The Nucleus & Cytoplasm (A Histological Perspective)

Identify the structure highlighted in yellow. A. Nucleus B. Endoplasmic Reticulum C. Mitochondria D. Golgi

*The answer is A.* The nucleus is highlighted.

In transmission EM preparations of cells the cell membrane often appears as a trilaminar structure having two parallel dark-staining components on either side of an unstained middle layer. This central poorly stained region of the membrane is primarily responsible for which of the following functions? a. Creation of a barrier to water-soluble molecules b. Binding by cellular receptions to specific ligands c. Catalyzing membrane-associated activities d. Transport of ions e. Connections to the cytoskeleton

*The answer is A.* The plasma membrane creates a barrier to water-soluble molecules.

Which of the following proteins is/are most likely to have initially contained a "signal peptide" that bound a "signal recognition particle" during its translation? a. An enzyme of the respiratory chain b. Lamins c. Proteins in secretory granules d. F-actin e. Proteins in the large ribosomal subunit

*The answer is C.* Certain proteins contain a signal peptide, which the signal recognition particle binds to and directs toward the rough ER. Proteins that enter the RER go through the Golgi where they are processed and sorted. They are either transported to the lysosomes or secretory vesicles where they are secreted from the cell or go through the cell membrane.

Release of cytochrome c from the organelle activates which of the following cellular processes? (A) Apoptosis (B) Autophagy (C) Cell division (D) Cell motility (E) Exocytosis

*The answer is A.* Apoptosis is a programmed pathway of cell death that is triggered by a variety of extracellular and intracellular signals. It is often a self-defense mechanism, destroying cells that have been infected with pathogens or those in which genomic alterations have occurred. Mitochondria play a key role in regulating apoptosis. In response to cellular stress, mitochondria open an outer membrane "permeability transition pore" that permits the release of cytochrome c from the inner mitochondrial membrane to the cytoplasm. Within the cytoplasm, cytochrome c triggers an apoptotic cascade that leads to the activation of effector enzymes (caspases) that degrade chromatin and destabilize the cytoskeleton. During development, apoptosis deletes unwanted cells in limb buds to form the digits. None of the other cellular processes are activated by the release of cytochrome c from mitochondria.

Key differences between apoptotic and necrotic cell death include which of the following? a. Apoptotic cells do not release factors that induce inflammation. b. Necrosis does not trigger inflammation. c. Apoptosis does not utilize intracellular proteases. d. Apoptosis usually follows lethal physical damage to a cell. e. Necrosis is involved in formation of some organs during embryonic development.

*The answer is A.* Apoptotic cells do not release factors that induce inflammation. Necrosis can trigger inflammation. Apoptosis uses intracellular proteases known as caspases.

The gene for green fluorescent protein is modified by the addition of a signal sequence that targets the translation product to the lumen of the endoplasmic reticulum (ER). The distribution of the rough ER in a transfected myoblast cell line is monitored by confocal fluorescence microscopy (shown in the image). Which of the following families of proteins facilitates proper protein folding in the ER, cytoplasm, and nucleus of this muscle stem cell? (A) Chaperones (B) Clathrins (C) Cyclins (D) Lamins (E) Ubiquitin ligases

*The answer is A.* For many proteins, polypeptide folding is prone to error and requires the assistance of molecular chaperones. Chaperones are a family of proteins found in the nucleus, cytoplasm, and ER that assist other proteins in assuming their correct three-dimensional conformation. They also prevent protein aggregation and target abnormally folded proteins for proteolytic degradation. Chaperones that are up-regulated in response to cellular stress are referred to as "heat shock proteins." Mutations in chaperone genes have been linked to a number of chronic diseases, termed chaperonopathies. None of the other proteins regulates protein folding.

Binding of histone H1 proteins to importins is important for which of the following? a. Transport through the nuclear pores complexes b. Properly directed vesicular transport through the Golgi apparatus c. Transport from the granular part of the nucleolus d. Further binding to the "linker DNA" and proper assembly of nucleosomes e. Phosphorylation of cyclins

*The answer is A.* Importins help to transport histone H1 proteins into the nucleus via nuclear pore complexes.

You are studying cell migration during embryonic development. Neural tubes are harvested from postimplantation mouse embryos and placed in culture on plastic dishes coated with fibronectin. Time-lapse imaging reveals neural crest cells migrating away from the explanted tissue. The cells are observed to undergo continuous changes in cell shape, including the formation and retraction of lamellipodia. What protein is the principal mediator of membrane ruffling and locomotion in these cultured cells? (A) Actin (B) Desmin (C) Lamin (D) Tubulin (E) Vimentin

*The answer is A.* Motility is a remarkable property of cells that is essential for embryonic development, wound healing, and lymphocyte trafficking. Cell locomotion involves the coordinated assembly and disassembly of actin microfilaments. Actin filaments are helical structures, with a growing end that adds globular (G-actin) to filamentous F-actin. Assembly of microfilaments can generate membrane protrusions, such as filopodia and lamellipodia. Changes in the shape of lamellipodia over time are referred to as "membrane ruffling." During cell locomotion, the leading edge of the plasma membrane displays cell-substrate adhesion proteins that bind glycoproteins in the extracellular matrix. Desmin and vimentin (choices B and E) are intermediate filament proteins found in mesenchymal cells. Lamins (choice C) are nuclear matrix proteins that stabilize the nuclear membrane and organize chromatin. Tubulins (choice D) form the spindle apparatus, regulate intracellular transport, and control the movement of cilia and flagella.

A 29-year-old woman presents with a 101°F fever, pericardial effusions and Libman-Sachs endocarditis, arthralgia, and facial rash across the malar region ("butterfly rash") that is accentuated by sun exposure. Laboratory tests show creatine 1.7 mg/dL (normal 0.5-1.1 mg/dL), high titers of antinuclear autoantibodies (ANA), Smith antigen, and antinucleosome antibodies in the blood. Which one of the following is most likely to be directly affected by the disruption of nucleosomes in this patient? a. Packaging of genetic material in a condensed form b. Transcribing DNA c. Forming pores for bidirectional nuclear-cytoplasmic transport d. Forming the nuclear lamina e. Holding together adjacent chromatids

*The answer is A.* Nucleosomes help package genetic material in a condensed form.

