Showing papers by "University of St Andrews published in 1972"

Review Lecture - Replication of DNA in the chromosomes of eukaryotes

Abstract: The evidence that each chromatid of a eukaryotic organism contains only one DNA double helix comes from a variety of observations. It begins with the autoradiographic demonstration by J. H. Taylor that tritiated thymidine, incorporated into chromosomes during one round of DNA synthesis, is present in both chromatids at the first division after labelling, but in only one chromatid after a further round of DNA synthesis accomplished in the absence of label. Further evidence comes from those experiments which demonstrate that when two sister chromatids break and fuse one with the other, each chromatid behaves as though it contained two chains of opposite polarity, fusion between chains being restricted to those of like polarity. J. G. Gall’s study of the kinetics of digestion of lampbrush chromosomes by pancreatic DNase also supports the view that each chromatid contains only two polynucleotide chains which are cleaved by this enzyme independently of one another; while O. L. Miller’s observations on the dimensions of the fibres remaining after lampbrush chromosomes have been digested by trypsin only allow for there being two polynucleotide chains per chromatid. By means of the technique of DNA fibre autoradiography devised by J. A. Huberman and A. D. Riggs, the units involved in replicating the chromosomal DNA of somatic cells of Xenopus have been compared with those of Triturus . Both these organisms have initiation points for DNA replication that are arranged in tandem, and from each initiation point replication proceeds in opposite directions at divergent forks. The intervals between initiation points in Xenopus range from about 20 to 125 µ m apart, whereas those of Triturus are much more widely separated. At 25 °C replication of DNA in Xenopus somatic cells proceeds at 9 µ m per hour one-way at each fork, whereas the corresponding rate in Triturus is 20 µ m per hour. Triturus somatic cells take about 4 times longer than comparable cells of Xenopus to replicate their DNA. The Triturus genome contains about 10 times as much DNA as the Xenopus genome, and comparison of the replication process in these two organisms indirectly adds weight to the view that the Triturus genome is 10 times longer than that of Xenopus , rather than that it contains 10 times as many DNA double helices per chromatid. DNA fibre autoradiography has also been used to study replication in Triturus spermato-cytes. The round of DNA synthesis just before meiosis in Triturus is an exceptionally long-drawn-out process, taking 9 to 10 days for completion at 16 °C. This lengthy S-phase is not occasioned by abnormally slow replication, the rate being 12 µ m per hour one-way at 18 °C, nor is it the result of an exceptional staggering of replication starts. Instead it appears to be correlated with a gross reduction in the number of initiation points for replication. i.e. with an increase in the lengths of the replicating units. A rough calculation suggests that each meiotic chromomere may correspond to a unit of replication during the pre-meiotic S-phase.

Neurobiology of echinodermata.

