Large Number Of Generations Research Articles

Utility experience with digital excitation systems

Ontario Hydro, a Canadian utility with an installed capacity of over 30000 MW, has utilized static excitation systems on a large number of generators, ranging from 3 MVA to 1100 MVA in size, since the early 1960's. Ontario Hydro has always attempted to take advantage of excitation system control as a means of eliminating or reducing stability constraints on their operation. The advent of digital excitation systems offered us the opportunity to work with manufacturers to incorporate special features or controls which had previously required complex additional hardware. This paper discusses the authors' experience with these systems, from 1988 to date, as well as providing a list of items which merit consideration by utilities and manufacturers as they move toward the second generation of digitally-implemented systems.

Fractal neutron optics multilayers in Cantor ternary set pattern

In the present paper, it is shown that the fractal notion can be used in neutron optics. More specifically, self-similar neutron multilayer mirrors can be made according to the pattern of the Cantor ternary set. These new multilayers have a fractional Hausdorff dimension. The self-similarity gives them new reflectivity properties. Compared with periodic neutron multilayers and supermirrors, these fractal multilayers, with a large number of generations, allow high reflectivity of the order of unity at very short neutron wavelengths.

Polymorphic microsatellite loci of the rat (Rattus norvegicus).

The EMBL and GenBank DNA databases were searched for microsatellite sequences of the rat containing dinucleotide repeats of (CA)n and (GA)n. Among those obtained, 23 sequences were analyzed by polymerase chain reaction to examine the size variation of the amplified fragment in inbred rat strains. All of the 23 microsatellite sequences varied in size among the strains tested. The 23 microsatellite loci in a pair of substrains separated from the same progenitor strain were then analyzed. Fragments identical in size were observed in all loci of the two substrains, indicating the stability of the microsatellite over a large number of generations. The microsatellite loci, therefore, should be useful markers for linkage analyses in the rat.

Mechanism of high-copy-number integration of pMIRY-type vectors into the ribosomal DNA of Saccharomyces cerevisiae

Targeted integration of the yeast plasmid pMIRY2 into the ribosomal DNA (rDNA) of Saccharomyces cerevisiae by homologous recombination results in transformants carrying 100–200 copies of the plasmid per cell which are stably maintained over a large number of generations [Lopes et al., Gene 79 (1989) 199–206]. These properties make pMIRY2 an attractive vector for high-level production of (heterologous) proteins by yeast cells. We have investigated the mechanism underlying high-copy-number (hen) integration of pMIRY-type plasmids and show that either targeting to a location outside the rDNA locus or use of the wild-type LEU2, instead of the deficient LEV2d gene, as selection marker reduces the copy number to the low value characteristic of standard integrating (YIp-type) yeast plasmids. Further experiments demonstrate that the hcn of pMIRY-type plasmids is achieved by amplification of a small number of copies initially integrated into the rDNA locus. Amplification depends upon the strong selection pressure created by the extremely low expression of the deficient LEU2d gene, but not on the presence of this gene per se. The hcn integration also occurs when either the TRP1 or URA3 gene is used as the selection marker, provided expression of the marker gene is severely curtailed e.g., by removal of most of its 5′-flanking region.

Selection in yeast I: Assessing genetic stability and relative fitness of commercial yeasts

The potential for changes in allele frequencies in yeast populations by selection was examined. Cells from the wine yeastSaccharomyces cerevisiae (strain Montrachet) were grown over a large number of generations using two different culturing techniques, each with two variations: serial transfers on WLN agar plates with and without UV irradiation, and continuous culture in autoclaved and in filter-sterilized grape must. A low frequency of variant isozyme patterns was found in samples taken at the end of the experiment. Growth rates in must and on agar plates were also examined, and it was found that all samples were faster-growing than the original strain, to varying degrees. Applications for the selection system developed are discussed.

The characterization of an hereditary stomatocytosis

batch experiments were conducted, limiting one or more of the components of the minimal medium used (Table I). Growth in fed-batch under nitrogen-limitation resulted in stabilization of the plasmid. Phosphorus-limitation, however, had a marked destabilizing effect on the plasmid, especially at low phosphate concentrations. This destabilizing effect was not as severe with a generation time of 3 h. As there was no phosphate present initially in the medium, its buffering capacity was quite poor and therefore the pH was quite low. However, when the pH was controlled at 7.0 during a phosphate-limited fed-batch, there was a marked increased in stability. To determine whether nutrient-limitation had a stabilizing effect over a large number of generations, B. subtilis IA42 (pPGVq5138) was grown in continuous culture for 70 generations. When grown in Luria-Bertani broth with a generation time of 1 h, only 10% plasmid loss was observed for up to 30 generations, but with further growth the plasmid became progressively less stable until after 70 generations more than 90% plasmid loss was observed. Increasing the generation time to 2 h did not enhance stability, in fact the plasmid was less stable with the lower dilution rate. The stability of the plasmid was also examined under nitrogen-limitation in continuous culture, but the specific limitation did not enhance stability any more than the corresponding run in Luria-Bertani broth.