You are asked to lead a seminar on intracellular protein trafficking. What organelle provides a microenvironment for the posttranslational modification and sorting of membrane and secretory proteins? (A) Golgi apparatus (B) Lysosome (C) Peroxisome (D) Plasma membrane (E) Smooth endoplasmic reticulum

*The answer is A.* The Golgi apparatus is an intracellular organelle that regulates posttranslational modification and sorting of membrane and secretory proteins. Like the ER, the Golgi apparatus is composed of flat membrane sacs (vesicles). Newly synthesized proteins leave the ER in small transport vesicles that fuse with the Golgi membrane network. Here, a variety of glycosyltransferase enzymes attach linear and branched oligosaccharide chains to the asparagine residues (N-linked glycans) and serine/threonine residues (O-linked glycans) of membrane and secretory proteins. The ultimate destination of each protein is determined by intrinsic signal peptides and patterns of protein glycosylation. Mature vesicles leave trans-Golgi membranes as secretory vesicles that may be stored in apical cytoplasm or as transport vesicles that deliver proteins/ glycoproteins to various organelles or membrane domains (e.g., apical, basal, or lateral membranes). Lysosomes (choice B) are vesicles filled with acid hydrolases that degrade cellular debris. Peroxisomes (choice C) are small vesicles filled with catalase and other enzymes that remove reactive oxygen species (e.g., hydrogen peroxide). None of the other organelles are involved in the posttranslational modification of membrane and secretory proteins.

Hepatocytes in a liver biopsy are examined by electron microscopy. The parallel lines with knob-like features (arrows, shown in the image) represent which of the following intracellular organelles? (A) Endoplasmic reticulum (B) Golgi apparatus (C) Mitochondria (D) Nucleus (E) Peroxisomes

*The answer is A.* This electron micrograph demonstrates ultrastructural features of rough endoplasmic reticulum (ER). These flat membrane vesicles provide a large surface area for protein synthesis (translation). The small knob-like features are ribosomes that are actively synthesizing membrane and secretory proteins. Signal peptides mediate the attachment of ribosomes to the rough ER. Signal recognition particles, docking proteins, and translocator proteins collaborate to shepherd these proteins through the lipid bilayer. Cytosolic proteins are synthesized by "free ribosomes." None of the other organelles exhibit the ultrastructural features of rough ER.

Identify the structure highlighted in red. A. Nucleus B. Endoplasmic Reticulum C. Mitochondria D. Golgi

*The answer is B.* The endoplasmic reticulum is highlighted.

A 14-year-old boy is diagnosed with epidermolysis bullosa simplex (EBS). His skin blisters easily with rubbing or scratching. Blisters occur primarily on his hands and feet and heal without leaving scars. Genetic analysis shows mutations in the KRT5 and KRT14 genes, which code keratin 5 and keratin 14. What is the primary function of those proteins? a. Generate movement b. Provide mechanical stability c. Carry out nucleation of microtubules d. Stabilize microtubules against disassembly e. Transport organelles within the cell

*The answer is B.* The primary function of keratin is to provide mechanical stability.

A 23-year-old man presents with a 6-month history of yellow skin and sclerae. Physical examination shows mild jaundice and peritoneal ascites. The patient is subsequently diagnosed with α-1-antitrypsin deficiency. A liver biopsy stained with PAS reveals globular inclusions of misfolded α-1-antitrypsin (shown in the image). The abundance of these abnormal glycoproteins has apparently overwhelmed normal degradation pathways. Which of the following cellular processes describes the normal mechanism for specifically targeting and degrading misfolded proteins within cells? (A) Activation of the caspase enzyme cascade (B) Activation of the ubiquitin-proteasome pathway (C) Delivery of acid hydrolases to lysosomes (D) Fusion of secretory vesicles with the plasma membrane (E) Generation of reactive oxygen species

*The answer is B.* Abnormal or unwanted proteins are degraded within lysosomes or targeted for degradation by proteasomes. Proteasomes are protein complexes that bind and degrade proteins that have been "tagged" with ubiquitin, a 76-amino-acid protein. Protein ubiquitination is a complex process that requires a variety of enzymes, including activators, conjugating enzymes, and ubiquitin ligases. α-1-Antitrypsin deficiency is a heritable disorder in which mutations in the gene for α-1-antitrypsin yield an insoluble protein. These globules stain red with PAS. The mutant protein is not easily exported from the cells. It accumulates causing cell injury and cirrhosis. α-1-Antitrypsin deficiency is the most common genetic cause of liver disease in infants and children and the most frequent genetic disease for which liver transplantation is indicated. Caspases (choice A) mediate apoptotic cell death. Reactive oxygen species (choice E) contribute to the lysis of bacteria and necrotic debris in the phagolysosomes of inflammatory cells.

Transitions in the cell cycle from one phase to the next are regulated by protein kinases whose activity depends on what other proteins? a. Tumor suppressors b. Cyclins c. Actins d. Lamins e. Importins

*The answer is B.* Cyclins are a family of cytoplasmic proteins present at different cell cycle phases, which activates a specific cyclin-dependent kinase.

You are studying the role of mitochondrial dysfunction in alcoholic liver disease. Genes for an inner mitochondrial membrane protein and a red fluorescent protein are spliced, and the fusion protein is expressed in mouse embryo fibroblasts. The distribution of mitochondria in the transfected cells is visualized by confocal fluorescence microscopy (shown in the image). Inhibition of the electron transport chain in this organelle leads to which of the following reversible changes in cell behavior? (A) Extension of filopodia (B) Hydropic swelling (C) Intracellular lipid storage (D) Membrane ruffling (E) Protooncogene activation

*The answer is B.* Fusion proteins containing fluorescent protein markers can be used to examine the distribution of organelles in living cells. In this experiment, mitochondria are identified as long, coiled, rope-like structures. These ATP energy-producing organelles are derived from the oocyte at the time of fertilization. They carry their own DNA, synthesize many of their own proteins, and replicate autonomously during interphase. Mitochondria can assume different sizes and shapes, and they often localize to sites within the cell where energy is most needed. When cellular levels of ATP are depleted (e.g., by exposure to toxins or lack of oxygen), cells undergo acute hydropic swelling. This increase in cell volume is caused by an inability of the plasma membrane Na/K ATPase to pump sodium out of the cell. Without adequate levels of ATP to fuel the membrane pump, sodium and water are retained within the cell, and the cell swells. Inhibition of the mitochondrial electron transport chain over an extended period of time will lead to cellular atrophy. ATP depletion does not lead to the other listed changes in cell morphology or behavior.

The retrieval of secretion granule membrane immediately after exocytosis occurs in which of the following cell components? (A) A caveolin-coated vesicle (B) A clathrin-coated vesicle (C) A coatomer I-coated vesicle (D) A coatomer II-coated vesicle (E) An early endosome

*The answer is B.* Membrane recycling after exocytosis of the contents of a secretion granule occurs via clathrin-coated vesicles.

Movement of acid hydrolases from the trans-Golgi network to a late endosome takes place in which of the following cell components? (A) A caveolin-coated vesicle (B) A clathrin-coated vesicle (C) A coatomer I-coated vesicle (D) A coatomer II-coated vesicle (E) An early endosome

*The answer is B.* Proteins targeted for lysosomes (via late endosomes) leave the trans-Golgi network in clathrin-coated vesicles.