Abstract: Summary During the past ten years much information has been added to our knowledge of nerve and muscle systems of echinoderms. 1. Electron-microscopy has shown that all the main nerve trunks consist of large numbers of small, parallel-running unmyelinated axons which are packed tightly together. Glial cells are generally absent. Discrete regions of neuropile are recognizable by the interweaving of axons, and the presence of vesicles. It has not yet been found possible to locate synapses with certainty in the nervous system, but it appears that they are chiefly confined to neuropile. The obvious nerve cords are massive accumulations of neurons which do not appear to interact locally. 2. Peripheral axons are difficult to distinguish because both interstitial and muscle cells have processes which often resemble axons. Ultrastructural analysis of this problem is aggravated by difficulties in fixation. However, the electron-microscope has shown that much of the echinoderm body wall contains a thick subepithelial plexus of processes from epithelial cells. Epithelial cells may thus act as sensory cells and supply axons to the plexus. 3. With the exception of striated muscles in some pedicellariae, all echinoderm muscles so far examined are of the smooth type. These muscles characteristically contain large filaments, and in this way do not resemble vertebrate smooth muscle. Some muscles are innervated by simple axonal contact, in others the muscles themselves send processes towards the nervous tissue. 4. Physiological studies of electrical activity in nerve and muscle systems have not added significantly to our knowledge of function. Several authors have demonstrated that massed electrical activity is conducted decrementally along the radial nerve cords, but this does not explain any known aspect of coordination. The only records of electrical activity from single neurons (Takahashi, 1964) have not been repeated. 5. There is strong evidence for two types of neurons in the central nervous system of echinoderms. One of these contains acetylcholine, the other dopamine and/or noradrenaline. Electron-microscopical histochemistry has given good indication that catecholamines are bound in echinoderm nerve tissue to particles similar to those reported in other invertebrate nervous tissues, and there is good evidence that acetylcholine is bound to synaptic vesicles which are morphologically identical to those present in the mammalian brain. The available data further indicate that acetylcholine is a transmitter in sensory and motor neurons, while dopamine and/or noradrenaline are transmitters in interneurons. Such interneurons may be involved in the coordination of the movement of the tube-feet. Other substances which have been implicated in neuro-effector mechanisms in other animal groups have not been found or are present in very small quantities. 6. Studies on the reproductive physiology of starfish have shown that several substances in the radial cords play important roles in its control. Such substances cannot at present be called neurosecretions because it is not known if they are derived from neurons. 7. Pharmacological studies on isolated muscle tissues have not added significantly to our knowledge of their control. The potency of ACh in causing contraction is well documented, and anticholinesterases are similar in effect. Catecholamines, although clearly very important in the nervous system, do not produce clear-cut effects. The published reports of relaxation to noradrenaline may well be due to direct effects on the muscle. No definite information has been obtained on the role of the adrenergic parts of the nervous systems of echinoderms, other than showing that they are not involved in motor responses. Extensive studies with a wide variety of drugs have produced inconsistent and largely negative results.

The immobilization of enzymes on nylon structures and their use in automated analysis.

Abstract: 1. Glucose oxidase (EC 1.1.3.4) and urease (EC 3.5.1.5) were covalently attached through glutaraldehyde to low-molecular-weight nylon powder. 2. Immobilized derivatives of glucose oxidase and urease were prepared by cross-linking the respective enzymes within the matrix of a nylon membrane. 3. An improved process is described for the immobilization of glucose oxidase and urease on the inside surface of partially hydrolysed nylon tube. 4. Automated analytical procedures are described for the determination of glucose with each of the three immobilized glucose oxidase derivatives and for the determination of urea with each of the three immobilized urease derivatives. 5. The efficiencies of the three immobilized enzyme structures as reagents for the automated determination of their substrates were compared.

A System of Electrically Coupled Small Cells in the Buccal Ganglia of the Pond Snail Planorbis Corneus

Abstract: 1. The buccal ganglia of Planorbis contain a population of electrically coupled small cells. This has been studied, in preparations of isolated ganglia, by recording intracellularly from the cells two at a time. 2. The population is usually silent but activity initiated in any one of its members tends to spread to the rest of the population in both ganglia. Failure of spread, or fatigue, gradually occurs on repetition. 3. The group has the properties of a trigger system, initiating prolonged patterned activity in large numbers of neurones in the buccal ganglia. This may normally initiate feeding. 4. In addition to central processes, both in the buccal ganglia and to the rest of the CNS, the system has peripheral axons in most of the buccal nerves. No synaptic input could be demonstrated. 5. Action potentials in some of the cells increase greatly in duration with repetition. The resulting electrotonic EPSP9s, recorded in closely coupled trigger cells, correspondingly increase in size. The possible integrative significance of this is discussed, especially its effect in offsetting fatigue. 6. In some preparations spontaneous bursting occurred in trigger cells and this elicited burst activity in large neurones, including motoneurones. The system may have an intrinsic pacemaker.

The fine structure of the heart of the teleost, Pleuronectes platessa L.

