hydrolysis of the membrane phospholipid phosphatidylinositol bisphosphate (PIP₂) into inositol trisphosphate (IP₃) and diacylglycerol (DAG) by the membrane-bound enzyme phospholipase C (PLC).Secondary hyperparathyroidism is a condition characterized by excessive PTH release in response to depressed serum calcium or elevated serum phosphate levels.  The novel peptide AMG 416, an hCaSR agonist, is being investigated as a possible treatment for secondary hyperparathyroidism caused by end-stage renal disease (ESRD).  IP-1, a downstream metabolite of IP₃, is a marker of hCaSR activity.  Researchers used embryonic kidney cells lacking hCaSR (HEK-293T) and HEK-293T clone cells stably transfected with hCaSR (hCaSR) to measure IP-1 levels following AMG 416 treatment (Figure 1). Figure 1  IP-1 concentration in embryonic kidney cells lacking hCaSR (HEK-293T) and HEK-293T clone cells transfected with hCaSR (hCaSR) following AMG 416 treatmentThe dependence of AMG 416 on extracellular calcium was investigated by incubating hCaSR cells with increasing concentrations of AMG 416 in the presence or absence of calcium treatment (Figure 2). Figure 2  IP-1 levels in hCaSR cells incubated with and without calcium and AMG 416 peptide  (Note:  IP-1 levels are normalized to the number of cells present in each culture.)In addition, PTH concentration was measured in 5/6 Nx mice, an animal model of secondary hyperparathyroidism in human ESRD.  Blood samples were taken from male and female 5/6 Nx mice for 8 hours following administration of varying concentrations of AMG 416 (Figure 3), and PTH levels were measured. Figure 3  Serum PTH concentration following administration of AMG 416 to 5/6 Nx mice -Which of the following is a potential complication of secondary hyperparathyroidism?

Researchers are studying the last two phases of mitosis, anaphase and telophase, in actively dividing cancer cells.  Different fluorescent probes are used to label various cellular components in these cells.  Under the microscope, fluorescently labeled DNA appears blue, the nuclear envelope appears green, and the mitotic spindle appears red.  Given this, green fluorescence would be most intense during:

Passage The fruit fly Drosophila melanogaster is frequently used as a model system to investigate neurological disorders.  The genes Abl, Enabled (Ena), and receptor protein tyrosine phosphatase (PTP) are involved in axonal guidance during neuronal development and are all believed to operate in the same pathway.  To investigate interactions between PTP and either Ena or Abl, fly lines were developed in which mutations of each gene were easily identifiable by eye characteristics.  In flies that would normally have white eyes, a dominant mutation that inactivates PTP results in red and white eyes; a dominant Ena mutation yields orange eyes (or red and orange if the PTP and Ena mutations are both present); and a dominant Abl mutation yields elongated eyes.  Using these fly lines, the following experiments were conducted.Experiment 1Females heterozygous for PTP (eePp) were crossed with males heterozygous for Ena (Eepp).  F1 generation males and females, each with red and orange eyes, were then selected and crossed with each other to produce an F2 generation.  Flies with two mutant Ena alleles (EE), as well as those with two mutant PTP alleles and at least one mutant Ena allele (EePP), did not survive past the larval stage.Experiment 2PTP and Abl are located on the same chromosome.  Females with the PTP mutation on one copy of the chromosome and the Abl mutation on the other copy were crossed with wild-type males.  In Drosophila, recombination occurs only in female gametes.  Progeny with parental and recombinant genotypes are shown in Table 1.  F1 males with red and white elongated eyes were then crossed with homozygous PTP mutant females.  This produced an F2 generation in which 50% of both male and female progeny had elongated eyes.Table 1  Phenotypes of 500 F1 Flies Like humans, male flies have a single X chromosome, whereas females have two X chromosomes.  Unlike humans, female Drosophila can store sperm and fertilize multiple eggs over the course of several days after a single mating.  Researchers ensured that females were virgins prior to setting up each cross to avoid contamination. Adapted from Song JK, Giniger E, Desai CJ. The receptor protein tyrosine phosphatase PTP69D antagonizes Abl tyrosine kinase to guide axons in Drosophila. Mech Dev. 2008;125(3-4):247-56. -Which evidence in Table 1 suggests that PTP and Abl are located close together on the chromosome?