Which of the following is found during meiosis but not mitosis? a. Chromatids b. Polar microtubules c. Metaphase d. Synapsis e. Cytokinesis

*The answer is D.* Synapsis only occurs during meiosis I.

A 55-year-old woman learns that she has high levels of serum cholesterol (greater than 280 mg/dL; normal less than 200 mg/dL) and is at increased risk for development of ischemic heart disease. The patient asks you to explain the normal pathway for serum cholesterol uptake and clearance. You explain to her that low-density lipoprotein (LDL) receptors present in her liver bind LDL cholesterol and internalize it by forming coated vesicles (endosomes). Which of the following structural proteins mediates LDL receptor internalization by organizing small buds of plasma membrane into endosomes? (A) Actin (B) Clathrin (C) Desmin (D) Laminin (E) Vimentin

*The answer is B.* The LDL receptor is a transmembrane glycoprotein that regulates plasma cholesterol by mediating endocytosis and recycling of apolipoprotein (apo) E. Lacking LDL receptor function, high levels of LDL circulate, are taken up by tissue macrophages, and accumulate to form arterial plaques (atheromas). Receptor-mediated endocytosis is a mechanism for uptake of specific ligands and receptors that is regulated by clathrin. Clathrin stabilizes small invaginations of the plasma membrane, forming coated vesicles (endosomes). Coated vesicles are transported to lysosomes, where ligands and receptors are separated, and receptors are recycled to the plasma membrane. None of the other proteins regulates receptor-mediated endocytosis.

As part of your research, you investigate the role of cyclins and cyclin-dependent kinases in regulating ES cell growth in vitro. These rapidly dividing cells spend most of their time in which phase of the mitotic cell cycle? (A) G0 (B) G1 (C) G2 (D) M (E) S

*The answer is B.* The cell cycle can be divided into discrete phases that are referred to as G1, S, G2, and M. Cells that have exited the cell cycle are said to reside in G0. Together, G1, S, and G2 constitute interphase. DNA is replicated for cell division during S-phase. Progression of cells through G1 and G2 are regulated by cyclins and cyclin-dependent kinases. These gap phases provide critical checkpoints for cell division. In most rapidly proliferating cells, G1 is the longest and most variable phase of the cell cycle (not choices A, C, D, and E). During G1, cells "evaluate" the integrity of their genome. DNA damage that cannot be repaired typically leads to programmed cell death. M phase is divided into prophase, metaphase, anaphase, and telophase.

A spinal cord smear preparation is obtained at autopsy and stained with Luxol fast blue/cresyl violet. The large octopus-like cells on this slide are multipolar motor neurons (shown in the image). A protein forms intracellular tracts that deliver organelles and vesicles to distant nerve terminals via anterograde axonal transport. The motor neurons described are labeled by immunocytochemistry using antibodies directed against a neuron-specific protein that helps maintain the shape of dendrites and axons. This structural protein forms which of the following intracellular organelles? (A) Endoplasmic reticulum (B) Intermediate filaments (C) Microfilaments (D) Microtubules (E) Plasma membrane

*The answer is B.* The cytoskeleton is an intracellular network of filamentous proteins that provides structural support, transports organelles, regulates cell motility, and controls cell division. It includes microtubules composed of tubulin, microfilaments composed of actin, and intermediate filaments composed of tissue-specific fibrous proteins. Unlike microtubules and microfilaments, intermediate filaments are nonpolar structures composed of protein building blocks that vary from one tissue to another. Intermediate filament protein families include keratins, lamins, vimentins, desmins, and neurofilament proteins. Keratins protect the external surface of the skin. Lamins stabilize the inner nuclear membrane, organize chromatin, and regulate gene expression. Neurons express neurofilament proteins that provide flexible, structural support to help maintain complex patterns of axons and dendrites within the central and peripheral nervous system. Microtubules and microfilaments are present in nerve axons and dendrites and contribute to cell structure, but they are not composed of neuron-specific fibrous proteins. None of the other organelles provide structural support to neurons or glial cells.

Which of the following is a region of chromatin that is well developed in large neurons active in protein synthesis? a. Heterochromatin b. The nucleolus c. The Nissl substance (neuronal RER) d. The Barr body e. The nucleosome

*The answer is B.* The nucleolus is a region in the nucleus where the synthesis of rRNA occurs. rRNA is used to form ribosomes, which are sites of protein synthesis.

As part of your research, you examine integral membrane proteins in cleavage-stage mouse embryos using fluorescence microscopy (shown in the image). A pulse of high-intensity UV light is directed at a small patch on the surface of one blastomere, thereby causing an immediate loss of fluorescence emission (photobleaching). Over the next 10 minutes, fluorescence emission from this patch of membrane recovers. Which of the following cellular properties/processes best explains these experimental findings? (A) Lipid raft assembly (B) Membrane fluidity (C) Patching and capping (D) Protein trafficking (E) Receptor-mediated endocytosis

*The answer is B.* The plasma membrane separates the cytoplasm and intracellular organelles from the external environment. Loss of plasma membrane integrity results in cell death (necrosis). The plasma membrane is a fluid mosaic of lipids and proteins. Integral proteins pass through the lipid bilayer, whereas peripheral proteins do not. Membrane proteins are essential for cell viability and differentiated cell functions. For example, membrane proteins serve as pumps, enzymes, channels, receptors, structural molecules, and attachment sites. Oligosaccharides and polysaccharides conjugated to membrane proteins and sphingolipids form a cell surface coat (glycocalyx). In polarized epithelial cells, the plasma membrane exhibits distinct apical, basal, and lateral domains. Fluorescence recovery after photobleaching (FRAP) is an experimental technique that can be used to measure the rate at which lipids and proteins move by lateral diffusion within the plane of the membrane. The viscosity of the plasma membrane has been compared to that of thick molasses. Tight junctions provide a barrier to the lateral diffusion of membrane proteins and lipids. In some cells, the plasma membrane forms microdomains (lipid rafts, choice A) that regulate cell signaling. Patching and capping (choice C) describe the clustering of cell surface molecules by specific cross-linking agents, such as antibodies or pollen. Protein trafficking and endocytosis (choices D and E) do not regulate the lateral diffusion of lipids and proteins in the plasma membrane.

Identify the structure highlighted in yellow. A. Nucleus B. Endoplasmic Reticulum C. Mitochondria D. Golgi

*The answer is C.* The mitochondria are highlighted.

Vesicles of a Golgi apparatus that are destined to become part of other organelles most likely have which of the following on their membranes? a. Channel proteins b. Clathrin c. COP II d. Actin e. GTP

*The answer is C.* Vesicles of a Golgi apparatus that are destined to become part of other organelles most likely have COP II on their membranes. COP II coated vesicles are vesicles that carry proteins from the ER to the ERGIC and on to the cis-Golgi network.

