ISSN:0265-9247    

DOI:10.1002/bies.950030102

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1985

数据来源: WILEY

摘要:

AbstractThe view is presented that extracellular architecture in plant cell walls results from an interplay between molecular self‐assembly and mechanical reorientation due to growth forces. A key initial self‐assembly step may involve hemicelluloses. It is suggested that hemicelluloses may self‐assemble into a helicoid via a cholesteric liquid crystalline phase; the detailed molecular structure of hemicelluloses (stiff backbone, bulky side chains, and the presence of asymmetric carbon atoms) is shown to be consistent with cholesteric requirements for such self‐assembly. Since hemicelluloses are hydrogen‐bonded to the periphery of cellulose microfibrils, the cellulose could then itself become helicoidally arranged. Such ‘universal plywood’ structure is found in the walls of a wide variety of plants, and in several types of cell (including wood). The permanent effects of growth stresses on patterns seen in sections of helicoids are displayed by computer graphics plots, and the expected changes in stiffness a

ISSN:0265-9247    

DOI:10.1002/bies.950030103

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1985

数据来源: WILEY

摘要:

AbstractIt is proposed that early in phylogeny a large proportion of amino acid substitutions were selectively neutral, but that bursts of adaptive substitutions during major radiations of life so increased selective constraints that most mutations in modern proteins are detrimental. Recent findings on DNA nucleotide sequences indicate that decreasing mutation rates further slowed the rate of molecular evolution in the lineage to humans.

ISSN:0265-9247    

DOI:10.1002/bies.950030104

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1985

数据来源: WILEY

摘要:

AbstractThe formation of mature large rRNAs from larger primary transcripts is very different in bacterial and mammalian cells. In both, cotranscription can help to assure the coordinated production of various rRNA species. However, in bacteria, processing is ordered, initiated by cleavages at double‐stranded stems which enclose the mature sequences; several cleavages are required to produce each mature terminus; and the final steps occur in polysomes, apparently linked to continued protein synthesis. In mouse cells, in contrast, cleavages generate nearly all mature termini stochastically and directly, with no evidence for double‐stranded stems as cleavage signals; and processing occurs in the nucleolus, long before any new rRNA chains arrive in polysomes. Spacer sequences also serve different potnetial functions at the evolutionary extremes. In bacteria, transcribed spacer sequences are associated with different secondary structures in the pre‐rRNA compared to the mature rRNA, and these may promote ribosome formation. In mammalian cells, analyses are too premature to assign any comparable function to transcribed spacer sequences, but the non‐transcribed spacers are probably critical in the organization and replication of nucleoli, functions that are absent from b

ISSN:0265-9247    

DOI:10.1002/bies.950030105

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1985

数据来源: WILEY

摘要:

AbstractThe behaviour and roles of the microtubule network and the microfilaments following fertilization in the mouse oocyte are described. The microtubule network is organized by multiple microtubule organizing centres (MTOCs) and these play a major role in establishing spindle structure and pronuclear movement following fertilization; in contrast to sea urchin and frog eggs, the sperm centriole plays little part in organization of the post‐fertilization spindle. The microfilaments are required for spindle rotation, polar body formation, certain changes in the egg cortex, and also for pronuclear movement. Influences of the chromosomes on microtubule and microfilament organisation are also discusse

ISSN:0265-9247    

DOI:10.1002/bies.950030106

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1985

数据来源: WILEY

摘要:

AbstractBacterial resistance mechanisms to the antibiotics known as β‐lactams, which include the penicillins and cephalosporins, can take several forms but frequently involve the production of β‐lactamases from either plasmid‐ or chromosomally‐encoded loci. Gram negative bacteria express a β‐lactamase from evolutionarily related chromosomal ampC genes. Genetic analysis of both inducible and constitutively expressed AmpC β‐lactamases provide insights into the mechanisms regulating production of the enzyme. Evolutionary relationships between the genes of different species are discussed, as well as the regulatory mechanisms that in both laboratory mutants and clinical isolates lead to overproduction

ISSN:0265-9247    

DOI:10.1002/bies.950030107

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1985

数据来源: WILEY

摘要:

AbstractMethionine aminopeptidases with a universal specificity have been revealed from the sequences of the amino‐terminal region of mutant forms of yeast iso‐1‐cytochrome c and from a systematic examination of the literature for amino‐terminal sequences formed at initiation sites. The aminopeptidase removes amino‐terminal residues of methionine when they precede certain amino acids, with a specificity that appears to be determined mainly by the residue adjacent to the methionine residue at the amino terminus. The result with the mutationally altered iso‐1‐cytochromes c and the results from published sequences of other proteins from a wide range of prokaryotes and eukaryotes suggest that the aminopeptidase usually cleaves amino‐terminal methionine when it precedes residues of alanine, cysteine, glycine, proline, serine, threonine and valine but not when it precedes residues of arginine, asparagine, aspartic acid, glutamine, glutamic acid, isoleucine, leucine, lysine or methionine. We suggest that the specificity is almost always determined simply by the size of the side chain of the penultimate residue; methionine is usually cleaved from residues with a side chain having a radius of gyration of 1·29 Å or less, but is not cleaved from residues with l

ISSN:0265-9247    

DOI:10.1002/bies.950030108

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1985

数据来源: WILEY

摘要:

AbstractProblems and Paradigms is a feature series devoted to discussions of major problems in biology. Past contributions have dealt with the selector gene hypothesis, problems of pattern formation, the phenomenon of paramutation, theories of aging, and the origins of life. In this article, Brian C. Goodwin discusses the problem of morphogenesis, in terms of the idea of the genetic program and in the light of recent findings.

ISSN:0265-9247    

DOI:10.1002/bies.950030109

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1985

数据来源: WILEY

摘要:

AbstractRoots presents articles on landmark discoveries that laid the basis for contemporary molecular and cellular biology. In this article, N. H. Horowitz, Professor Emeritus at the California Institute of Technology, and a former associate of George Beadle's, reviews the work that led to the one gene–one enzyme hypothesi

ISSN:0265-9247    

DOI:10.1002/bies.950030110

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1985

数据来源: WILEY

ISSN:0265-9247    

DOI:10.1002/bies.950030111

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1985

数据来源: WILEY