VARIANCE AND COVARIANCE OF HOMOZYGOSITY IN A STRUCTURED POPULATION

The variance of homozygosity for a K-allele model with n partially isolated subpopulations is derived numerically using identity coefficients. The variance of homozygosity within a subpopulation is shown to depend strongly upon the migration rates between subpopulations but is not strongly influenced by the number of alleles possible at a locus. The variance of homozygosity within a subpopulation, given the value of expected homozygosity, is approximately equal to the value of the variance of homozygosity given by Stewart's formula for a single population. If the population is presumed to be panmictic, but is actually subdivided, and the gametes are sampled at random from the total population, the apparent variance of homozygosity depends on the number of alleles possible. With small migration rates and K large, the apparent variance of homozygosity is much smaller than in a single population with the same expected homozygosity. However, when K is small, the variance of homozygosity is approximately given by Stewart's formula. The transient behavior of the variance of homozygosity shows that a large number of generations may be required to approach equilibrium values.

Monolayer organization by serially cultured bovine corneal endothelial cells: Effects of a retina-derived growth-promoting activity

This study describes the ability of bovine corneal endothelial cells to proliferate and organize monolayers when cultured in the presence of a saline-soluble fraction of the retina (RE). This strongly stimulates cell proliferation by increasing the growth rate. In the continuous presence of RE, the cells organize a confluent monolayer composed of closely apposed polygonal cells lying on a basement membrane. Thus, over a large number of generations, these cells express most of their differentiated traits: contact inhibition of movement, proliferation arrest at monolayer formation, and the extracellular deposition of a material organized as a typical basement membrane. The presence of RE increases by a factor of 10 the number of cellular generations expressing these differentiated traits, and subcultured cells maintained in a medium lacking Re are unable to build up a similar organization at confluency, even if this is extended over several months. If these cultures are transferred to a medium containing RE, they behave like endothelial cells which have been always cultured with RE. We suggest that the expression of the endothelial differentiated state is related to the raised mitotic activity induced by the retinal factor.

VIRUS REPLICATION – AN INTRODUCTION

Viruses can only multiply in an internal cellular environment. They lack subcellular organelles such as nuclei, mitochondria, ribosomes as well as cytoplasmic components that are necessary for the synthesis of their own structural components: nucleic acids (RNA or DNA), proteins, carbohydrates and lipids. Thus, viruses cannot multiply but must be replicated by the cells that they invade. The essential steps of virus replication are similar for all virus classes: first, a virus particle (virion) must attach to the outer surface of the host cell. Next, it penetrates the cell membrane and enters the cytoplasmic environment. Some or all of the outer surface layers of the virus (envelope and capsid) are removed, so that the viral genome (DNA or RNA) becomes accessible to the cellular organelles and enzymes which will initiate the replication process. Some of the genetic information is used to synthesize virus-specific enzymes (polymerases) which are necessary for the replication of the viral nucleic acid. Other parts of the viral genome code for the structural proteins. The newly synthesized viral nucleic acid and proteins are then assembled (maturation) into new viral particles which may leave the cells either by simple cell lysis or by a budding process at the membranes of the outer cell surface or in the endoplasmic reticulum. This basic strategy of virus infections leaves ample space for variations depending on whether the viral genome is composed of DNA or RNA, on whether it is double- or single-stranded, linear or circular, monolithic or fragmented. The parental RNA may either be directly used by ribosomes (+ strand viruses), or it may first need transcription into a complementary strand (- strand viruses). Enveloped viruses probably require the synthesis of specific carbohydrates and lipids. Depending on the genetic make-up of the host cell and on the conditions of infection, the virus cycle may be productive (i.e. many new infectious particles are released by each infected cell) or it may be restrictive, or even abortive.Some viruses may remain latently present in cells over a long time and over a large number of generations. Several mechanisms for such chronicity of virus infection have been discovered. Certain taxa of viruses have the unique capability to integrate their genome, or a fragment thereof, into that of the cell. Chronic infections may also be maintained by the generation of defective-interfering (DI) particles, which contain only part of the virus genome, but which are replicated when they are present in cells, along with complete virus particles. In general the small size of the DI-genome allows it to be replicated more rapidly than the intact genome and hence competes strongly for the polymerases. In certain conditions, DI-particles can alter the lytic pathway of infection and rapidly establish persistent infections in which the cells survive but carry in them considerable amounts of virus antigens. This is an important area in virology to-day as it may be related to the development of long-term degenerative diseases which may have a viral etiology.