Which of the following proteins contributes to the structural matrix that anchors chromatin to the nuclear membrane during interphase of the cell cycle? (A) Desmin (B) Keratin (C) Lamin (D) Perlecan (E) Vimentin

*The answer is C.* A network of intermediate filament proteins is associated with the inner nuclear membrane. This nuclear (fibrous) lamina stabilizes the nuclear membrane, organizes chromatin, and regulates gene expression. It is composed largely of lamin A and lamin C proteins that form intermediate filaments. Lamin receptors bind these filamentous proteins to the nuclear membrane. During cell division, the nuclear lamina and nuclear membrane disintegrate to facilitate chromosome segregation and separation. Lamin gene mutations are associated with a variety of diseases (laminopathies) including Hutchinson-Gilford progeria. Patients with progeria undergo accelerated aging. Perlecan (choice D) is a basement membrane protein. None of the other intermediate filament proteins (choices A, B, and E) anchors chromatin to the nuclear membrane.

Which of the following organelles divides by fission? (A) Golgi complex (B) RER (C) Peroxisome (D) SER (E) Centriole

*The answer is C.* A peroxisome originates from preexisting peroxisomes. It imports specific cytosolic proteins and then undergoes fission. The other organelle that divides by fission is the mitochondrion.

Which of the following facilitates breakdown of the nuclear envelope during the onset of mitosis? a. Disassembly of nucleosomes in the associated constitutive heterochromatin b. Increased export of material by the nuclear pore complexes into the perinuclear space c. Phosphorylation of lamin subunits by a cyclin-dependent kinase (CDK) d. Activities triggered at a restriction point late in G1 e. The activity of proteasomes

*The answer is C.* Cyclin-dependent kinase (CDK) phosphorylates lamin subunits in order to breakdown the nuclear envelope during the onset of mitosis.

You are studying the differentiation of epithelial cells lining the intestinal mucosa and identify a common stem cell for the secretory lineage that gives rise to Paneth cells, enterocytes, and goblet cells. Which of the following terms describes the developmental potential of these gastrointestinal stem cells? (A) Embryonic (B) Metaplastic (C) Multipotent (D) Nullipotent (E) Pluripotent

*The answer is C.* Development proceeds from clusters of self-renewing stem cells to beautiful networks of highly differentiated cells. How stem cells acquire instructions for differentiation remains a mystery. When these instructions are revealed, stem cell-based therapies may transform medicine, providing a source of replacement cells and tissues for patients with chronic diseases. The zygote and early cleavage stage blastomeres are totipotent cells, meaning that they have the ability to form all embryonic and extraembryonic tissues. The inner cell mass of the blastocyst is composed of pluripotent embryonic stem cells (choices A and E) that give rise to all embryonic cells and tissues. Pluripotent embryonic stem (ES) cells can be isolated from human blastocysts and cultured in vitro. ES cells that lose the ability to undergo differentiation are said to be nullipotent (choice D). Metaplastic cells (choice B) have undergone a change in differentiation from one pathway to another. Examples of metaplasia include squamous metaplasia in the lungs of smokers and glandular metaplasia in the esophagus of patients with acid reflux. The correct answer for this question is multipotent (choice C). Gastrointestinal stem cells that have the ability to differentiate into a limited number of derivatives are best described as multipotent, adult stem cells.

A soft tissue biopsy is examined in the pathology department. Normal adipocytes are examined at high magnification (shown in the image). The clear space that has pushed the cytoplasm and nucleus to the periphery of these cells is best described by which of the following terms? (A) Endosome (B) Granule (C) Inclusion (D) Vacuole (E) Vesicle

*The answer is C.* Differentiated cells synthesize a wide variety of proteins, lipids, and carbohydrates that are stored, transported, or secreted. Adipocytes synthesize and store large quantities of triglycerides. Lipid droplets in the cytoplasm coalesce to form a large inclusion that pushes the cytoplasm and nucleus to the periphery of the cell (shown in the image). Glycogen, hemosiderin (denatured ferritin), and lipofuscin (crosslinked lipids and proteins) are also stored as cytoplasmic inclusions. Other metabolic products are packaged within membrane-bound organelles, termed vesicles (choice E). The cytoplasm of most cells is filled with innumerable small vesicles. With the help of microtubules and motor proteins, vesicles transport proteins, lipids, and carbohydrates from one organelle to another (e.g., from ER to Golgi or plasma membrane to lysosome). Large membrane-bound organelles are referred to as vacuoles (choice D). Examples of vacuoles include phagolysosomes and autophagic vacuoles. Endosomes (choice A) are vesicles that internalize ligands and cell surface receptors and transport them to lysosomes for degradation or for recycling back to the plasma membrane. Granules (choice B) are secretory vesicles that are commonly stored in apical cytoplasm. During exocytosis, secretory granules fuse with the plasma membrane, releasing their contents to the extracellular space.

A 23-year-old woman complains of recurrent bone pain and increasing abdominal girth. Physical examination reveals enlargement of the patient's liver and spleen (hepatosplenomegaly). A spleen biopsy reveals large macrophages, with a fibrillar appearance reminiscent of "wrinkled tissue paper" (shown in the image). The patient is subsequently diagnosed with Gaucher disease. She carries mutations in the genes for glucocerebrosidase. Without this hydrolytic enzyme, glucocerebroside accumulates within which of the following cellular organelles? (A) Autophagic vacuoles (B) Endoplasmic reticulum (C) Lysosomes (D) Mitochondria (E) Peroxisomes

*The answer is C.* Gaucher disease is characterized by the accumulation of glucosylceramide in the lysosomes of macrophages. The underlying abnormality in Gaucher disease is a deficiency in glucocerebrosidase— a lysosomal acid hydrolase. The hallmark of this disorder is the presence of lipid-laden macrophages (Gaucher cells) in the spleen, liver sinusoids, lymph nodes, lungs, and bone marrow. Gaucher cells are derived from resident macrophages in the respective organs (e.g., Kupffer cells in the liver and alveolar macrophages in the lung). None of the other organelles stores glucosylceramide in patients with Gaucher disease.

Which of the following consists of globular actin monomers linked into a double helix? (A) Keratin (B) Lamin A (C) Microfilament (D) Microtubule (E) Neurofilament

*The answer is C.* Globular actin monomers (G actin) polymerize into a double helix of filamentous actin (F actin), also called a microfilament, in response to the regulatory influence of a number of actin-binding proteins.

