Chiroptical and stoichiometric evidence of a specific,primary dimerisation process in alginate gelation

The stoichiometry of calcium-ion chelation to alginate chains has been investigated by circular dichroism (c.d.), and by equilibrium dialysis in the presence of various concentrations of sodium chloride. C.d. intensity in the carboxylate π → π * spectral region increases linearly with calcium-ion concentration up to a level equivalent to half the total poly-L-guluronate stoichiometric requirement, and thereafter shows little further change. Similarly, the level of bound calcium resistant to displacement by swamping concentrations of sodium ions is equivalent to half the stoichiometric requirement of poly-L-guluronate chain-sequences alone. In terms of the previously developed “egg-box” model of co-operative junction-zone formation in alginate gelation, these results are interpreted as showing that the primary mechanism of interchain association is by dimerisation of poly-L-guluronate chain-segments in a regular, buckled, two-fold conformation related to that characterized for the free acid in the solid state, with tight interchain chelation of calcium to the carboxylate groups on the interior faces of the dimer (i.e., half the carboxylate residues of the participating chain-sequences). This interpretation is entirely consistent with previous evidence from electron microscopy, and offers a simple rationalisation of experimental results from competitive-ion binding studies.     

The proteins of polytene chromosomes of Drosophila hydei

It is reported that chromatin can be prepared from highly purified polytene nuclei from the salivary glands of third instar larvae of Drosophila hydei; such chromatin differs from that of diploid nuclei mainly by deficiencies in certain nonhistone chromosomal proteins. It is suggested that these proteins are important components of constitutive heterochromatin, which is severely underrepresented in polytene chromosomes. Chromosome morphology, including the pattern of induced puffs, is maintained throughout the mass isolation of glands and sucrose gradient purification of nuclei, as indicated by studies on temperature-shocked and control larvae. No significant alteration in the chromosomal proteins following puff induction by heat shock could be detected on analysis of the isolated protein fractions by disc gel electrophoresis. More sensitive techniques must be developed to study the apparent rearrangement or accumulation of protein at puff sites, and to elucidate the role of this protein in gene activation.     

Specific binding of covalently cross-linked mouse nerve growth factor to responsive peripheral neurons.

The binding characteristics of [¹²⁵I]nerve growth factor, covalently cross-linked with dimethyl suberimidate, to chick embryonic dorsal root ganglia are indistinguishable from the iodinated native hormone. Both show non-saturability, non-linear Scatchard plots and acceleration of dissociation of hormone-receptor complexes by native hormone which is reflected in the binding constants calculated. These results demonstrate that dimerization of the native hormone at the receptor is not responsible for the negatively cooperative behavior observed for native nerve growth factor. Further, experiments with amino-silylated glass tubes also eliminate interaction between hormone and reaction vessel as an explanation of the non-saturable and multiple affinity properties of the observed binding.     

Characterization of Postreplication Repair in Mutagen-Sensitive Strains of DROSOPHILA MELANOGASTER

Mutants of Drosophila melanogaster, with suspected repair deficiencies, were analyzed for their capacity to repair damage induced by X-rays and UV radiation. Analysis was performed on cell cultures derived from embryos of homozygous mutant stocks. Postreplication repair following UV radiation has been analyzed in mutant stocks derived from a total of ten complementation groups. Cultures were irradiated, pulse-labeled, and incubated in the dark prior to analysis by alkaline sucrose gradient centrifugation. Kinetics of the molecular weight increase in newly synthesized DNA were assayed after cells had been incubated in the presence or absence of caffeine. Two separate pathways of postreplication repair have been tentatively identified by mutants derived from four complementation groups. The proposed caffeine sensitive pathway (CAS) is defined by mutants which also disrupt meiosis. The second pathway (CIS) is caffeine insensitive and is not yet associated with meiotic functions. All mutants deficient in postreplication repair are also sensitive to nitrogen mustard. The mutants investigated display a normal capacity to repair single-strand breaks induced in DNA by X-rays, although two may possess a reduced capacity to repair damage caused by localized incorporation of high specific activity thymidine-³H. The data have been employed to construct a model for repair of UV-induced damage in Drosophila DNA. Implications of the model for DNA repair in mammals are discussed.     

The role of causal knowledge in the evolution of traditional technology

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