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PubMed1992

Luminescent screens for general X-ray diagnostics.

This article gives the main parameters of amplifying screens manufactured in the USSR: the sensitivity, the resolution, the modulation depth, the noise. The technical level of Soviet screens is compared with the level of those manufactured by certain foreign companies. The measures taken to increase the technical level of today's products and the further evolution directions are discussed. Data are given on screen production in the USSR during the last 5 years and their supposed production up to 2000.

Evaluation Studies as Topic↗
PubMed1988

Directional mutation pressure and neutral molecular evolution.

A quantitative theory of directional mutation pressure proposed in 1962 explained the wide variation of DNA base composition observed among different bacteria and its small heterogeneity within individual bacterial species. The theory was based on the assumption that the effect of mutation on a genome is not random but has a directionality toward higher or lower guanine-plus-cytosine content of DNA, and this pressure generates directional changes more in neutral parts of the genome than in functionally significant parts. Now that DNA sequence data are available, the theory allows the estimation of the extent of neutrality of directional mutation pressure against selection. Newly defined parameters were used in the analysis, and two apparently universal constants were discovered. Analysis of DNA sequence has revealed that practically all organisms are subject to directional mutation pressure. The theory also offers plausible explanations for the large heterogeneity in guanine-plus-cytosine content among different parts of the vertebrate genome.

Animals↗
PubMedDate not supplied

[Biopolymers and evolution].

Biopolymers are usually studied being extracted from the whole system of a cell or of an organism. Some important features are lost during such a procedure. It is necessary to take into account the behavior of proteins and nucleic acids in metabolic networks and to investigate their evolution. The substitutions of amino-acids metabolic networks residues are biologically possible in the polypeptides and proteins if they do not influence their spatial structure and function. The correlations of the primary structure with these properties are degenerate. The protein can be treated as "an edited statistical copolymer" (Ptitsyn). In the process of "edition" an important role is played by the ions of transient metals. Nucleic acids possess similar properties. It can be shown that the deleterious mutations of proteins can be compensated by the changes of their amount, spatial and temporal characteristics of the synthesis. Not only the structure of the protein is important but also the exact answers of the questions: how much, when and where? The contemporary theory of evolution unites phylogeny and onthogeny. The directionality of evolution is determined both by natural selection and by the already existing structure of an organism. Hence many characters are not adaptive. This is valid also for the molecular level of the structure. Thus three independent groups of facts and suggestions are presented, which confirm the neutral theory of evolution (Kimura) and elucidate its physical meaning. The molecular evolution does not coincide with the biological evolution.

Amino Acid Sequence↗
PubMed1988

Directional selection and the evolution of breeding date in birds.

In many bird species, those pairs that breed earlier in the season have higher reproductive success than those that breed later. Since breeding date is known to be heritable, it is unclear why it does not evolve to an earlier time. Under assumptions outlined by Fisher, a model is developed that shows how breeding date may have considerable additive genetic variance, appear to be under directional selection, and yet not evolve. These results provide a general explanation for a persistent correlation of fitness with a variety of traits in natural populations.

Animals↗
PubMed1976

Evolution of DNA structure: direction, mechanism, rate.

