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Biomedical subjects

M D Morris

Publications and source records attributed to M D Morris.

At least 55 records · Page 3Linked to original sources
PubMed1995

Optical melting of 128 octamer DNA duplexes. Effects of base pair location and nearest neighbors on thermal stability.

The use of short oligonucleotide probes is finding increased application in DNA sequencing and genome characterization techniques, but a lack of knowledge of the hybridization properties of short duplexes hinders their use. Melting data were acquired on 128 DNA duplexes based on the length proposed in sequencing by hybridization procedures and formed from the general sequences 5'-XYZTGGAC-3',5'-GTCCAXYZ-3',5'-GCXYZGAC-3', and 5'-GTCXYZGC-3' where X, Y, and Z are either A, T, G, or C. These molecules were designed to elucidate the effects of location and nearest-neighbor stacking on the stability of base pairing in short DNA duplexes. The type of base pairs present had a major effect on stability, but was insufficient to predict stability without the inclusion of nearest-neighbor terms. Furthermore, the addition of information on position, or distance from the end, of the nearest-neighbor doublets led to statistically better fitting of the melting data. However, the positionally dependent stabilization differences are small compared with the contributions of base pairing and stacking.

Base Composition↗
PubMed1995

DNA conformational dynamics in polymer solutions above and below the entanglement limit.

Video microscopy of nucleic acids (DNA) undergoing electrophoresis in hydroxyethyl cellulose (HEC) sieving buffers demonstrates previously unobserved shape-changing interactions between DNA and HEC molecules. We provide the first visual demonstration of entanglement between DNA and one or several discrete HEC molecules, which has been postulated to occur in ultradilute polymer solutions. Typically, nucleic acids appear to become entangled with HEC at a single region only, in both dilute and fully entangled HEC solutions. Fluctuations of the center of mass velocity of a DNA molecule and its correlation with conformation are revealed from analyses of the image data. These observations account for the success of recently reported rapid, high-resolution dc and pulsed-field capillary electrophoretic separations of nucleic acids in ultradilute hydroxyethyl cellulose solutions and hydroxyethyl cellulose/poly(ethylene oxide) solutions.

Cellulose↗
PubMed1995

Rapid pulsed field capillary electrophoretic separation of megabase nucleic acids.

Pulsed field capillary electrophoresis in ultradilute solutions of sieving polymers has been used to separate nucleic acid fragments as long as 1.6 million base pairs. Hydroxyethyl cellulose solutions are used for separations in the size range 8000-50,000 base pairs. Longer chain nucleic acids are separated in mixed hydroxyethyl cellulose/poly(ethylene oxide) solutions. Separations are rapid (12-13 min).

Cellulose↗
PubMed1995

Characterization of the chemical interactions betwen 4-MET and enamel by Raman spectroscopy.

OBJECTIVES: The goal of this investigation was to use Raman spectroscopy to study the chemical interactions between 4-methacryloxyethyltrimellitic acid (4-MET), a hydrolized form of 4-methacryloxyethyltrimellitate anhydride (4-META) and powdered substrates such as bovine enamel powder and synthetic carbonate-containing apatite. METHODS: Powdered samples were prepared by immersing them in 4-MET/methylmethacrylate (MMA) solution for 15 min, 3 h and 24 h. They were then dried for spectroscopy. An experimental Raman microprobe equipped with a cryogenically-cooled camera was employed to acquire spectra of the samples. RESULTS: Spectra of samples wetted for 24 h strongly indicate that there is strong ionic interaction between Ca2+ ions on the powdered sample surface and 4-MET. Spectra of the 15-min samples, however, suggest only some degree of hydrogen bonding. The spectrum of the 3-h 4-MET/enamel powder sample showed features intermediate between the two. SIGNIFICANCE: Results presented in this paper show that salt formation between 4-MET and dental substrates is not likely to be instantaneous. On the contrary, the rate of such reaction is probably too slow relative to that of polymerization of the co-monomers to be important under clinical conditions. The function of the 4-MET in the 4-MET/MMA resin system appears, therefore to be wetting the tooth surface through hydrogen bonding, which then allows MMA monomers to diffuse and penetrate deeper through the surfaces. The primary bonding mechanism of this system is more likely due to a micro-mechanical locking mechanism.