A 32-year-old man and his 30-year-old wife are referred for a reproductive endocrinologist infertility (REI) consult after 2 years of "trying to get pregnant." He is diagnosed with oligozoospermia. Ejaculated mature sperm are collected and undergo genetic analysis. Using gene linkage analysis, his REI specialist determines that he has aberrations in spermatogenic meiotic recombination, including both diminished frequency and suboptimal location, resulting in high frequency of aneuploid sperm. In explaining the diagnosis, she explains meiosis and recombination attributing the problem to a specific phase of the meiosis. Which part of meiosis is most closely associated with recombination? a. Metaphase I b. Anaphase I/Telophase I c. Prophase I d. Prophase II e. Anaphase II/Telophase II

*The answer is C.* In prophase I, synapsis and crossing over occurs.

An 85-year-old woman with Alzheimer disease dies in her sleep. At autopsy, hepatocytes are noted to contain golden cytoplasmic granules that do not stain with Prussian blue (shown in the image). This "wear-and-tear" pigment of aging (lipofuscin) accumulates primarily within which of the following cellular organelles? (A) Endosomes (B) Golgi apparatus (C) Lysosomes (D) Peroxisomes (E) Vacuoles

*The answer is C.* Lysosomes are acidic vesicles that degrade proteins, lipids, and carbohydrates. They are filled with a variety of acid hydrolases that degrade macromolecules to their constituent parts (e.g., amino acids and simple sugars). In some situations, lysosomes are unable to degrade cellular debris. Examples include (1) endogenous substrates that are not catabolized because a key enzyme is missing (lysosomal storage diseases), (2) insoluble endogenous pigments (lipofuscin and melanin), and (3) exogenous particulates (silica and carbon). Examination of this patient's liver at autopsy reveals insoluble "wear-and-tear" pigment of aging. These pigments are composed of cross-linked lipids and proteins (peroxidation products) that accumulate over time. Lipofuscin is stored within the lysosomes of long-lived cells in the brain, heart, and liver. None of the other organelles store lipofuscin.

Movement of protein from trans to cis Golgi cisternae occurs in which of the following cell components? (A) A caveolin-coated vesicle (B) A clathrin-coated vesicle (C) A coatomer I-coated vesicle (D) A coatomer II-coated vesicle (E) An early endosome

*The answer is C.* Transfer of material among the cisternae of the Golgi complex in a retrograde direction takes place via (COP-I) coatomer-coated vesicles.

Cytoplasm often stains poorly because its lipid content is removed by the organic solvents used in the clearing step in routine histological preparations. This problem is most likely to occur with cytoplasmic regions rich in which of the following organelles? a. Free polysomes b. Mitochondria c. Lysosomes d. Smooth endoplasmic reticulum e. Rough endoplasmic reticulum

*The answer is D.* Cytoplasmic regions rich in smooth ER would most likely have this problem because it has a high lipid content.

Which of the following best defines the term "exocytosis"? a. The discharge of ions or small molecules from a cell by protein pumps in the cell membrane b. The uptake of material at one domain of a cell's surface and its discharge from the opposite side of the cell c. The process by which proteins move from one cytoplasmic compartment to another d. The discharge of proteins in cytoplasmic vesicles from a cell following fusion of the vesicles with the plasmalemma e. Diffusion of lipid-soluble molecules from a cell across the cell membrane

*The answer is D.* Exocytosis is a process by which the contents of a cell vacuole are released to the exterior through fusion of the vacuole membrane with the cell membrane.

About 3 years ago, a 39-year-old construction worker became increasingly uncoordinated. His wife describes bouts of depression and apathy beginning about a decade ago. Laboratory tests are normal. MRI and CT reveal striatal and caudate atrophy with "boxcar ventricles." His mini-mental status examination score is 24/30. The cranial nerve examination shows dysarthria, saccadic extraocular eye movements, and a hyperactive gag reflex. There is increased tone in all extremities. Polymerase chain reaction reveals one normal band with 20 CAG (trinucleotide) repeats and the other with 49 CAG repeats. Modulation of respiration and mitochondrial membrane potential, and bioenergetic failure are associated with the abnormal gene in this disease. Which of the following mechanisms used to establish the mitochondrial electrochemical gradient may be altered in this disease? a. The action of ATP synthase b. Transfer of electrons from NADH to O2 in the intermembrane space c. Pumping of protons into the mitochondrial matrix by respiratory chain activity d. Proton-translocating activity in the inner membrane e. Transport of ATP out of the matrix compartment by a specific transporter

*The answer is D.* Proton-translocating activity in the inner membrane of the mitochondria is used to establish the mitochondrial electrochemical gradient.

A 56-year-old man has been taking atorvastatin because of a poor lipid profile and a family history of cardiovascular disease. The statin family of drugs enhances endocytosis of low density lipoprotein (LDL) from the blood. Endocytosis of LDL differs from phagocytosis of bacterial cells in which of the following ways? a. Use of membrane-enclosed vesicles in the uptake process b. Coupling with the lysosomal system c. Dependence on acidification d. Use of clathrin-coated pits e. Use of hydrolases

*The answer is D.* The endocytosis of LDL uses clathrin-coated pits, whereas the phagocytosis of bacterial cells does not.

Which of the following has a rigid wall composed of 13 protofilament strands? (A) Keratin (B) Lamin A (C) Microfilament (D) Microtubule (E) Neurofilament

*The answer is D.* A microtubule consists of - and -tubulin dimers polymerized into a spiral around a hollow lumen to form a fairly rigid tubule. When cross-sectioned, the microtubule reveals 13 protofilament strands, which represent the tubulin dimers present in one complete turn of the spiral.

The genes for green fluorescent protein and tubulin are spliced, and the fusion protein is expressed in a myoblast cell line. The distribution of microtubules is monitored by confocal fluorescence microscopy (shown in the image). During mitosis, these cytoskeletal proteins are reorganized to coordinate chromosome separation. Which of the following organelles is the principal microtubule-organizing center in these myoblasts? (A) Astral fibers (B) Basal body (C) Centromeres (D) Centrosomes (E) Kinetochores

*The answer is D.* As mentioned above, microtubules are rigid hollow tubes composed of repeating units of αβ-tubulin dimers. They regulate diverse cellular activities, including chromosome separation during mitosis and meiosis, intracellular vesicle transport, and the movement of cilia and flagella. These polymeric structures grow from nucleation sites within centrosomes. Centrosomes are composed of two centrioles positioned at right angles and a zone of pericentriolar proteins that regulate microtubule nucleation. Centrosomes are associated with the nuclear membrane during interphase and replicated during S-phase of the cell cycle. Basal bodies (choice B) are modified centrioles located at the base of cilia and flagella. Kinetochores (choice E) are protein complexes on chromosomes that provide attachment sites for the spindle apparatus during cell division. None of the other organelles is a primary microtubule-organizing center in nonciliated, muscle stem cells.