Avian Prehatching Behavior: Functional Aspects of the Tucking Pattern

The domestic chick's position at the time of pipping (initial cracking of the eggshell) is important to its hatching success. Normally the anterior end of the chick is at the large end of the egg; the legs, feet, and left wing are flexed; the neck is depressed ventrally; the head is turned to the right, its left side resting on the right pectoral muscle, and is tucked under the partially extended right wing (Fig. la). Six malpositions generally reducing hatchability have been recognized by poultry scientists. Malposition VI, head over the right wing (Fig. lb), reduces hatching, but so slightly that Waters (1935), Byerly and Olsen (1937), Munday (1953) and others did not consider this position abnormal. Hamburger and Oppenheim (1967) detailed the specific prehatching pattern of tucking, the placing of the head under the right wing 1-2 days before pipping. This attitude is maintained throughout the hatching stage of cutting the cap from the shell. It is a genetically consistent behavior pattern of the chick and probably of most birds (see Oppenhelm 1972). Thus I was interested to learn that its absence (producing malposition VI) does not seriously hamper pipping or hatching. This could be explained by the fact that only a very slight selective advantage is necessary to fix an adaptation in a species over a large number of generations, but it still remains to elucidate the function(s) of the wing and the actual selective advantage produced by tucking. What is the function of the normal wing-overhead position? Kuo's (1932) supposition that it is important for the wing to protect the face from the air chamber membrane has been shown invalid by Waters (1935) and Hamburger and Oppenheim (1967). Protection of the head or eyes from sharp shell fragments during the head thrusts of climax could be a function, but the eyes generally remain closed until hatching is completed in precocial birds (Hamburger and Oppenheim 1967), longer in altricial birds, and the eyes may need no extra protection. Also, Oppenheim (1972) stated that the wing in altricial birds does not cover much of the relatively large head. Normal pipping convulsions (Brooks and Garrett 1970) might deform the delicate wing bones if the wing were between thorax and beak in malposition VI. Wing protection is not needed in the hatching climax, however: the thorax does not push against the beak, now, because there is sufficient space in which to elevate the head; and the force does not have to be as great to break the eggshell (which is already broken) further. Yet the wing covers the head throughout. Narayanan and Oppenheim (1968) hypothesized that tucking inhibits head movements. In their experiments involving extirpation of the right wing bud, a significant increase in frequency of head movements was demonstrated, but the cause of the increase was obscured by possible neural modifications due to limb extirpation. Later Oppenheim (1970) re-tested this by manually un-tucking chicks and ducklings, but no significant increase in frequency of head movements was shown this time. In any case, it seems doubtful that a decrease in frequency (rather than amplitude) of head movements would produce the selective pressure necessary to evolve the tucking pattern. In fact, the opposite might be more beneficial.

Five years' review of cardiac pacing in Glasgow.

In this 5-yr review of experience in Glasgow with implantable cardiac pacemakers, the performance of both generators and electrode-lead systems are discussed. Many examples are then given of the use of the pacemaker frontal plane vector technique to assess implanted cardiac pacemakers. The value of pacemaker clinics is also assessed in terms of the reduced unplanned workload on hospitals and hospital staff. Finally suggestions are made to minimize the large number of generators and electrode-lead systems whose implant lifetimes are ‘curtailed’.

Heavy traffic approximations for the Galton-Watson process

The behaviour of the Galton-Watson process in near critical conditions is discussed, both with and without immigration. Limit theorems are obtained which show that, suitably normalized, and conditional on non-extinction when there is no immigration, the number of individuals remaining in the population after a large number of generations has approximately a gamma distribution. The error estimates are uniform within a specified class of offspring distributions, and are independent of whether the critical situation is approached from above or below. These results parallel those given for continuous time branching processes by Sevast'yanov (1959), and extend recent work by Nagaev and Mohammedhanova (1966), Quineand Seneta (1969), and Seneta (1970).