Abstract: Tissue from the four chambers of the heart of the plaice (Pleuronectes platessa, L.) has been examined in the electron microscope in order to describe the morphology of the heart at a fine structural level. The sinus venosus is a thin walled chamber between 60–90 μ thick consisting of a connective tissue matrix in which are situated the plexus of the parasympathetic cardiac ganglion and localised bundles of myocardial cells. The myocardial cells do not form a continuous layer but are associated in particular with the region of the cardiac ganglion and are innervated by it. The sino-auricular junction has hitherto been described as a pacemaker region but the myocardial cells in this region are identical in morphology to myocardial cells in other parts of the heart. There is a large complex of nerves, derived from the cardiac plexus, that runs around the junction before branching to innervate the auricle. The myocardial tissues consist of an outer layer of myocardium forming the wall of the heart and a profusion of trabeculae. The endocardium invaginates into the endocardium to divide up the cells into populations of approximately 25 cells in profile. There is no well-defined coronary blood supply although capillaries are occasionally seen. The myocardial cells themselves are small in diameter (3.5–5.5 μ) and show some primitive features which are: a short sarcomere (1.4–2.0 μ), the absence of any sarcoplasmic reticulum, and very scarce fasciae occludentes. In the atrium in particular, there are many groups of 1500 A membrane-bound, dense-cored vesicles in the myocardial cells. Ventricular cells contain more myofilaments and mitochondria than do atrial cells and have many vesicles of 0.1–0.3μ diameter whose function and contents are unknown. Connective tissue is very evident in the plaice heart, being an integral part of the sinus venosus and the auriculo-ventricular junction and being the sole constituent of the auriculoventricular valve and the bulbus arteriosus.

Command fibres from the supra-oesophageal ganglion to the stomatogastric ganglion in Panulirus argus

Abstract: In the higher Decapoda Crustacea a small inferior ventricular nerve (ivn) directly connects the supra-oesophageal ganglion (S.O.G.) with the stomatogastric nervous system. Collision experiments (Fig. 3) in the isolated preparation of the stomatogastric nervous system ofPanulirus argus demonstrated that two fibres (ivn through fibres) pass directly from the ivn to the stomatogastric nerve (sgn). The sgn is considered to be the sole input pathway to the stomatogastric ganglion. The 28–30 neurones of the stomatogastric ganglion (St. G.) form a motor relay station which directly controls much of the activity of the foregut in these animals. The stomatogastric neurones form two distinct groups. One of these groups controls the activity of the gastric mill (gastric mill cycle) and the other group controls the activity of the pyloric stomach filtering apparatus (pyloric cycle). The mechanism of formation of the pyloric cycle is intrinsic to the St. G. and apparently normal pyloric cycle activity is recorded in the isolated preparation. The situation with regard to the gastric mill cycle is less clear and normal activity was not recorded in our isolated preparations. Stimulation of the ivn selectively activated the two through fibres in the sgn and produced marked frequency-dependent changes in the output of both groups of stomatogastric ganglion neurones (Table 1). The ivn through fibres are therefore regarded as command fibres. The effects of the two fibres were indistinguishable and additive when both fibres were stimulated simultaneously. Low frequency stimulation of the ivn (Figs. 4, 5) produced increased pyloric cycle frequency with more pronounced activity in the dilator group of neurones (PD, VD, AB). Higher frequency stimulation (5–10 per second) effectively disrupted the pyloric cycle (Figs. 6, 7), and at 20 per second almost all activity ceased. Only the activity of the GM neurones of the gastric cycle was examined. These neurones were relatively unaffected by low frequency stimulation but were progressively inhibited by higher frequency stimulation (Fig. 8) and at 20 per second they were subject to rapid, total and sustained inhibition. The mechanisms producing these changes were investigated by intracellular recordings from identified neurones of the St. G. In the pyloric cycle at least the PD pacemaker and the VD neurones of the dilator group received direct epsps from the command fibres (Figs. 11–14). The antagonistic pyloric constrictor elements (PY) were subject to increased indirect inhibition from the more active dilator neurones and in some cases there was probably also direct inhibition of these constrictor neurones (Figs. 9, 10). The GM neurones received a possible direct ipsp and at least one indirect effect by an increase in an ongoing ipsp during stimulation (Figs. 15, 16). There was little facilitation of the command ipsps at the frequencies used but at higher frequencies there was a summation which produced strong hyperpolarization. The innervation pattern of the ivn command fibres in the St. G. is summarized in Fig. 17. The ivn through fibres have very strong direct effects. The weight of present evidence is that in this system most presynaptic fibres have much smaller modulating effects. The function of the ivn through fibres is unknown at this time.