The North American flying squirrel and the Draco lizard of Southeast Asia have both evolved a membrane between their limbs that allows them to glide between trees in their habitats.  Given that the species are distantly related, which of the following patterns of evolution would explain the development of this anatomical structure in both organisms?Parallel evolutionDivergent evolutionConvergent evolution

Passage In the central nervous system, myelin produced by oligodendrocytes (glial cells) functions as an insulating sheath surrounding certain nerve fibers.  To identify candidate genes involved in the myelination process, researchers collected oligodendrocytes from zebrafish and purified the messenger RNA (mRNA) transcripts expressed by these glial cells.  The mRNA was converted to complementary DNA (cDNA) by reverse transcriptase, and the cDNA was fluorescently labeled and assessed for hybridization on a microarray.  The expression of mRNA in other zebrafish cells was similarly measured.Transcripts detected in zebrafish oligodendrocytes at levels greater than 3 times those found in other cells were selected as candidate myelination genes.  Some of the detected genes, including plp1a, Sox10, and mbp, were previously known to be specific to oligodendrocytes, validating the procedure.  The cDNA of newly identified candidates was amplified by PCR, and the ends were digested with the restriction enzymes EcoRI and XhoI.  Genes were then ligated into the multiple cloning site (MCS) of the pSKII vector, shown in Figures 1 and 2, which had also been digested by EcoRI and XhoI. Figure 1  Overview of the pSKII vector (plasmid) Figure 2  The pSKII MCS, showing the restriction sites of several restriction enzymesTable 1 shows the functional parameters of several restriction enzymes. To confirm that the candidate genes originated in oligodendrocytes, complementary RNA (cRNA) strands were synthesized from the cloned plasmids and hybridized to fixed zebrafish sections.  One of the genes that localized to oligodendrocytes, known as cldnk, produces a 915 base pair mRNA transcript from two exons.  The cldnk gene is involved in tight junction formation and may be required for myelin sheath development around axons. -What is the size of the pSKII plasmid after it is digested by EcoRI and XhoI and the cldnk gene is inserted?

Passage Renal reabsorption of glucose is facilitated by sodium-glucose linked transporters (SGLTs), which are secondary active transport proteins located in the apical membrane of the proximal tubular cells in the kidney.  SGLT activity determines the transport maximum of glucose T_m, the maximum rate at which the kidneys can reabsorb glucose.Excretion of glucose in the urine (glycosuria) occurs when the glomerular filtration rate (GFR) of glucose exceeds the T_m.  Glycosuria is a symptom of diabetes mellitus, a disease marked by hyperglycemia (elevated serum glucose).  Uncontrolled hyperglycemia can damage the nerves that innervate urinary tract musculature, resulting in urinary retention (inability to fully empty the bladder) or incontinence (involuntary urine leakage).Canagliflozin is a SGLT inhibitor used for the treatment for diabetes.  The effect of canagliflozin on renal glucose reabsorption was tested in a glucose infusion experiment in patients with type 2 diabetes mellitus (T2DM).  First, researchers measured the participants' fasting serum glucose levels (average ~80 mg/dL) and then began a continuous intravenous infusion of a glucose solution.  The rate of glucose infusion was adjusted such that blood glucose concentration steadily rose to 300 mg/dL.  Blood and urine glucose concentrations were measured at 30-minute intervals.  The experiment was later repeated in the same patients following canagliflozin administration (100 mg, once a day for seven days).  The results of both trials are shown in Figure 1. Figure 1  Mean urine glucose excretion versus blood glucose concentration in patients with T2DM, with and without canagliflozin treatmentResearchers then estimated the new T_m of glucose following canagliflozin treatment (Figure 2).  Canagliflozin was found to also lower fasting blood glucose in individuals with T2DM.  Side effects included weight loss, increased urinary frequency, excessive thirst, and blood pressure reduction. Figure 2  Mean rate of renal glucose reabsorption versus plasma glucose concentration in patients with T2DM, with and without canagliflozin treatment -In individuals with glycosuria, glucose that is not reabsorbed in the proximal tubule next flows into which of the following nephron structures?

Passage Bone remodeling is the continuous, microscopic process by which bone tissue throughout the body is resorbed and re-deposited.  Proper regulation of bone remodeling is necessary to maintain bone strength and prevent the onset of conditions of fragile bone such as osteoporosis.  Patients with such conditions have increased fracture risk and generally exhibit decreased activity of osteoblasts (bone-depositing cells), leading to reduced bone density and weakening of the bone matrix.  However, some cases can occur due to overactive osteoclasts (multinucleated cells that mediate bone resorption).  Osteoclasts resorb bone by secreting proteolytic enzymes and acids into the adjacent extracellular space.  These enzymes degrade the organic matrix of bone, and the acid causes the dissolution and release of the mineral components of bone, such as calcium, potassium, and magnesium.  Generally, less than 1% of adult bone is occupied by active osteoclasts.Osteoclast differentiation and activation is thought to be related to the RANK/RANKL signaling pathway.  RANKL, expressed as a membrane-bound or secreted protein, serves as the ligand for RANK, a transmembrane receptor.  In bone tissue, RANK is expressed by precursors of mature osteoclasts, also known as osteoclast progenitor cells (OPCs).  RANKL is expressed by osteoblasts adjacent to OPCs, and the binding of RANKL to RANK in these neighboring cells causes downstream effects through recruitment of TRAF6, an intracellular protein.  TRAF6 activates multiple downstream signaling cascades, but only four of these directly mediate the development of mature osteoclasts from OPCs.  In one of the four pathways, the transcription factor NF-kB is activated, which leads to increased expression of the proto-oncogene c-Fos.  Subsequently, c-Fos interacts with another transcription factor known as NFATc to promote the expression of genes related to osteoclast-specific development and function in OPCs (Figure 1). Figure 1  RANK/RANKL signaling induces differentiation of OPCs into osteoclasts -Cathepsin K is a proteolytic enzyme secreted by osteoclasts.  In a clinical trial, patients with osteoporosis due to increased RANK-RANKL signaling are divided into an untreated group (saline placebo administered) and a treated group (cathepsin K inhibitor administered).  Given the information in the passage, bone density after the trial will most likely be:

Passage Renal reabsorption of glucose is facilitated by sodium-glucose linked transporters (SGLTs), which are secondary active transport proteins located in the apical membrane of the proximal tubular cells in the kidney.  SGLT activity determines the transport maximum of glucose T_m, the maximum rate at which the kidneys can reabsorb glucose.Excretion of glucose in the urine (glycosuria) occurs when the glomerular filtration rate (GFR) of glucose exceeds the T_m.  Glycosuria is a symptom of diabetes mellitus, a disease marked by hyperglycemia (elevated serum glucose).  Uncontrolled hyperglycemia can damage the nerves that innervate urinary tract musculature, resulting in urinary retention (inability to fully empty the bladder) or incontinence (involuntary urine leakage).Canagliflozin is a SGLT inhibitor used for the treatment for diabetes.  The effect of canagliflozin on renal glucose reabsorption was tested in a glucose infusion experiment in patients with type 2 diabetes mellitus (T2DM).  First, researchers measured the participants' fasting serum glucose levels (average ~80 mg/dL) and then began a continuous intravenous infusion of a glucose solution.  The rate of glucose infusion was adjusted such that blood glucose concentration steadily rose to 300 mg/dL.  Blood and urine glucose concentrations were measured at 30-minute intervals.  The experiment was later repeated in the same patients following canagliflozin administration (100 mg, once a day for seven days).  The results of both trials are shown in Figure 1. Figure 1  Mean urine glucose excretion versus blood glucose concentration in patients with T2DM, with and without canagliflozin treatmentResearchers then estimated the new T_m of glucose following canagliflozin treatment (Figure 2).  Canagliflozin was found to also lower fasting blood glucose in individuals with T2DM.  Side effects included weight loss, increased urinary frequency, excessive thirst, and blood pressure reduction. Figure 2  Mean rate of renal glucose reabsorption versus plasma glucose concentration in patients with T2DM, with and without canagliflozin treatment -Administration of canagliflozin most directly increases urinary output by increasing the:

Passage Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disease affecting 1 in 10,000 live births.  The pathological hallmark of SMA is the loss of α-motor neurons in the spinal cord, which causes progressive paralysis and, ultimately, premature death in infants.  SMA is caused by insufficient levels of survival motor neuron (SMN), a regulatory protein required for the maintenance of the spinal cord nerve cells that control voluntary muscle movement.The SMN protein is generated from the SMN1 and SMN2 genes, which differ in sequence by only a single nucleotide in exon 7.  In healthy individuals, the gene product of SMN1 is the full-length (FL) SMN protein, and the gene products of SMN2 are both the FL-SMN protein as well as a truncated unstable protein (SMNΔ7) that is rapidly degraded.  Approximately 90% of the SMN2 gene product is SMNΔ7, and only 10% is FL-SMN (Figure 1).  In most patients with SMA, SMN1 is mutated such that its protein product is nonfunctional or deleted.  The disease phenotype arises because the SMN2 gene cannot fully compensate for the deficiency in FL-SMN protein synthesis. Figure 1  Structure of SMN1 and SMN2 transcriptsAntisense oligonucleotides (ASOs) that bind pre-mRNA were injected into SMA mice (SMN1^−/−, SMN2^+/+) exhibiting progressive tail necrosis, a characteristic measure of SMA severity.  ASO therapy effectiveness was assessed by measuring the tail length of wild-type (WT) mice and SMA mice exposed to increasing concentrations of ASOs (Figure 2). Figure 2  Tail length progression of WT mice compared to SMA mice treated with either saline or ASOs Adapted from Lorson CL, Hahnen E, Androphy EJ, Wirth B. A single nucleotide in the SMN gene regulates splicing and is responsible for spinal muscular atrophy. Proc Natl Acad Sci USA. 1999;96(11):6307-11. -Valproic acid (VA) is a histone deacetylase inhibitor that has been investigated for SMA therapy.  Valproic acid most likely improves motor neuron survival by:

Passage The nicotinic acetylcholine receptor (nAChR) is a protein embedded in the muscle fiber membrane that facilitates cell-to-cell communication across the neuromuscular junction.  When acetylcholine (ACh) binds nAChR, electrical depolarization within skeletal muscle fibers is induced and ultimately gives rise to contractile activity.  The nAChR is composed of five protein subunits symmetrically organized around a central ion pore.  Previous studies demonstrated that these subunits may serve as epitopes (antibody binding sites) in autoimmune disease.  By reversibly binding nAChR subunits, anti-nAChr antibodies may temporarily block the attachment site for ACh, resulting in muscle weakness.Researchers harvested muscle samples from mice to investigate the effects of anti-nAChR antibodies on muscle tension, the pulling force transmitted by muscles during contraction.Experiment 1First, skeletal muscle samples (tibialis anterior) of adult male mice were surgically harvested (n = 30), washed, and stripped of connective tissues.  Next, the isolated muscle samples were placed in a 150-mL oxygen-infused, electrolyte-rich bath containing glucose and Ca²⁺ ions that were able to enter the muscle fibers and enable maximal contractile force.  One end of the submerged muscle was anchored in place while the other was attached to an apparatus that measures muscle tension and modifies muscle length.  Using electrodes, action potentials were elicited in the harvested muscle tissue to evaluate contractile function (Figure 1). Figure 1  Action potential elicited from a harvested skeletal muscle sampleExperiment 2The researchers divided the 30 muscle samples into three groups of 10.  Resting muscle tension was measured, and the electrolyte-rich baths of each group were further infused with a 7.5-mL solution containing mouse-specific anti-nAChR antibodies, ACh, or a mixture of both (Table 1).  Post-infusion (peak) muscle tension was measured for 30 minutes, with results displayed in Figure 2.Table 1  Solutions Infused into the Electrolyte-Rich Bath Figure 2  Pre-infusion resting muscle tension and post-infusion peak muscle tension in harvested muscle samples (Note: * indicates p < 0.05.) -A mutant mouse is found to exhibit normal musculoskeletal function following the administration of anti-nAChR antibodies.  Compared to a wild-type mouse, this mutant mouse is most likely to show which of the following changes at the neuromuscular junction?

Passage Neurulation refers to the early formation of the nervous system in vertebrate embryos and begins when the notochord, a rodlike mesodermal structure, induces neural tube and neural crest cell formation in the ectoderm.  Neural tube defects (NTDs) arise from abnormalities in the development of the neural tube and can range from mild to severe, depending on which portion of the tube is affected.  Spina bifida, a form of NTD, is divided into three categories.  The most serious subclassification of spina bifida is myelomeningocele, which occurs when the lower spine fails to close, which leaves the spinal cord and nerve roots in the lower back damaged and exposed.NTDs are thought to be caused in part by folate deficiency.  Folate plays a critical role in normal cell division and growth as it is a necessary coenzyme for DNA synthesis and methylation.  Mutations in several genes involved in folate metabolism have been associated with an increased risk for NTDs.  It has been hypothesized that folate decreases NTD risk by ameliorating oxidative stress, thereby reducing damage to developing cells in the embryo.  Certain maternal conditions, such as high blood glucose, increase the risk of NTD development by enhancing embryonic oxidative stress during crucial developmental periods.A study was conducted to examine how supplementation with folic acid, the dietary form of folate, affected pregnant women who were identified to have preexisting diabetes, pregestational obesity (another risk factor for NTD), or no preexisting conditions (control group).  Higher folic acid intake decreased the incidence of embryonic NTDs among pregnant women with diabetes, but no significant effect was found for pregnant women who were obese. Adapted from Parker SE, Yazdy MM, Tinker SC, Mitchell AA, Werler MM. The impact of folic acid intake on the association among diabetes mellitus, obesity, and spina bifida. Am J Obstet Gynecol. 2013;209(3):239.e1-8. -Neural crest cells migrate away from the neural tube to give rise to most of the peripheral nervous system.  Given this information, would neural crest cells be involved in the pathology of myelomeningocele?

The sequence below represents a mature mRNA molecule in a eukaryotic cell.5′-GCGGACAUGCGGCCACGAUAGACUCCAAAAAAA-3′If translation of this mRNA molecule were to occur, which of the following most likely corresponds to the anticodons of the first and last tRNAs that would be brought into the ribosome, respectively?

The diagram below shows the results of a northern blot analysis that tested for the expression of four peptide hormones in two endocrine tissues. The endocrine glands that most likely correspond to tissues A and B are the: