Chapter Questions
Cells in the stem of a seedling that is grown in the dark orient their microtubules horizontally. How would you expect this to affect the growth of the plant?
Mutations in the genes encoding collagens often have detrimental consequences, resulting in severely crippling diseases. Particularly devastating are mutations that change glycines, which are required at every third position in the collagen polypeptide chain so that it can assemble into the characteristic triple-helical rod (see Figure $20-9$ ).A. Would you expect collagen mutations to be detrimental if only one of the two copies of a collagen gene is defective?B. A puzzling observation is that the change of a glycine residue into another amino acid is most detrimental if it occurs toward the amino terminus of the rod-forming domain. Suggest an explanation for this.
Proteoglycans are characterized by the abundance of negative charges on their sugar chains. How would the properties of these molecules differ if the negative charges were not as abundant?
Analogs of hemidesmosomes are the focal contact sites described in Chapter $17,$ which are also sites where the cell attaches to the extracellular matrix. These junctions are prevalent in fibroblasts but largely absent in epithelial cells. On the other hand, hemidesmosomes are prevalent in epithelial cells but absent in fibroblasts. In focal contact sites, intracellular connections are made to actin filaments, whereas in hemidesmosomes connections are made to intermediate filaments. Why do you suppose these two different cell types attach differently to the extracellular matrix?
Gap junctions are dynamic structures that, like conventional ion channels, are gated: they can close by a reversible conformational change in response to changes in the cell. The permeability of gap junctions decreases within seconds, for example, when the intracellular $\mathrm{Ca}^{2+}$ concentration is raised. Speculate why this form of regulation might be important for the health of a tissue.
Why does ionizing radiation stop cell division?
Why do you suppose epithelial cells lining the gut are renewed frequently, whereas most neurons last for the lifetime of the organism?
About $10^{16}$ cell divisions take place in a human body during a lifetime, yet an adult human body consists of only about $10^{13}$ cells. Why are these two numbers so different?
Which of the following statements are correct? Explain your answers.A. Gap junctions connect the cytoskeleton of one cell to that of a neighboring cell or to the extracellular matrix.B. A wilted plant leaf can be likened to a deflated bicycle tire.C. Because of their rigid structure, proteoglycans can withstand a large amount of compressive force.D. The basal lamina is a specialized layer of extracellular matrix to which sheets of epithelial cells are attached.E. Skin cells are continually shed and are renewed every few weeks; for a permanent tattoo, it is therefore necessary to deposit pigment below the epidermis.F. Although stem cells are not differentiated, they are specialized and therefore give rise only to specific cell types.
Which of the following substances would you expect to spread from one cell to the next through (a) gap junctions and (b) plasmodesmata: glutamic acid, mRNA, cyclic AMP, $\mathrm{Ca}^{2+},$ G proteins, and plasma membrane phospholipids?
Discuss the following statement: "If plant cells contained intermediate filaments to provide the cells with tensile strength, their cell walls would be dispensable."
Through the exchange of small metabolites and ions, gap junctions provide metabolic and electrical coupling between cells. Why, then, do you suppose that neurons communicate primarily through synapses rather than through gap junctions?
Gelatin is primarily composed of collagen, which is responsible for the remarkable tensile strength of connective tissue. It is the basic ingredient of jello; yet, as you probably experienced many times yourself while consuming the strawberry-flavored variety, jello has virtually no tensile strength. Why?
"The structure of an organism is determined by the genome that the egg contains." What is the evidence on which this statement is based? Indeed, a friend challenges you and suggests that you replace the DNA of a stork's egg with human DNA to see if a human baby results. How would you answer him?
Leukemias-that is, cancers arising through mutations that cause excessive production of white blood cells- have an earlier average age of onset than other cancers. Propose an explanation for why this might be the case.
Carefully consider the graph in Figure $Q 20-16,$ showing the number of cases of colon cancer diagnosed per 100,000 women per year as a function of age. Why is this graph so steep and curved, if mutations occur with a similar frequency throughout a person's life-span?
Heavy smokers or industrial workers exposed for a limited time to a chemical carcinogen that induces mutations in DNA do not usually begin to develop cancers characteristic of their habit or occupation until $10,20,$ or even more years after the exposure. Suggest an explanation for this long delay.
High levels of the female sex hormone estrogen increase the incidence of some forms of cancer. Thus, some early types of contraceptive pills containing high concentrations of estrogen were eventually withdrawn from use because this was found to increase the risk of cancer of the lining of the uterus. Male transsexuals who use estrogen preparations to give themselves a female appearance have an increased risk of breast cancer. High levels of androgens (male sex hormones) increase the risk of some other forms of cancer, such as cancer of the prostate. Can one infer that estrogens and androgens are mutagenic?
Is cancer hereditary?