Microtubule-Arms and Propulsion of Food Particles Inside a Large Feeding Organelle in the Ciliate Phascolodon Vorticella

Abstract: Attention is drawn to the apparently invariable presence of rows of arm-bearing microtubules lining the sucking mouthparts of ciliates. Their arrangement inside the cytopharyngeal basket of Phascolodon is described and compared with that in the tentacles of suctorians and the sucking tubes and baskets of other ciliates. It is suggested that arms are involved in the propulsion of materials alongside microtubules when these ciliates ingest food. Mechanisms by which arms may accomplish this are outlined. The different functions of links and other intertubular materials in the several microtubular basket components of Phascolodon are considered. The form and arrangement of microfilarnents and pointed, sharp-edged structures around the mouth are also described. The possibility that the pointed structures are used for biting is discussed.

Ultrastructural and histochemical studies on the innervation of the heart of a teleost, Pleuronectes platessa L.

Abstract: The innervation of the heart of the plaice has been studied ultrastructurally and histochemically in order to describe the anatomy, and the nature of the neuro-transmitters involved in the regulation of the heart. A single cardiac branch of the vagus runs over the Duct of Cuvier and sinus venosus where it forms a plexus extending into the sinoatrial region. Many ganglion cells are scattered throughout this plexus upon which are seen cholinergic-type terminals at which AChE can be localised. It is therefore concluded that this is the parasympathetic cardiac ganglion. The innervation of the atrium is dense at the sinoatrial end but decreases towards the ventricle. The ventricle is probably aneural, possibly a consequence of the lack of a coronary blood supply to act as a pathway for an innervation of this chamber. Most of the intramural axon profiles contain large numbers of small agranular vesicles and are cholinergic, AChE being localised at their membranes. Other profiles contain small numbers of larger granular vesicles amongst the agranular vesicles but these are not adrenergic since these vesicles are not depleted by reserpine, do not load with 6-OHDA, and AChE can be localised at the membranes of the profiles. No adrenergic-type profiles were seen in any part of the heart, nor were any fluorescent, catecholamine-containing fibres observed.

Post-meiotic events in the Homobasidiomycetidae

Abstract: Cytological investigations of the Homobasidiomycetidae reveal the existence of four patterns of post-meiotic events which differ as regards the site of the third nuclear division and the fate of the resultant nuclei. The state of knowledge concerning post-meiotic events is reviewed and attention drawn to the possibility of utilizing such data in the fields of taxonomy and phylogeny.

A Discussion on volcanism and the structure of the Earth - Kimberlites and their relation to the mantle

Abstract: Kimberlite is a rock-type that originates within the Earth’s upper mantle, and which has sampled en route to the surface a variety of upper mantle rocks that now occur as xenoliths, or, in a fragmented form, xenocrysts in the kimberlite. Kimberlite is, in fact, a hybrid rock resulting from the incorporation of the mantle xenoliths and xenocrysts into a fine-grained matrix with which they may interact. This matrix, which must result from modification of primary mantle material, consists mineralogically mainly of calcite, serpentine, ilmenite, perovskite, magnetite, phlogopite and apatite; chemically, the matrix is rich (relative to garnet peridotite) in TiO 2 , Al 2 O 3 , total iron, CaO, K 2 0 , Na 2 O, H 2 O, CO 2 and P 2 O 5 , and a large number of the so-called ‘ incompatible ’ trace elements. The kimberlite matrix focuses attention upon the presence of various elements the source of which in the upper mantle is not adequately explained by the currently popular garnet peridotite model. The source difficulties are to some extent resolved by accepting titaniferous phlogopite as an additional upper mantle phase, although the source of carbon, sulphur, nitrogen and phosphorus is still a matter for speculation.

