Chapter_19_Solutions - Chapter 19 Cell Junctions, Cell...

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DEFINITIONS 19–1 Selectin 19–2 Homophilic 19–3 Desmosome junction 19–4 Integrin 19–5 Adherens junction 19–6 Cadherin 19–7 Adhesion belt TRUE/FALSE 19–8 False. Although cells can be readily dissociated by removing Ca 2+ from the external medium, it is unlikely that Ca 2+ -dependent cell–cell adhesions are regulated by changes in Ca 2+ concentration. Cells have no way to control the Ca 2+ concentration in their environment. 19–9 True. This like-to-like binding is the reason that cadherin–cadherin interac- tions between cells are referred to as homophilic. 19–10 True. Selectins are transmembrane proteins that bind to cell-surface carbo- hydrates on other cells. These proteins (lectins) mediate a variety of tran- sient cell–cell interactions in the bloodstream. 19–11 False. The adhesions mediated by cadherins are much stronger than those mediated by Ig family members. Thus, cadherins are mainly responsible for holding cells together, segregating cell collectives into discrete tissues, and maintaining tissue integrity. THOUGHT PROBLEMS 19–12 This quote is correct in spirit, though incorrect in detail. Warren Lewis was trying to draw attention to the importance of the adhesive properties of cells in tissues at a time when the problem had been largely ignored by the biol- ogists of the day. The quote is incorrect because a large fraction of our bod- ies is made up of connective tissue such as bone and tendon, whose integrity depends on the quality of the matrix rather than on the cells that inhabit it. It is not at all easy to dissociate cells from tissues, as anyone who has eaten a tough piece of steak can testify. 19–13 IgG antibodies contain two identical binding sites; thus, they are able to cross- link the molecules they recognize (this is the basis for immune precipitation). A437 CADHERINS AND CELL CELL ADHESION In This Chapter CADHERINS AND A437 CELL–CELL ADHESION TIGHT JUNCTIONS A441 AND THE ORGANIZATION OF EPITHELIA PASSAGEWAYS FROM A444 CELL TO CELL: GAP JUNCTIONS AND PLASMODESMATA THE BASAL LAMINA A446 INTEGRINS AND A448 CELL–MATRIX ADHESION THE EXTRACELLULAR A450 MATRIX OF ANIMALS THE PLANT CELL WALL A454 Chapter 19 19 Cell Junctions, Cell Adhesion, and the Extracellular Matrix
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If whole antibodies were used to block aggregation, they might cross-link the cells rather than inhibit their aggregation. By contrast, monovalent Fab fragments cannot cross-link cells. They bind to the cell adhesion molecules and prevent them from binding to their partners, thus preventing cell aggre- gation (Figure 19–23). Reference: Beug H, Katz FE & Gerisch G (1973) Dynamics of antigenic mem- brane sites relating to cell aggregation in Dictyostellium discoideum . J. Cell Biol. 56, 647–658. 19–14 In outline, you would need to isolate RNA from the adherent cell line, con- vert it into DNA using reverse transcriptase, and clone the cDNAs into a plasmid or viral expression vector to make an expression library. To identify individual clones that express the target of your antibody (presumably E-
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Chapter_19_Solutions - Chapter 19 Cell Junctions, Cell...

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