On the basis of the results of an analysis of frequencies of pyrimidine oligonucleotides, the degree of pyrimidine clustering of DNA in species from different taxa has been determined. A tendency for an increase in the index of clustering of DNA was revealed in the sequence: invertebrates, fishes, amphibians, reptiles, birds, mammals. A mechanism is postulated, according to which the increase in the degree of clustering of DNA d-ring the evolution may be associated with the accumulation of mutations, Purine equalibrium Pyrimidine transversions, resulting in a selective enrichment of one of the chains of DNA with pyrimidines and the other- with purines, i.e. in an increase in the degree of purine-pyrimidine imbalance (asymmetry) of DNA complementary chains. This mechanism of DNA evolution is supported by the presence of positive correlation between the degree of clustering and the degree of the chain asymmetry of natural DNAs, as well as the character of the amino acid substitutions in cytochromes c in different species. The progressive evolution of different groups of organisms on the whole may have been accompanied by an acceleration of the rates of evolution of the DNA structure. On the basis of the amino acid sequence of cytochromes c in different species the degree of clustering and the degree of the chain asymmetry of the corresponding structural genes of DNA was found to have a general tendency towards an increase in the following order: invertebrates, fishes, amphibians, reptiles, birds, mammals. Thus, evolution of cytochrome c cistron is a vector process based on a selection of mutations which, on the one hand, are neurtral to protein, and, on the other hand, result in the sense chain of DNA being enriched with pyrimidines and the nonsense one (and the corresponding mRNA)- with purines. Hence, it is the polynucleotide template rather than protein, that must have been the "object of selection". The frequency of substitutions in cytochromes c cistron for vertebrates is 1.56x13(-9) per nucleotide per year. It is believed that the evolutionary modification of the DNA structure may be associated with an increase in the interference resistance of the translation, i.e. with selection for codons of highest readout stability.

Amino Acid Sequence↗
PubMed1991

Prebiotic co-evolution of self-replication and translation or RNA world?

A prebiotic scenario is proposed, based on the recent "domain hypothesis" model (Lahav, 1989, J. molec. Evol. 29, 475-479), suggested for domain propagation of RNA-like molecules in a fluctuating environment. The same system is suggested now not only for the evolution of ribozymes, but also for the evolution of directed peptide synthesis, as follows: Short, self-structured strands (termed prebioectons), each possessing a templatable domain which is chargeable by an amino acid, are the predecessors of tRNA (proto-tRNA). Complementary domains are formed on these prebioectons during an environmental cycle such as wetting-drying, followed by their dissociation from their template domain and ligation, to form the predecessor of mRNA (proto-mRNA). The evolution of directed peptide synthesis is suggested to be based on the ability of the charged prebioectons to attach preferentially to their complementary domains on the proto-mRNA. Two stages of this process are envisioned, namely: (a) Template-directed, random peptide synthesis taking place when non-specifically-charged prebioectons are sequentially attached each to its complementary domain on the proto-mRNA, followed by peptide bond formation. (b) Template-and-sequence-directed peptide synthesis, which can be realized after the "invention" of a catalytic molecule capable of specifically charging a proto-tRNA by an amino acid; this is the crucial evolutionary stage, where a crude genetic code becomes functional. Gradually, catalytic peptides and ribozymes are selected for their functions and evolve, while being encoded in the primitive "memory" of the emerging system. Thus, rather than the RNA monopoly postulated by the RNA World hypothesis, an early co-evolution of primitive enzymes and ribozymes is suggested.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1976

Evolution of continuous variation: direct approach through joint distribution of genotypes and phenotypes.

The evolutionary dynamics of the joint distribution of genotypes and phenotypes is studied. The model, originally devised to study the joint effects of Mendelian and other types of transmissions, provides results of interest also to the theory of direct Mendelian transmission with natural selection. Assuming bivariate normal distributions, it is shown that in the latter case genotypic and phenotypic means and variances, and genotype-phenotype correlation can be expressed recursively as functions of the parameters for the selection, environmental, and mutation variance. Equilibria and rates of approach for these moments are calculated. It is also proved that in the presence of selection the heritability,defined as the ratio of expected genotypic to expected phenotypic variance after selection, is greater than that before selection by a predictable amount and that it can be greater than unity.

Biological Evolution↗
PubMed1992

Human cytochromes P450: evolution and cDNA-directed expression.