Heavy traffic approximations for the Galton-Watson process

The behaviour of the Galton-Watson process in near critical conditions is discussed, both with and without immigration. Limit theorems are obtained which show that, suitably normalized, and conditional on non-extinction when there is no immigration, the number of individuals remaining in the population after a large number of generations has approximately a gamma distribution. The error estimates are uniform within a specified class of offspring distributions, and are independent of whether the critical situation is approached from above or below. These results parallel those given for continuous time branching processes by Sevast'yanov (1959), and extend recent work by Nagaev and Mohammedhanova (1966), Quineand Seneta (1969), and Seneta (1970).

Factors Influencing Alanine Aminotransferase Activity in Leaves ofLolium temulentumL.

Factors Influencing Alanine Aminotransferase Activity in Leaves of<i>Lolium temulentum</i>L.

Inbreeding with selection and linkage. II. Sibmating.

SummaryThe generation matrix is derived for the effects of sib‐mating on two linked loci (a, b) and (A, B), when selection against homozygosity acts at the (A, B) locus so that only fractions x of A A and BB survive, but the (a, b) locus is not directly affected by selection. The matrix is of order 19 × 19. Using an electronic digital computer, the rates of inbreeding progress at the (a, b) locus have been calculated for various survival values x and recombination frequencies y, under the following types of selection:Case I–selection acts within lines only.Case II–selection acts equally within and between lines.The results quoted include, for various (x, y), the final rate of change in heterozygosity λ (Table 4), the relative speed of inbreeding after 10,20 and a large number of generations (Table 5), and the number of heterozygotes per 1000 remaining after 5, 10 and 20 generations (Table 6).Graphs illustrate the effect of changing x, when y is constant, on the relative speed of inbreeding after 20 generations (Fig. 1) and after a large number of generations when this speed has become constant (Fig. 2). At generation 20 the speed declines fairly steadily with x in case I, reaching a minimum with x between 0–1 and 0‐0, after which it rises slightly; but in case II the speed falls rapidly to a minimum with x about 0–6, and then rises slowly to the same value as in case I when x= 0.The final speed of inbreeding for constant y does not generally fall below 100 % in case I until x is well below unity, and then falls fairly rapidly to a minimum with x close to zero. The case II lines drop below 100 % with x a little below unity, fall very rapidly to a minimum at x= 0–7633, and then rise slowly to join the case I lines at x= 0. Thus, under case II selection, inbreeding progress at a linked locus is reduced most when the selection pressure is only of moderate strength.The 19 × 19 generation matrix contains 4 submatrices along the leading diagonal with zeros below each, so that their roots are the latent roots of the complete matrix. These matrices are A1 (both loci fixed), A2 (locus (a, b) fixed), A3 (locus (A, B) fixed) and A4 (neither locus fixed). It is shown that the final rate of change in heterozygosity (A) is the largest of the latent roots of A3 and A4 in case I, and is the ratio of the largest latent root of A3 and A4 to the largest latent root of A1 and A2 in case II, for any particular (x, y).

RANDOM GENETIC DRIFT IN MULTI-ALLELIC LOCUS

In the theory that evolution is anl irregularly shifting state of balance, especially that of a subtle balance between the evolutionary pressures of mutation, immigration and selection as a group and random or stochastic processes which give rise to processes of trial and error (Wright, 1950), the process of random genetic drift is one of the problems first to be clarified. Actually the problem of the random drift has been discussed by various authors. As early as 1921, and pointed out that the group of organisms destined to become the parents of the next generation is usually considerably smaller than the number of individuals of their species so that some genes will be lost by chance. According to their opinion, reduction of potential variability is automatic, being independent of any sort of selection. They considered that this is the most important gain in knowledge that we owe to Mendel's work and to the biomechanical interpretation of it. The first mathematical treatment of this so-called Hagedoorn Effect was carried out by Fisher (1922). It was rather unfortunate that an abbreviation of a term in his differential equation led him to the erroneous result that the rate of decrease of the variance in the population is 1/4n per generation, where n is the number of individuals in the population. Independently, \Vright worked on this problem and obtained the correct answer of 1/2N for the first time, where N is the effective population number (see Wright, 1931). Using an integral equation he also arrived at the flat distribution of unfixed classes for the state of steady decay. This stimulated Fisher to re-examine his results and he found himself in entire agreement with Wright's results. Furthermore, Fisher elaborated the terminal part of the distribution for the case of steady decay by his method of a functional equation (Fisher, 1930a, b). This method was followed by Haldane (1939) to treat the more general case where the number of descendants does not necessarily follow the Poisson distribution. Until now, Fisher's method remains as an unique and powerful tool to find the exact distribution at the terminal part. In 1945, Wright introduced the Fokker-Planck equation to solve the problem of the gene frequency distribution and applied the equation to the case of steady decay. The problem of random drift was also treated as a problem of a finite Markov chain by Malecot (1944), who got the asymptotic formula for a very large number of generations which is essentially the same as the previously known formula for steady decay. His result on the characteristic roots was later corrected by Feller (1951), who succeeded in getting all characteristic values of the matrix of transition probabilities. In spite of all these works, the theoretical results have been largely at the level of asymptotic formulae, and no complete solution has been published until the present