A spinal cord smear preparation is obtained at autopsy and stained with Luxol fast blue/cresyl violet. The large octopus-like cells on this slide are multipolar motor neurons (shown in the image). What protein forms intracellular tracts that deliver organelles and vesicles to distant nerve terminals via anterograde axonal transport? (A) Actin (B) Clathrin (C) Lamin (D) Tubulin (E) Ubiquitin

*The answer is D.* Axons are cellular processes that convey electrochemical signals away from neuronal cell bodies. These elongated structures are largely dependent on the neuronal cell body for the delivery of organelles and vesicles and for the removal of cellular waste (e.g., abnormal proteins). Axonal transport is an intracellular shuttle/delivery system that uses microtubules and motor proteins (e.g., kinesin and dynein) to transport vesicles to and from the synaptic membrane. Microtubules are rigid hollow tubes composed of repeating units of αβ-tubulin dimers. These polymeric structures grow from nucleation sites within centrosomes. Tubulins comprise a family of proteins that regulate diverse cellular activities, including: (1) chromosome separation during mitosis and meiosis, (2) intracellular vesicle transport, and (3) the whip-like movement of cilia and flagella. Clathrin (choice B) forms coated membrane vesicles during receptor-mediated endocytosis. Ubiquitin (choice E) is a protein that tags other proteins for degradation by proteasomes. None of the other proteins mediate axonal (axoplasmic) transport.

A 30-year-old man with very high blood cholesterol levels (290 mg/dL) has been diagnosed with premature atherosclerosis. His father died of a heart attack at age 45, and his mother, aged 44, has coronary artery disease. Which of the following is the most likely explanation of his condition? (A) He has a lysosomal storage disease and cannot digest cholesterol. (B) He has a peroxisomal disorder and produces low levels of hydrogen peroxide. (C) The SER in his hepatocytes has proliferated and produced excessive amounts of cholesterol. (D) He has a genetic disorder and synthesizes defective LDL receptors. (E) He is unable to manufacture endosomes.

*The answer is D.* Cells import cholesterol by the receptor-mediated uptake of LDLs in coated vesicles. Certain individuals inherit defective genes and cannot make LDL receptors, or they make defective receptors that cannot bind to clathrin-coated pits. The result is an inability to internalize LDLs, which leads to high levels of LDLs in the bloodstream. High LDL levels predispose a person to premature atherosclerosis and increase the risk of heart attacks.

A kidney biopsy from a 44-year-old man is examined by electron microscopy. The nucleus of an endothelial cell exhibits a peripheral ring of dark-stained chromatin (arrow, shown in the image). Which of the following best describes the functional significance of the dark-stained ring of marginal chromatin observed in this electronmicrograph? (A) DNA replication center (B) Kinetochore complex assembly (C) Nucleosome assembly (D) Organization of inactive chromatin (E) Ribosomal RNA biosynthesis

*The answer is D.* Chromatin is composed of DNA, RNA, and protein. In routine H&E slide preparations, nuclear chromatin binds hematoxylin and is said to be basophilic. Patterns of gene expression are regulated, in part, by global changes in chromatin packing. Inactive chromatin, heterochromatin, is highly condensed and deeply basophilic. Much of th heterochromatin in this endothelial cell is found along the periphery of the nucleus. This is referred to as marginal chromatin. By contrast, active chromatin, euchromatin, is dispersed within the nucleoplasm and lightly stained. Chromatin is supported and organized by structural proteins that provide points of attachment between chromatin and the inner n clear membrane. None of the other cell processes describe the function of heterochromatin.

In which of the following nuclear structures is DNA actively transcribed to rRNA? (A) Envelope (B) Lamina (C) Matrix (D) Nucleolus (E) Pore

*The answer is D.* DNA is actively transcribed to rRNA int he nucleolus.

A digital slide of a sympathetic chain ganglion is examined in the histology laboratory. Large multipolar neurons are surrounded by nerve fibers and connective tissue (shown in the image). Identify the dark basophilic region within the nucleus of these ganglion cells. (A) Basal body (B) Centrosome (C) Golgi apparatus (D) Nucleolus (E) Peroxisome

*The answer is D.* In eukaryotic cells, the nucleus maintains the integrity of the genome and regulates complex patterns of gene expression. The nucleus provides a microenvironment for the myriad structural proteins and enzymes that control DNA replication, DNA repair, chromatin assembly, and RNA synthesis. The nucleoplasm contains many different types of RNAs, including messenger (mRNA), ribosomal (rRNA), transfer (tRNA), and small nuclear (snRNA). Genes that encode the rRNAs are clustered in a nonmembranous region of the nucleus, termed the nucleolus. The nucleolus stains intensely with basic dyes and is visible within the nucleus of these ganglion cells. The outer margin of the double-membrane nuclear envelope is visible in this H&E slide preparation. Basal bodies (choice A) are modified centrioles located at the base of cilia and flagella. Centrosomes (choice B) and Golgi apparatus (choice C) are perinuclear, but these organelles cannot be identified by light microscopy. Peroxisomes (choice E) are located in the cytosol.

Which of the following cytoskeletal components is associated with kinesin? (A) Keratin (B) Lamin A (C) Microfilament (D) Microtubule (E) Neurofilament

*The answer is D.* Kinesin is a force-generating protein associated with microtubules. It serves as a molecular motor for the transport of organelles and vesicles outward, away from the centrosome.

You are involved in a translational research project to develop small-molecule inhibitors of pepsin secretion by chief cells in the stomach mucosa. Chief cells store precursor enzymes within zymogen granules. By electron microscopy, these "protein factory" cells would most likely show an abundance of which of the following intracellular organelles? (A) Centrosomes (B) Endosomes (C) Phagolysosomes (D) Rough endoplasmic reticulum (E) Smooth endoplasmic reticulum

*The answer is D.* The endoplasmic reticulum (ER) is composed of parallel membrane sheets and sacs that are specialized for protein and lipid biosynthesis. Smooth ER lacks ribosomes, and its surface appears smooth when examined by electron microscopy. Smooth ER is particularly abundant in cells that synthesize lipids. By contrast, cells that are actively synthesizing proteins feature an abundance of rough ER. Rough ER features bound ribosomes, and its surface appears rough when examined by electron microscopy. Signal sequences, recognition particles, docking proteins, and translocator proteins collaborate to guide proteins destined for secretion through the lipid bilayer. Chief cells store precursor proteins (e.g., pepsinogen) in membrane-bound dense-core granules.