Animals↗
PubMed1994

Raman spectroscopic measurement of spatial and temporal temperature gradients in operating electrophoresis capillaries.

Steady-state and transient intracapillary temperature gradients are measured by Raman microthermometry during capillary zone electrophoresis. The axial temperature gradient away from a heat sink extends for several millimeters, depending upon the contact of the capillary and heat sink. In free-air convection, small radial gradients, 2-4 degrees C from the center of the lumen to the wall, are observed at 0.85 kW/cm3. The temperature profile is adequately described by a parabola. With forced cooling, the center-wall temperature difference is less than 1 degrees C. The time to steady-state temperature after power-on is found to be 20 s. The measurements are compared to the results of heat transport calculations.

Electrophoresis↗
PubMed1994

Pulsed field capillary electrophoresis of multikilobase length nucleic acids in dilute methyl cellulose solutions.

Pulsed field capillary gel electrophoresis in dilute methyl cellulose solutions is used to separate nucleic acid fragments in the size range 75-23,000 base pairs. Field inversion is shown to increase resolution for fragments longer than about 500 base pairs. Methyl cellulose solutions as dilute as 0.01% can be used. Intermolecular hydrogen bonding is suggested as the cause of apparent cellulose fiber entanglement at concentrations below the calculated entanglement limit. The 1-kb DNA ladder and the lambda DNA/HindIII restriction fragment mixtures are each baseline resolved in a 28-cm capillary in less than 9 min at 180 V/cm (dc component).

Bacteriophage lambda↗
PubMed1994

Separation of nucleic acids by capillary electrophoresis in cellulose solutions with mono- and bis-intercalating dyes.

The effects of bis- and mono-intercalating dyes on the capillary electrophoretic separation of double-stranded DNA have been investigated in buffers containing (hydroxypropylmethyl)-cellulose. Broad bands and incomplete separations of phi X 174 DNA HaeIII digest were obtained with the bis-intercalators ethidium homodimer 1 (EthD-1) and ethidium homodimer 2 (EthD-2), under a wide range of dye and polymer concentrations. The above results were attributed to the presence of several intramolecular dye/DNA complexes for each fragment. For EthD-2/DNA complexes, but not for EthD-1/DNA complexes, additional bands beyond the expected number of fragments were observed. The extra bands were attributed to formation of intermolecular dimers. Use of the monomeric intercalating dyes, ethidium and propidium, allowed complete separation of all fragments of phi X 174 DNA HaeIII digest. A low-power green He-Ne laser (543.6 nm) was shown to be satisfactory for laser-induced fluorescence detection with any of these dyes.

Cellulose↗
PubMed1994

Variable frequency modulation in DNA separations using field inversion capillary gel electrophoresis.

Variable frequency pulsed field capillary gel electrophoresis of nucleic acid restriction fragment mobility and resolution is reported. The frequency of the sine wave superimposed on the dc driving field was randomly varied throughout the duration of the electrophoresis in order to enhance the fragment separations. By frequent changes of frequency, fragments of various sizes experienced optimum conditions for separation sometime during the electrophoresis. The mobilities of phi X 174 HaeIII fragments in 9% T linear polyacrylamide were primarily dependent on power dissipation in the capillary, independent of whether the driving field was dc only or modulated. Resolution of long fragments exhibited a strong modulation frequency dependence. Modulated driving fields provided better resolution of the 603-1353 base pair fragments than dc only fields. Modulation protocols with center frequencies near 100 Hz gave the highest resolution.

DNA↗
PubMed1994

Electric birefringence imaging of DNA in agarose electrophoresis gels: noise limits and prolonged electric field effects.

Electric birefringence imaging (EBI) maps stresses in agarose electrophoresis gels. Both DNA and gel alignment can be monitored using this technique. Previously, we have reported birefringence images associated with resolved small (< 48 kbp) and partially resolved large 48 kb-(2 Mbp) DNA fragments oriented with relatively short electric field pulses. In this paper, differences between images acquired using short and long (DC) voltage pulses are presented. It is shown that stress signals distribute over long distances when exposed to prolonged electric fields. Resolution also affects band shapes. Refinements in instrumental design and evaluation of limiting noise sources of the technique are also described.

