Questions and Answers of Molecular Cell Biology

Researchers have isolated two E-cadherin mutant isoforms that are hypothesized to function differently from the isoform of the wild-type E-cadherin. An E-cadherin negative mammary carcinoma cell line
Many of the proteins that regulate transit through the cell cycle have been characterized. Xnf7, identified in extracts of Xenopus eggs, binds to the anaphase-promoting complex/ cyclosome (A PC/C).
Antibody labeling of proteins like that used in immune-fluorescence analysis can be applied to electron microscopy, but instead of using fluorescent labels attached to antibodies, investigators use
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Meiosis segregates the paternal homologs into sperm and the maternal homologs into eggs.Which statements are true? Explain why or why not.
In normal adult tissues, cell death usually balances cell division.Which statements are true? Explain why or why not.
Several negative-strand viruses carry their genome as a set of discrete RNA segments. Examples include influenza virus (eight segments), Rift Valley fever virus (three segments), Hantavirus (three
In the early days of penicillin research, it was discovered that bacteria in the air could destroy the penicillin, a big problem for large-scale production of the drug. How do you suppose this occurs?
CD4 proteins on helper and regulatory T cells serve as co-receptors that bind to invariant parts of class II MHC proteins. CD4 is thought to increase the adhesion between T cells and
At first glance, it would seem a dangerous strategy for the thymus to actively promote the survival, maturation, and emigration of developing T cells that bind weakly to self peptides bound to self
Before exposure to a foreign antigen, T cells with receptors specific for the antigen are a tiny fraction of the T cells—on the order of 1 in 105 or 1 in 106 T cells. After exposure to the antigen,
Working out the rules by which T cells interact with their target cells was complicated. Some of the key observations came from studying the way cytotoxic T cells killed cells infected with
For many years it was a complete mystery how cytotoxic T cells could see a viral protein that seemed to be present only in the nucleus of the virus-infected cell. The answer was revealed in a classic
Based on its sequence similarity to Apobec1, which deaminates Cs to Us in RNA, activation-induced deaminase (AID) was originally proposed to work on RNA. But definitive experiments in E. coli
It would be disastrous if a complement attack were not confined to the surface of the pathogen that is the target of the attack. Yet, the proteolytic cascade involved in the attack liberates
Why do living trees not rot? Redwood trees, for example, can live for centuries, but once they die they decay fairly quickly. What might this suggest?
The antibody diversity created by the combinatorial joining of V, D, and J segments by V(D)J recombination pales in comparison to the enormous diversity created by the random gain and loss of
To guarantee that the antigen-presenting cells in the thymus will display a complete repertoire of self peptides to allow elimination of self-reactive T cells, the thymus recruits dendritic cells
T cells whose receptors strongly bind a self-peptide– MHC complex are killed off in peripheral lymphoid organs when they encounter the self peptide on an antigen- presenting dendritic cell.Which
When the Oxford team of Ernst Chain and Norman Heatley had laboriously collected their first two grams of penicillin (probably no more than 2% pure!), Chain injected two normal mice with 1 g each of
The majority of antibiotics used in the clinic are made as natural products by bacteria. Why do you suppose bacteria make the very agents we use to kill them?
Avian influenza viruses readily infect birds, but are transmitted to humans very rarely. Similarly, human influenza viruses spread readily to other humans, but have never been detected in birds. The
Influenza epidemics account for 250,000 to 500,000 deaths globally each year. These epidemics are markedly seasonal, occurring in temperate climates in the northern and southern hemispheres during
John Snow is widely regarded as the father of modern epidemiology. Most famously, he investigated an outbreak of cholera in London in 1854 that killed more than 600 victims before it was finished.
The intracellular bacterial pathogen Salmonella typhimurium, which causes gastroenteritis, injects effector proteins to promote its invasion into nonphagocytic host cells by the trigger mechanism. S.
The Gram-negative bacterium Yersinia pestis, the causative agent of the plague, is extremely virulent. Upon infection, Y. pestis injects a set of effector proteins into macrophages that suppresses
What are the three general mechanisms for horizontal gene transfer?
Clostridium difficile infection is the leading cause of hospital-associated gastrointestinal illness. It is typically treated with a course of antibiotics, but the infection recurs in about 20% of
In order to survive and multiply, a successful pathogen must accomplish five tasks. Name them.
You should not take antibiotics for diseases caused by viruses.Which statements are true? Explain why or why not.
Viruses replicate their genomes in the nucleus of the host cell.Which statements are true? Explain why or why not.
Pathogens must enter host cells to cause disease.Which statements are true? Explain why or why not.
The microbiomes from healthy humans are all very similar.Which statements are true? Explain why or why not.
Our adult bodies harbor about 10 times more microbial cells than human cells.Which statements are true? Explain why or why not.