Fluorescent fusion proteins are used to monitor the distribution of organelles in a myoblast cell line. The distribution of mitochondria and microfilaments is examined by confocal fluorescence microscopy (shown in the image). In this composite image, DNA is colored blue, microfilaments are colored green, and mitochondria are colored red. Which of the following cell adhesion proteins forms anchoring junctions that link actin microfilaments to adhesive glycoproteins on the surface of the culture dish? (A) Cadherins (B) Cloudins (C) Connexins (D) Integrins (E) Selectins

*The answer is D.* This fluorescent image shows the subcellular location of microfilaments, mitochondria, and chromatin in a cultured myoblast. The actin bundles (colored green) are aligned along an axis of cell polarity and migration. These microfilaments make connections with the plasma membrane at sites of cell-substrate adhesion. Microfilaments attach to proteins along the inner leaflet of the plasma membrane. These attachment proteins include α-actinin, vinculin, paxillin, talin, and integrin. Integrins are transmembrane receptors that mediate cell signaling and cell-substrate adhesion. They link microfilaments of the cytoskeleton to various proteins in the extracellular matrix, including laminin, vitronectin, fibronectin, and collagen. Integrins help regulate cell shape, motility, and differentiation. Cadherins (choice A), cloudins (choice B), and selectins (choice E) mediate cell-cell adhesion. Connexins (choice C) form intercellular pores that permit gap junction communication.

Hepatocytes from a liver biopsy are examined by electron microscopy. Identify the elongated organelles shown in the image. (A) Endoplasmic reticulum (B) Golgi apparatus (C) Lysosomes (D) Mitochondria (E) Peroxisomes

*The answer is D.* This transmission electron micrograph reveals elongated, tubular mitochondria in the cytoplasm of hepatocytes. Mitochondria have inner and outer membranes that provide compartments for the enzymes and cytochromes that mediate glycolysis and oxidative phosphorylation. Folds of the inner mitochondrial membrane (cristae) provide additional surface area for energy production (shown in the image). Electron transport proteins and ATP synthase are associated with the inner membrane. Enzymes that carry out the citric acid (Krebs) cycle are present within the mitochondrial matrix that is surrounded by the inner membrane. When cells are deprived of oxygen, mitochondria swell and their cristae become less prominent. None of the other organelles exhibit the ultrastructural features of mitochondria.

Movement of protein from the RER to the VTC takes place in which of the following cell components? (A) A caveolin-coated vesicle (B) A clathrin-coated vesicle (C) A coatomer I-coated vesicle (D) A coatomer II-coated vesicle (E) An early endosome

*The answer is D.* Transport of protein from the RER to the VTC occurs via (COP-II) coatomer-coated vesicles.

Polarity in microtubules is important in determining which of the following? a. The strength of vinblastine binding to microtubules b. The velocity of transport along microtubules with myosin motors c. The overall dynamic instability of the microtubules d. The linkage of microtubules to intermediate filaments e. The direction of vesicular transport along microtubules

*The answer is E.* Polarity in microtubules is important in determining the direction of vesicular transport along microtubules.

You are investigating maternal factors that regulate the cell cycle during early development. A mouse embryo is flushed from the uterine tube, treated with acid Tyrode solution to remove its zona pellucida, and examined by phase microscopy (shown in the image). The embryo exhibits a cleavage furrow and appears to be undergoing cytokinesis. These events take place during what phase of mitosis? (A) Anaphase (B) Interphase (C) Metaphase (D) Prophase (E) Telophase

*The answer is E.* After fertilization, the male and female pronuclei join to form the nucleus of the zygote. Maternal enzymes and transcription factors regulate nuclear reprogramming and activate zygotic gene transcription. The first cleavage division takes place about 24 hours after fertilization. During this mitotic cell division, sister chromatids are partitioned to genetically identical daughter cells (blastomeres). Mitosis consists of four phases: prophase, metaphase, anaphase, and telophase. Chromosome condensation occurs during prophase (choice D). The mitotic spindle organizes sister chromatids during metaphase (choice C). Chromosomes are pulled apart during anaphase (choice A). Cytokinesis, nuclear membrane formation, and DNA unwinding occur during telophase. A contractile ring of actin and nonmuscle myosin forms the cleavage furrow. After telophase, the daughter cells enter interphase of the cell cycle (choice B). The blastomeres at this stage are totipotent. They become smaller in size with each subsequent cell division. Totipotency of the blastomeres is lost after the third cleavage division (eight-cell stage) as the embryo undergoes compaction to form the blastocyst.

A skin biopsy is examined at a double-headed microscope. The surgical pathologist directs your attention to waxy/lipid material filling the cytoplasm of secretory cells forming a sebaceous gland (shown in the image). Secretion of this waxy material to the pilosebaceous canal involves programmed cell death (apoptosis). Which of the following cytologic features provides evidence of apoptosis in this gland? (A) Aggregation of intermediate filaments (B) Disaggregation of polyribosomes (C) Membrane blebs (D) Mitochondrial swelling (E) Nuclear pyknosis

*The answer is E.* Apoptosis is a programmed pathway of cell death that is activated by a variety of extracellular and intracellular signals. It is often a self-defense mechanism, destroying cells that harbor viruses or have acquired genetic alterations. In this example, secretory cells of the sebaceous gland initiate programmed cell death in order to release their intracellular stores of lipid and wax. This process of exocytosis is referred to as holocrine secretion. Cytologic features of cells undergoing apoptosis include nuclear condensation (pyknosis) and chromatin fragmentation (karyorrhexis and karyolysis). Pyknotic nuclei are small, shrunken, and deeply basophilic (shown in the image). The other cytologic findings are features of acute reversible cell injury.

A newborn boy is diagnosed with Apert syndrome. He has craniosynostosis, hypoplasia of the middle part of the face with retrusion of the eyes, and syndactyly that includes fusion of the skin, connective tissue, and muscle of the first, middle, and ring fingers with moderate fusion of those digits. There is very limited joint mobility past the first joint. Which one of the following is most likely decreased in cells of the interdigital regions of the developing hand of this newborn child? a. Random DNA degradation b. Inflammation c. Cell swelling d. Bcl-2 e. DNA degradation by endonucleases

*The answer is E.* DNA degradation by endonucleases.

A cervical biopsy is obtained from a 42-year-old woman with a history of abnormal Pap smears. The tissue is tested for human papillomavirus (HPV) by in situ hybridization using cDNA probes. Evidence of HPV viral genome is detected in cells in the cervical biopsy (dark blue spots, shown in the image). The patient is told that she is at increased risk for the development of cervical cancer. She asks you to elaborate. You explain that HPV encodes an early gene (E6) that activates a cellular protein that, in turn, accelerates the degradation of the p53 tumor suppressor protein. Name the protein that is activated by HPV E6. (A) β-Catenin (B) Cathepsin (C) Glucuronyl transferase (D) GTP-activating protein (E) Ubiquitin ligase

*The answer is E.* Epithelial cells in this cervical biopsy exhibit distinct perinuclear vacuoles (shown in the image). These sharply demarcated, clear zones surround the nuclei of HPV-infected cells. The vacuoles are filled with actively replicating virus particles (virions). The gene products of oncogenic DNA viruses, like HPV, are known to inactivate tumor suppressor proteins. Recent studies indicate that they do so, by accelerating the degradation of p53 via the ubiquitin-proteasome pathway (see Question 24). Loss of p53 permits cells to escape cellular senescence and proliferate. Mutations in GTP-activating protein (choice D) are associated with neurofibromatosis. None of the other proteins accelerates the degradation of p53 in cervical epithelial cells.