Birefringence↗
PubMed1994

Bone marrow equivalent prompt dose from two common fallout scenarios.

A cell-kinetics model for radiation-induced myelopoiesis has been derived for mice, rats, dogs, sheep, swine, and burros. The model was extended to humans after extensive comparisons with molecular and cellular data from biological experiments and an assortment of predictive/validation tests on animal mortality, cell survival, and cellular repopulation following irradiations. One advantage of the model is that any complex pattern of protracted irradiation can be equated to its equivalent prompt dose. Severity of biological response depends upon target-organ dose, dose rate, and dose fractionation. Epidemiological and animal data are best suited for exposures given in brief periods of time. To use those data to assess risk from protracted human exposures, it is obligatory to model molecular repair and compensatory proliferation in terms of prompt dose. Although the model is somewhat complex both mathematically and biologically, this note describes simple numerical approximations for two common exposure scenarios. Both approximations are easily evaluated on a simple pocket calculator by a health physicist or emergency management officer.

Animals↗
PubMed1994

Dose-rate RBE factors for photons: hematopoietic syndrome in humans vs. stromal cell cytopenia.

Traditionally, dose-response modeling has been on a strict experiment-by-experiment basis. Such an approach greatly restricts understanding of complex biological systems affected by numerous confounding factors that individually vary from experiment to experiment. In contrast, work described in this manuscript relies on a new analytical process (that considers both pooled and experiment-specific considerations) that was used to jointly analyze the bone marrow cell kinetics from a large data base on six species of test animals irradiated by protracted schedules of ionizing photon radiations. From this approach, we have modeled how the human LD50 may vary with dose protraction and how the dose rate efficiency or RBE factors for x rays, 137Cs, and 60Co change for irradiations given at constant rate over one minute, hour, day, week, and month.

Animals↗
PubMed1994

A chicken achaete-scute homolog (CASH-1) is expressed in a temporally and spatially discrete manner in the developing nervous system.

We have identified a basic helix-loop-helix encoding cDNA from embryonic chicken retina which shares sequence similarity with the achaete-scute family of genes of Drosophila. The deduced amino acid sequence of this chicken achaete-scute homolog (CASH-1) is identical, over the region encoding the basic helix-loop-helix domain, to the recently identified mammalian achaete-scute homolog (MASH-1) and to the Xenopus homolog (XASH1), and 70% identical, over the same region, to Drosophila achaete-scute complex members. The expression of CASH-1 is restricted to subsets of neuronal progenitor cells in the developing chicken nervous system, similar in distribution to that reported for MASH-1 and XASH1. In addition, in situ localization in the retina reveals a dynamic character of expression of the gene in a particular region of the CNS, and suggests that the expression of CASH-1 may be important in defining a particular stage in the progenitor cell necessary for the differentiation of particular neuronal phenotypes.

Amino Acid Sequence↗
PubMed1993

Mathematical models of marrow cell kinetics: differential effects of protracted irradiations on stromal and stem cells in mice.

UNLABELLED: It is known that hematopoiesis is supported by bone-marrow stem cells, but those cells must seed and grow on a stromal microenvironment. Typically, studies have shown that a surviving fraction of about 30 hematopoietic stem cells (HSCs) (i.e., about 0.04%) correspond to the LD50, although other studies have shown that marrow can repopulate from a single viable cell under strong regiments of antibiotics and infusions of irradiated blood elements. PURPOSE: This paper describes comparisons between our results (from maximum-likelihood estimation techniques for cellular damage, repair, and compensatory repopulation) and published experimental data on marrow stromal cells. METHODS AND MATERIALS: After biophysical consideration of the rate constants that were derived by maximizing the likelihood function (a consideration necessary to extend the model to cell populations not indicated by the model as "critical" for recovery), the rate constants for cellular damage to stem cells are fitted to experimental data. Rate constants for repair and proliferation of stem cells are assigned based on published data on repair/proliferation half-times, and these assignments affect the evaluation of the rate constants for cellular damage. From the two models, that is one for "critical" cells (having radiosensitive and repopulation characteristics similar to stromal cells) and another for stem cells, effects on two cell populations of different radiosensitivities and repopulation rates can be demonstrated for complex schedules of protracted irradiations which could reduce either cell population below a critical need for marrow repopulation. RESULTS: Our analysis of animal mortality data has indicated that recovery of an animal from potentially lethal irradiation is dominantly regulated by cells with survival and repopulation characteristics similar to those of stroma cells. CONCLUSION: In contrast to the surviving fraction of hematopoietic stem cells, it appears that the probability of an animal's recovery is high if the "critical" population of cells is above 1% (our "best" maximum likelihood estimate, from mouse data, with the corresponding lower confidence bound at about 0.2%). Of course, a few stem cells--perhaps only one--must maintain a potential for repopulation of blood and marrow.