The Distribution of Heterogeneity upon Inbreeding

Summary In this paper, it is shown how the variance of the map length of the germ plasm, heterogeneous after a given amount of inbreeding, can be calculated. Matrix methods are used in the evaluation of the variance for several systems of inbreeding after a large number of generations. The frequency distribution of the map length heterogeneous after many generations is developed in terms of the modified Bessel function of the first kind, the probability of complete homogeneity being represented by a condensation at the origin. A comparison of these distributions for sib and parent-offspring matings, two inbreeding systems which in general give the same speed of approach to homogeneity at a single genetic locus, reveals interesting differences in the probability statements that can be made in these two cases.

Determination of Generator Stand-by Reserve Requirements

During the past few years there has been a trend toward the use of statistics of forced (unscheduled) generator outages as a basis for calculating the proper amount of stand-by reserve generation for a given power system. This paper presents a method of analysis which was developed to fit the needs of a system having a large number of generators, but which, it is believed, can be used to advantage for small systems also. The method is based partially on previously published results.1,2,3 This method lends itself readily to calculations, involves few approximations, and has sufficient flexibility to take account of several types of machines having different outage factors. Although the discussion is confined to generator outages, the method of analysis here presented may be used for calculation of simultaneous overlap of events of other types for which corresponding assumptions can be made. Examples are random outages of transmission lines and other types of equipment, random use of parallel communication channels, and supply of current to a number of items of equipment, each having a random duty cycle. The method is applicable to any situation which can be represented on a graph similar to Figure 1(A), provided that necessary statistical data are available.

The frequencies of natural power oscillations in interconnected generating and distribution systems

THE distribution of power in the steady state of interconnected electric networks with large numbers of generators and of consumers of power is well known for a given system. The flow of energy in detail depends on the distribution of the loads, on the admission of power to the prime movers, and on the data for all the connecting lines. With every change of the power distribution in any branch of the system, transient phenomena are initiated which influence the entire complex of prime movers and generators, motors and other consumers, and intermediate lines. As is often observed in the larger systems, the changes in the flow of energy lead to oscillations of power and frequency, depending on the electrical and mechanical data of the generation and the use of the a-c energy. Also we will exclude such very slow fluctuations as may be caused by the control of the steam plant proper or the regulation of the flow of water and which have no direct relation to the electrical part of the system. We shall concentrate our attention on those power oscillations which influence the system frequency and change the distribution of the flow of the active energy within and between the various parts of interconnected systems. These electromechanical fluctuations affect the speed of the generators with their prime movers and speed governors as well as the speed of the numerous motors of the consumers.

Age of parents and vitality in Uroleptus mobilis

Previous studies on the vitality of Uroleptus mobilis have shown that all ex-conjugants start with an initial high vitality as measured by the division rate. From this initial high rate, vitality gradually decreases during the life cycle until the race dies a natural death. After the first fifty days of a cycle endogamous conjugation may occur at any time with a resulting renewal of vitality in the ex-conjugants. Sufficient data have now been obtained to indicate that there is a difference in vitality of offspring given off at different age periods of the parental series, and a summary of such data is shown in the accompanying table. Line 1 gives the total number of generations, or divisions, in in each series; line 2 the total number of days required for those divisions; line 3, compared with line 5, indicates the extent of the renewal of vitality resulting from conjugation; line 4 gives the average division rate for any ten days of the entire life cycle on the assumption that divisions are equally distributed; line 6 gives the age of parents in days, at the time of conjugation which resulted in the offspring series, and line 8 the age in generations; lines 7 and 9 give the number of days and the number of divisions subsequent to the time of conjugation which resulted in the offspring series and represent the vitality remaining in the parental protoplasm. High vitality in a series is indicated (I) by a high division rate which represents high metabolic activity; (2) by the large number of generations, or divisions, which the series undergoes, indicating a combination of high metabolic activity and endurance or lasting capacity; and (3) by endurance or long length of life of a series in days which may be combined with, or independent of, (I) and (2).