Which of the following provides structural support to astrocytes? (A) Keratin (B) Lamin A (C) Microfilament (D) Microtubule (E) Neurofilament

*The answer is E.* Glial filaments are a type of intermediate filament composed of glial fibrillary acidic protein and present in fibrous astrocytes. These filaments are supportive, but they may play additional roles in both normal and pathologic processes in the central nervous system.

A sample of adrenal cortex obtained at autopsy is fixed with formalin, embedded in paraffin, sectioned at 6 μm, stained with H&E, and examined by light microscopy (shown in the image). Cells of the zona fasciculata appear washed out and "spongy" due to an accumulation of cholesterol and other precursors for steroid hormone biosynthesis. Electron microscopic examination of these "steroid factory" cells would be expected to show an abundance of which of the following organelles? (A) Autophagic vacuoles (B) Dense-core secretory granules (C) Golgi apparatus (D) Rough endoplasmic reticulum (E) Smooth endoplasmic reticulum

*The answer is E.* Intracellular membranes establish compartment boundaries and organelles that serve different cellular functions. Examples of membrane-bound intracellular organelles include the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, peroxisomes, lysosomes, endosomes, and secretory vesicles. The endoplasmic reticulum (ER) is composed of parallel membrane sheets and sacs that are specialized for protein and lipid biosynthesis. Smooth ER lacks ribosomes, and its surface appears smooth when examined by electron microscopy. Smooth ER is particularly abundant in cells that synthesize lipids (e.g., fatty acids, phospholipids, cholesterol, and steroid hormones). In skeletal and cardiac muscle, smooth ER sequesters calcium and regulates muscle contraction. In the liver, smooth ER provides a large surface area for oxidative enzymes (e.g., cytochromes) that degrade toxins and carcinogens. The other organelles may be present in steroid-producing cells, but they would not be abundant.

Mitotic figures visible in a tissue section from the lining of the small intestine are most likely to belong to which of the following categories? a. Terminally differentiated cells b. Partially differentiated cells c. Blood cells d. Stem cells e. Progenitor cells

*The answer is E.* Progenitor cells are cells that lose the capacity for self-renewal and are committed to the generation of a particular cell lineage.

You attend a national meeting on regenerative medicine. One of the talks focuses on cellular senescence and cancer. Reactivation of the gene for which of the following nuclear proteins may enable some cancer cells to escape cellular senescence, continue to proliferate, and maintain genomic stability? (A) DNA helicase (B) Lamin A (C) Oct 4 transcription factor (D) Rb tumor suppressor protein (E) Telomerase

*The answer is E.* Telomerase is a nuclear enzyme that adds repetitive DNA sequences to maintain the length of chromosome telomeres. Somatic cells that are undergoing cellular senescence (i.e., loss of proliferative capacity) do not normally express telomerase. With each round of somatic cell replication, the telomere shortens. The length of telomeres may act as a "molecular clock" that governs the life span of replicating cells, providing a mechanism for cellular senescence. Because cancer cells and embryonic cells express high levels of telomerase, reactivation of this enzyme is thought to enable these cells to escape senescence, proliferate, and maintain genomic stability. Mutations affecting DNA helicase and lamin A (choices A and B) are associated with accelerated aging syndromes (progeria). Oct 4 (choice C) is a transcription factor that is essential for pluripotency and self-renewal in embryonic stem cells. Tumor suppressor proteins (choice D) restrain the cell cycle. The genes for Rb and p53 tumor suppressor proteins are among the most commonly mutated genes in human cancers.

What intracellular protein complex links microtubules of the spindle apparatus to sister chromatids during mitosis and meiosis? (A) Astral fibers (B) Centrioles (C) Centromere (D) Centrosome (E) Kinetochore

*The answer is E.* The spindle apparatus organizes and separates chromosomes during mitosis and meiosis. Microtubules of the spindle apparatus link chromosomes to microtubule organizing centers and mediate the movement of paired chromosomes to opposite poles of the cell during anaphase. Centromeres (choice C) are repetitive DNA sequences that provide a point of attachment between the sister chromatid and a nucleation site for the assembly of the kinetochore protein complex. Kinetochores are attachment sites for microtubules of the spindle apparatus. Each kinetochore binds 15 to 20 microtubules. Bundles of microtubules (spindle fibers) originate from microtubule-organizing centers (centrosomes, choice D). Centrosomes are composed of two centrioles (choice B) and a zone of pericentriolar proteins that regulate microtubule nucleation. Centrosomes are associated with the nuclear membrane during interphase and replicated during S phase of the cell cycle. They move to opposite poles of the cell during mitotic prophase as the nuclear envelope disintegrates. Astral fibers (choice A) are microtubules that anchor centrosomes to the plasma membrane. Dyneins are molecular motor proteins that move chromosomes along the spindle apparatus. Failure of sister chromatids to separate during anaphase is referred to as nondisjunction. The resulting embryos are said to exhibit genetic mosaicism.

Uncoupling of endocytosed ligands from receptors takes place in which of the following cell components? (A) A caveolin-coated vesicle (B) A clathrin-coated vesicle (C) A coatomer I-coated vesicle (D) A coatomer II-coated vesicle (E) An early endosome

*The answer is E.* The uncoupling of ligands and receptors internalized by receptor-mediated endocytosis occurs in the early endosome.

Which of the following cellular processes describes the uptake of extracellular fluids and small particles by the cell? (A) Autophagy (B) Exocytosis (C) Involution (D) Phagocytosis (E) Pinocytosis

*The answer is E.* Uptake of fluid and macromolecules at the cell surface is referred to as endocytosis. This energy-dependent cellular activity provides cells with essential fluids, nutrients, and proteins. It also enables specialized cells to internalize large particles (e.g., cellular debris and bacteria) for degradation within phagolysosomes. Endocytosis involves the formation of vesicles at the plasma membrane by a process of vesicle budding. Three general mechanisms of endocytosis are described: (1) pinocytosis (constitutive uptake of fluid and small particles), (2) phagocytosis (uptake of large particles by macrophages and other phagocytic cells; choice D), and (3) receptor-mediated endocytosis (clathrin-dependent uptake of specific ligands). Pinocytotic vesicles can be identified by electron microscopy. They are particularly abundant in the cytoplasm of vascular endothelial cells. Autophagy (choice A) enables cells to degrade and eliminate unwanted or damaged organelles. Exocytosis (choice B) is an energy-dependent process of secretion that involves fusion of secretory vesicles with the plasma membrane.