Animals↗
PubMed1993

Spatially resolved temperature measurements in electrophoresis capillaries by Raman thermometry.

Local temperatures inside a 75-microns-i.d. capillary under electrophoresis conditions are measured noninvasively with a Raman microprobe. The method is based on the temperature dependence of the water O-H stretch equilibrium between weakly bent and strongly bent hydrogen bonded species. With calibration against a known temperature standard, this technique is shown to be capable of obtaining spatially resolved intracapillary temperature measurements with an accuracy of +/- 1.0 degree C and a precision of +/- 0.1 degree C. Intracapillary temperatures ranging from 25 to 70 degrees C are observed over the range of buffer compositions and electric fields used. Differences between local temperatures and average capillary temperatures are observed at all operating conditions. The difficulty of an accurate theoretical description of heat dissipation under CE conditions is discussed.

Electrophoresis↗
PubMed1993

A cell kinetics model of radiation-induced myelopoiesis: rate coefficient estimates for mouse, rat, sheep, swine, dog, and burro irradiated by photons.

Rate coefficients in the model of cell kinetics and mortality introduced by Jones et al. (Radiat. Res. 128, 258-266, 1991) are estimated using mortality data from 27 animal experiments. Adjustments are made for the six species and three nominal classes of gamma radiation represented in these studies. The model fits most of the mortality data quite well when the rate coefficient representing cellular proliferation is fitted to individual species and each of the other coefficients is given a single value across the entire data set. Results are qualitatively similar to those reported by Morris et al. (Radiat. Res. 128, 267-275, 1991) who estimated the rate coefficients from a limited number of mouse studies involving only 250 kVp X rays. As in the earlier study, estimates here lead to greater cell survival than is observed for marrow cells in the amplification division compartment.

Animals↗
PubMed1993

A cell-kinetics model for radiation-induced myelopoiesis.

A mathematical model of time-dependent cellular damage, repair, killing and repopulation of bone marrow following treatments with ionizing radiations is described. Effects from variable dose rates, multiple exposures, different radiation sources and arbitrary intervals between treatments can be modeled by ordinary differential equations. Of several unique features, the most unusual is that rate constants for injury, repair, killing and proliferation of cells are evaluated by likelihood analysis of animal mortality data. Results indicate that a relatively radioresistant pool of bone marrow cells mediates the proliferation of the hematopoietic stem cells. Applications include modeling of 1) myelopoietic integrity as a function of time and dose rate, 2) the whole-body survival curve (at any point in the treatment protocol) for cells critical to myelopoiesis, 3) a prompt dose equivalence from any completed portion of a therapeutic schedule and 4) potential gain from schedule changes during the course of the treatment.

Animals↗
PubMed1992

Acrylamide polymerization kinetics in gel electrophoresis capillaries. A Raman microprobe study.

The formation of 3.5% T, 3.3% C cross-linked polyacrylamide is monitored in 75-microns-i.d. electrophoresis capillaries by Raman microprobe spectroscopy. The disappearance of the acrylamide 1292-cm-1 band is followed with 60-s time resolution for 30 min, and 2-4 min resolution for up to 10 h. Polymerization is 98% complete in 1.5 h and greater than 99% complete after 2 h. In the 900-1700-cm-1 region no bands attributable to cross-linking are observable. Reaction in the capillary follows second-order kinetics. The reaction is faster in the bulk system because heat dissipation is not sufficient to maintain a constant temperature.

Acrylamide↗