Generation of induced pluripotent stem (iPS) cells was first accomplished using retroviral vectors to carry the OSKM (Oct4, Sox2, Klf4, and Myc) set of transcription regulators into cells. The
It is possible to purify hematopoietic stem cells using a combination of antibodies directed against cellsurface targets. By removing cells that expressed surface markers characteristic of specific
One of the earliest assays for hematopoietic stem cells made use of their ability to form colonies in the spleens of heavily irradiated mice. By varying the amounts of transplanted bone marrow cells,
The origin of new β cells of the pancreas—from stem cells or from preexisting β cells—was not resolved until a decade ago, when the technique of lineage tracing was used to decide the issue.
At any given time, intestinal crypts of mice comprise about 15 stem cells and 10 Paneth cells. After cell division, which occurs about once a day, the daughter cells remain stem cells only if they
In the 1950s, scientists fed 3H-thymidine to rats to label cells that were synthesizing DNA, and then followed the fates of labeled cells for periods of up to a year. They found three patterns of
Disturbance of the balance in the activities of osteoblasts and osteoclasts in favor of osteoclasts can give rise to the condition known as osteoporosis, the brittle- bone syndrome of the
Every tissue that can be renewed is renewed from a tissue-specific population of stem cells.Which statements are true? Explain why or why not.
Stem cells, being stem cells, are by definition the same in all tissues.Which statements are true? Explain why or why not.
In the small intestine, stem cells in the crypts divide asymmetrically to maintain the population of cells that make up the villi; after each division, one daughter remains a stem cell and the other
The highly branched structures of neurons would seem to make it almost inevitable that they should make unproductive synapses with themselves, yet they manage to avoid this outcome very effectively.
The extracellular protein factor Decapentaplegic (Dpp) is critical for proper wing development in Drosophila (Figure Q21–3A). It is normally expressed in a narrow stripe in the middle of the wing,
The oscillatory clock that drives somite formation in vertebrates involves three essential components Her7 (an unstable repressor of its own synthesis), Delta (a transmembrane signaling molecule),
The segmentation of somites in vertebrate embryos is thought to depend on oscillations in the expression of the Hes7 gene. Mathematical modeling explains these oscillations in terms of the delays in
The mouse HoxA3 and HoxD3 genes are paralogs that occupy equivalent positions in their respective Hox gene clusters and share roughly 50% identity in their protein coding sequences. Mice with defects
It was clear from the early days of studying development that certain “morphogenetic” substances were present in the egg and segregated asymmetrically into cells of the developing embryo. One
Two adjacent cells in the nematode worm normally differentiate into an anchor cell (AC) and a ventral uterine precursor (VU) cell, but which of the two becomes the AC and which becomes the VU cell is
Morphogens play a key role in development, creating concentration gradients that inform cells of where they are and how to behave. Examine the simple patterns represented by the flags in Figure
In the early Drosophila embryo, there seems to be no requirement for the usual forms of cell–cell signaling; instead, transcriptional regulators and mRNA molecules move freely between nuclei. How
What are the three germ layers formed during gastrulation, and what are the principal structures each gives rise to in the adult?
Name the four processes that are fundamental to animal development, and describe each of them in a single sentence.
The cell cycle is the ticking clock that sets the tempo of developmental processes, with maturational changes in gene expression being dependent on cell-cycle progression.Which statements are true?
Changes in the coding regions of genes involved in development are primarily responsible for the differences between species.Which statements are true? Explain why or why not.
At different stages of embryonic development, the same signals are used over and over again by different cells, but with different biological outcomes.Which statements are true? Explain why or why
As development progresses, individual cells become more and more restricted in the range of cell types they can give rise to.Which statements are true? Explain why or why not.
Because of the many later developmental transformations that produce the elaborately structured organs, the body plan set up during gastrulation bears little resemblance to the body plan in the
In the early cleavage stages, when the embryo cannot yet feed, the developmental program is driven and controlled entirely by the material deposited in the egg by the mother.Which statements are
The Tasmanian devil, a carnivorous Australian marsupial, is threatened with extinction by the spread of a fatal disease in which a malignant oral–facial tumor interferes with the animal’s ability
PolyADP-ribose polymerase (PARP) plays a key role in the repair of DNA single-strand breaks. In the presence of the PARP inhibitor olaparib, single-strand breaks accumulate. When a replication fork
One major goal of modern cancer therapy is to identify small molecules anticancer drugs—that can be used to inhibit the products of specific cancer-critical genes. If you were searching for such
Virtually all cancer treatments are designed to kill cancer cells, usually by inducing apoptosis. However, one particular cancer—acute promyelocytic leukemia (APL)— has been successfully treated
A small fraction—2 to 3%—of all cancers, across many subtypes, displays a quite remarkable phenomenon: tens to hundreds of rearrangements that primarily involve a single chromosome, or
Mortality due to lung cancer was followed in groups of males in the United Kingdom for 50 years. Figure Q20–1 shows the cumulative risk of dying from lung cancer as a function of age and smoking
In contrast to colon cancer, whose incidence increases dramatically with age, incidence of osteosarcoma— a tumor that occurs most commonly in the long bones peaks during adolescence. Osteosarcomas
The main environmental causes of cancer are the products of our highly industrialized way of life such as pollution and food additives.Which statements are true? Explain why or why not.