Expansion and contraction of the oral disc in the sea anemone tealia felina

Abstract: 1. Electrical stimulation of the SS 1 of Tealia felina causes inhibition of spontaneousactivity and increase in length of oral disc radial muscle preparations. This response is elicited over a wide stimulus frequency range (1 every 2 sec to 1 every 30 sec). The response shows a slow onset and a long recovery period. 2. Stimulation of the nerve net at frequencies between 1 shock every 5 sec and 1 shock every 20 sec produces slow contraction. The radials also show fast contractions to shocks less than 2 sec apart. 3. Dissolved food substances excite the SS 1 in the column. The sensory response to application of food extract to a small area of the column shows evidence of sensory adaptation. 4. These observations are related to the pre-feeding response of Tealia and a model for oral disc expansion is described.

New tropical seed oils. IV. Component acids of leguminous and other seed oils including useful sources of crepenynic and dehydrocrepenynic acid.

Abstract: The component acids of seed oils from 40 legumes and from 43 other species are reported. Attention is drawn to a series of Afzelia oils which provide a useful source of crepenynic acid and dehydrocrepenynic acid and to several Khaya and other oils of potential commercial value.

Electroluminescence in reverse-biassed zinc selenide Schottky diodes

Abstract: Schottky diodes consisting of a metal layer in contact with an n-type ZnSe crystal are electroluminescent when biassed in the reverse direction. Electrons tunnel from the metal into the semiconductor, are accelerated in the depletion layer field and then impact-ionize luminescent centres. Experiments confirming each of these mechanisms are described. A simple theory is propounded to give a quantitative understanding of the processes and of the performance of the diodes as light sources.

Amine and amino acid microanalysis of two identified snail neurons with known characteristics

Abstract: La microanalyse de l'amine et de l'amine acide de deux neurones identifies a prouve la presence de serotonine dans l'un des neurones et son absence dans l'autre, celui qui contenait plus de methionine. A l'exception d'un interessant compose non identifie, les autres substances etaient essentiellement semblables dans les deux neurones.

Equilibrium Models of Cities: 1. An Axiomatic Theory

Abstract: A study is undertaken of the concept of a city as an ‘urban gravitational plasma’ consisting of one or more species of civic matter (populations, activity rates, and so on) interacting on themselves and each other, and, at the same time, responding to relocation coercions induced by satisfaction potentials of various kinds (housing rentals, amenity levels, and so on). The latter are assumed to be coupled to the territorial densities of the individual species of civic matter through equations of state, for which the housing rental-population density relation in market equilibrium theory is a prototype.The study is divided into four parts. The first part (presented here) approaches the problem from a formal axiomatic viewpoint, and the axioms and definitions are discussed in relation to the real urban situations from which they are abstracted. The notion of equilibrium configurations for a city is introduced, and the general equilibrium equations necessary for their existence are developed. Three particular...

Ionic relationships of enteromorpha intestinalis

Abstract: Summary Concentrations of the major component ions (K, Na and Cl) of sea water and cells of Enteromorpha intestinalis (L.) Link, were determined. Measurements of the electric potential difference across the plasmalemma and the influx and efflux of each ion indicate that K and Cl are actively imported and Na actively exported. Estimates of membrane permeability to the passive diffusive fluxes of these three ions alone have been used to explain the origin of the cell membrane potential. Tracer ion exchange kinetics are taken as showing that the tonoplast is more permeable than the plasmalemma to each ion.

Selective uptake of 5-hydroxytryptamine by axonal processes in Helix pomatia.

Abstract: THE neurotransmitter substances released from presynaptic terminals may be inactivated in different ways: (1) by metabolic conversion to inactive substances, (2) by diffusion away from the synaptic area, or (3) by cellular uptake from the region of the synaptic cleft; for example, by re-uptake into the presynaptic terminals1.

The Dependence of Population Distribution on Location Costs

Abstract: Part of a household's constrained budget is assumed to be comprised of a general cost of location. By using a simple generalisation of Muth's economic theory of the spatial pattern of urban housing, the population density distribution in a plane city is shown to be dependent on the distribution of this generalised cost of location. If housing demand is elastic, then this dependence is shown to be exponential, a conclusion which agrees with a familiar result obtained by maximising the entropy of a population system subject to a cost constraint. If the housing demand is less or more elastic, then the dependence is binomial. Implications of the theory are discussed and illustrations given.