The extreme specificity of the phenotype found in

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The extreme specificity of the phenotype found in nematode worms lacking Cdh3 is presumably due to functional redundancy in other cells and tissues where it is expressed; that is, other cell adhesion molecules present in those tissues compensate for the loss of Cdh3. Reference: Pettit J, Wood WB & Plasterk RHA (1996) cdh-3 , a gene encoding a member of the cadherin superfamily, functions in epithelial cell morpho- genesis in Caenorhabditis elegans . Development 122, 4149–4157. 19–17 The additional force required to remove P-selectin from the plasma mem- brane presumably reflects its attachments to other proteins on the cytoplas- mic side of the membrane. Thus, to remove a P-selectin from a bilayer requires only that the hydrophobic interactions with the bilayer lipids be overcome; to remove it from the plasma membrane requires, in addition, that all bonds on the cytoplasmic side also be broken. (In case you were wondering, it takes a force of 10,000 pN to break a carbon–carbon bond.) CALCULATIONS 19–18 The weight of an individual cell in seawater is 0.42 pN: W = 4.2 ¥ 10 –13 N, which is 0.42 pN Thus, an individual bond with a strength of 125 pN could support nearly 300 cells (125 pN/0.42 pN = 297) against the force of gravity in seawater. Given that there are multiple interactions between cells, the cells of a sponge are firmly tied together. Reference: Dammer U, Popescu O, Wagner P, Anselmetti D, Guntherodt H-J & Misevic GN (1995) Binding strength between cell adhesion proteoglycans measured by atomic force microscopy. Science 267, 1173–1175. 19–19 A. The fraction of the phage population that will be attached by at least one tail fiber at any one instant is equal to one minus the fraction not attached by any tail fibers, which is (0.5) 12 = 0.00024 for wild-type bacteria and (0.5) 6 = 0.0156 for ompC bacteria. Thus, at any instant 99.98% of the phage popula- tion will be attached to wild-type bacteria and 98.44% will be attached to ompC bacteria. B. The tiny difference in the fraction of the phage population attached to wild- type and ompC bacteria at first seems too small to account for the 1000- fold difference in infectivity. However, since T4 must wander around the surface of a bacterium to find an appropriate place to attach its baseplate, CADHERINS AND CELL CELL ADHESION A439 1100 kg – 1018 kg 9.81 N kg 4 p 3 W = ¥ ¥ m 3 ¥ (5 m m) 3 ¥ m 3 10 18 m m 3 W = g V ( r cell r sea w ater )
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the instantaneous calculation is misleading. If, for example, T4 must stay bound to the bacterial surface for 500 ‘instants’ during its wandering, then (0.9998) 500 = 90% will remain attached to wild-type bacteria, but only (0.984) 500 = 0.03% will remain attached to ompC bacteria. This difference would be more than enough to account for the 1000-fold difference in infectivity. By associating with the bacterial surface through multiple weak interac- tions, bacteriophage T4 can wander around the surface without falling off.
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