Cancer therapies directed solely at killing the rapidly dividing cells that make up the bulk of a tumor are unlikely to eliminate the cancer from many patients.Which statements are true? Explain why
In the cellular regulatory pathways that control cell growth and proliferation, the products of oncogenes are stimulatory components and the products of tumor suppressor genes are inhibitory
The chemical carcinogen dimethylbenz[a]anthracene (DMBA) must be an extraordinarily specific mutagen since 90% of the skin tumors it causes have an A-to-T alteration at exactly the same site in the
A plant must be able to respond to changes in the water status of its surroundings. It does so by the flow of water molecules through water channels called aquaporins. The hydraulic conductivity of a
Your boss is coming to dinner! All you have for a salad is some wilted, day-old lettuce. You vaguely recall that there is a trick to rejuvenating wilted lettuce, but you cannot remember what it is.
At body temperature, L-aspartate in proteins racemizes to D-aspartate at an appreciable rate. Most proteins in the body have a very low level of D-aspartate, if it can be detected at all. Elastin,
The glycosaminoglycan polysaccharide chains that are linked to specific core proteins to form the proteoglycan components of the extracellular space are highly negatively charged. How do you suppose
The affinity of integrins for matrix components can be modulated by changes to their cytoplasmic domains: a process known as inside-out signaling. You have identified a key region in the cytoplasmic
Discuss the following statement: “The basal lamina of muscle fibers serves as a molecular bulletin board, in which adjoining cells can post messages that direct the differentiation and function of
It is not an easy matter to assign particular functions to specific components of the basal lamina, since the overall structure is a complicated composite material with both mechanical and signaling
The food-poisoning bacterium Clostridium perfringens makes a toxin that binds to members of the claudin family of proteins, which are the main constituents of tight junctions. When the C-terminus of
Cell adhesion molecules were originally identified using antibodies raised against cell-surface components to block cell aggregation. In the adhesion-blocking assays, the researchers found it
Comment on the following (1922) quote from Warren Lewis, who was one of the pioneers of cell biology. “Were the various types of cells to lose their stickiness for one another and for the
Integrins can convert mechanical signals into intracellular molecular signals.Which statements are true? Explain why or why not.
The elasticity of elastin derives from its high content of α helices, which act as molecular springs.Which statements are true? Explain why or why not.
Tight junctions perform two distinct functions: they seal the space between cells to restrict paracellular flow and they fence off plasma membrane domains to prevent the mixing of apical and
Given the numerous processes inside cells that are regulated by changes in Ca2+ concentration, it seems likely that Ca2+-dependent cell–cell adhesions are also regulated by changes in
Fas ligand is a trimeric, extracellular protein that binds to its receptor, Fas, which is composed of three identical transmembrane subunits (Figure Q18–3). The binding of Fas ligand alters the
When human cancer cells are exposed to ultraviolet (UV) light at 90 mJ/cm2, most of the cells undergo apoptosis within 24 hours. Release of cytochrome c from mitochondria can be detected as early as
In contrast to their similar brain abnormalities, newborn mice deficient in Apaf1 or caspase-9 have distinctive abnormalities in their paws. Apaf1-deficient mice fail to eliminate the webs between
Imagine that you could microinject cytochrome c into the cytosol of wild-type mammalian cells and of cells that were doubly defective for Bax and Bak. Would you expect one, both, or neither type of
Development of the nematode Caenorhabditis elegans generates exactly 959 somatic cells; it also produces an additional 131 cells that are later eliminated by apoptosis. Classical genetic experiments
One important role of Fas and Fas ligand is to mediate the elimination of tumor cells by killer lymphocytes. In a study of 35 primary lung and colon tumors, half the tumors were found to have
Mammalian cells that do not have cytochrome c should be resistant to apoptosis induced by DNA damage.Which statements are true? Explain why or why not.
The human genome consists of 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes). During meiosis, the maternal and paternal sets of homologs pair, and then are separated
Down syndrome (trisomy 21) and Edwards syndrome (trisomy 18) are the most common autosomal trisomies seen in human infants. Does this fact mean that these chromosomes are the most difficult to
A living cell from the lung epithelium of a newt is shown at different stages in M phase in Figure Q17–2. Order these light micrographs into the correct sequence and identify the stage in M phase
How many kinetochores are there in a human cell at mitosis?
High doses of caffeine interfere with the DNA damage response in mammalian cells. Why then do you suppose the Surgeon General has not yet issued an appropriate warning to heavy coffee and cola
The yeast cohesin subunit Scc1, which is essential for sister-chromatid cohesion, can be artificially regulated for expression at any point in the cell cycle. If expression is turned on at the
Hoechst 33342 is a membrane-permeant dye that fluoresces when it binds to DNA. When a population of cells is incubated briefly with Hoechst dye and then sorted in a flow cytometer, which measures the

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