As the first step in the process of carcinogenesis, most chemical carcinogens require metabolic activation by cytochromes P450 for conversion to highly reactive electrophiles that bind covalently to DNA. Studies in rodents suggest that low or high levels of expression of a single P450 can determine susceptibility or resistance to chemically induced cancer. Although rodent systems have been used to explore the molecular basis of chemical carcinogenesis and to identify chemicals capable of damaging genes and causing cancer, it has been understood that marked species differences exist in the expression, regulation, and catalytic activities of different P450s. Thus, large efforts are underway to study the catalytic activities of human P450s directly by expression of their cDNAs in cultured cells. Two systems are being used: a) transient high-level P450 production in HepG2 cells for analysis of catalytic activities, and b) stable expression in human B-lymphoblastoid cells to study promutagen and procarcinogen activation. These studies define the relative contributions of individual P450 forms to the activation of various chemical carcinogens. The B-lymphoblastoid cDNA expression system can also be used to determine whether a chemical will be hazardous or toxic to humans. The most intriguing aspects of P450s are the occurrence of human genetic polymorphisms in P450 expression, which could be a risk factor for chemical carcinogenesis. The best-studied P450 genetic polymorphism is the debrisoquine/sparteine polymorphism which is due to mutant CYP2D6 alleles. Four mutant alleles have been characterized that account for most of the defective CYP2D6 genes in Caucasians. These can be detected by polymerase chain reaction assays.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2026

Direct link between convergent evolution at sequence level and phenotypic level of septal pore cap in Agaricomycotina.

Several homologous morphological characters, despite sharing apparently similar features, are known to have independently evolved in different lineages multiple times. However, the genetic backgrounds of such morphological convergences remain poorly understood. To detect any correlated amino acid substitutions potentially responsible for morphological convergence at the phenotypic level, we focused on the morphology of the septal pore cap (SPC), a structure involved in mycelia's complex multicellularity in fungi. SPCs are classified into 3 morphological types: perforate, imperforate, and vesiculate. To understand the evolutionary events that occurred at the sequence level during the morphological convergence of perforate SPCs in Agaricomycotina, we examined sequence differences among species with different SPC types by comparative genomic analysis using a single-copy gene dataset from 12 Agaricomycotina genomes with morphological literature of SPC. Our analysis revealed that sequences of 8 genes, including an SPC-related gene spc33, were clustered based on SPC morphology rather than species relationship. Additionally, same amino acid substitutions independently occurred in both lineages in which species with perforate SPCs emerged. These findings suggest that specific amino acid substitutions in spc33 were critical for the emergence of perforate SPCs in multiple lineages. Further, our gene search for spc33 across organisms suggests that spc33 evolved shortly before the emergence of imperforate SPC. This study represents the first step toward elucidating the genetic basis of the morphological evolution of SPC. It contributes to both clarifying the genetic basis underlying morphological convergence and advances the study of fungal evolutionary morphology.

Evolution, Molecular↗
PubMed1985

The evolution and vicissitudes of directional scanning.

The relationship between Directional Scanning and Eye/Hand Dominance is confused. Both seem to have emerged in the Bronze Age, when patterns of artistry and cursive writing became fixed; but, by the time the alphabet was invented, the patterns became complicated by human perversity and racial rivalries, with an interesting, often damaging, legacy to the civilisations and cultures that followed.

Art↗
PubMed1972

Nonmathematical concepts of selection, evolutionary energy, and levels of evolution.

The place of mathematics in hypotheticodeductive processes and in biological research is discussed. (Natural) Selection is defined and described as differential elimination of performed sets at any level. Sets and acting sets are groups of units (themselves sets of smaller units) at any level that may or do interact. A pseudomathematical equation describes directional change (evolution) in sets at any level. Selection is the ram of evolution; it cannot generate, but can only direct, evolutionary energy. The energy of evolution is derived from molecular or chemical levels, is transmitted upwards through the increasingly complex sets of sets that form living systems, and is turned in directions determined by the sum of selective processes, at different levels, which may either supplement or oppose each other. All evolutionary processes conform to the pseudomathematical equation referred to above, use energy as described above, and have a P/OE (ratio of programming to open-endedness) that cannot be measured, but can be related to other P/OE values. Phylogeny and ontogeny are compared as processes af directional change with set selection. Stages in the evolution of multi-cellular individuals are suggested, and are essentially the same as stages in the evolution of some multi-individual insect societies. Thinking is considered as a part of ontogeny involving an irreversible, nonrepetitive process of set selection in the brain.

Biological Evolution↗