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At least 37 records · Page 2Linked to original sources
PubMed1999

Song learning from playback in zebra finches: is there an effect of operant contingency?

Social interaction is often regarded as crucial for song copying in zebra finches, Taeniopygia guttata. Contingencies in the interaction between pupil and tutor might be essential for the song-copying process. The effect of contingency between a pupil's operant behaviour and tutor song has been studied previously, but with contradictory results. Our aim in this experiment was to provide a more rigorous test of the effect of operant contingent exposure to song playback in zebra finches. Eight experimental males were trained to expose themselves to tutor song by operant key pecking during their sensitive phase for song learning. Each bird had a yoked control, which heard the same tutor song at the same time. All birds were acoustically isolated. The results were surprising in two ways: (1) the control birds copied song to which they were passively exposed; and (2) the experimental birds did not copy more than the controls did. So, we found no effect of operant contingency on song learning. Furthermore, when tested as adults all but one male preferred the tutor song to an unfamiliar one. We conclude that zebra finches can copy playback song, and that social interaction is not crucial for song copying, although it might still be facilitating. Copyright 1999 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1995

A computer model for time-based simulation of electrophoresis systems with freely defined initial and boundary conditions.

A PC-based program has been developed that allows the user to perform one-dimensional computer simulations of electrophoresis systems with freely definable initial- and boundary conditions. The program can handle n constituents with n pK-values and calculates constituent concentrations and derived parameters as a function of time. Results are displayed graphically on screen and are stored as data files for graphical hard-copy processing.

Computer Simulation↗
PubMed1982

Globular proteins, GU wobbling, and the evolution of the genetic code.

It has previously been shown that the formation of GU base pairs in RNA copying processes leads to an accumulation of G and U in both strands of the replicating RNA, which results in a non-random distribution of base triplets. In the present paper, this distribution is calculated, and, using the X2-test, a correlation between the distribution of triplets and the amino acid composition of the evolutionarily conservative interior regions of selected globular proteins is established. It is suggested that GU wobbling in early replication of RNA could have led to the observed amino acid composition of present-day protein interiors. If this hypothesis is correct, then GU wobbling must have been very extensive in the imprecisely replicating RNA, even reaching values close to the critical for stability of its double-helical structure. Implications of the hypothesis both for the evolution of the genetic code and of proteins are discussed.

Biological Evolution↗
PubMed1996

A reiterative mode of DNA synthesis adopted by HIV-1 reverse transcriptase after a misincorporation.

Amplification of oligonucleotide repeats is a major cause of variability and instability of genomes. This phenomenon is probably due to an aberration in the copying process of polymerases. We show here that in the presence of MnCl2, mismatch formation commits HIV-1 reverse transcriptase to a new mode of DNA synthesis which generates repetitive products. This activity is distinct from terminal transferase since it requires specific DNA motifs in the template. This mechanism, which is processive, also works on homologous RNA templates where it generates reiterative products more than 150 nucleotides long. The corresponding mechanism, which involves extensive primer misalignment, is strikingly similar to that postulated for telomerases.

DNA Primers↗
PubMed1966

The inhibition of ribonucleic acid polymerase by acridines.

1. The aminoacridines, proflavine (3,6-diaminoacridine) and 9-aminoacridine, and a hydrogenated derivative, 9-amino-1,2,3,4-tetrahydroacridine, were shown to inhibit in vitro the DNA-primed RNA polymerase of Escherichia coli. The inhibition is strong with both proflavine and 9-aminoacridine, but weak with 9-amino-1,2,3,4-tetrahydroacridine. 2. The extent to which the three acridines bind to calf-thymus DNA in the enzyme medium was studied spectrophotometrically. The extent of binding decreases in the order: proflavine, 9-aminoacridine, 9-amino-1,2,3,4-tetrahydroacridine. Some evidence was also obtained for interaction between the nucleoside triphosphate substrates and proflavine or 9-aminoacridine; no such interaction was detectable with 9-amino-1,2,3,4-tetrahydroacridine. 3. Although the amount of acridine bound to DNA increases with increasing inhibition, a stage is reached where an increase in acridine concentration still causes an increase in inhibition, with practically no increase in the amount bound to DNA. 4. Plots of reciprocal rates against the reciprocal of DNA concentration were linear and had a common intercept when proflavine or 9-aminoacridine was present. Similar relations were obtained when the reciprocal concentration of nucleoside triphosphates was plotted. The observations are interpreted kinetically in terms of a competitive inhibition of the enzyme by proflavine or 9-aminoacridine and of a kinetic role for the DNA analogous to ;activation'. 5. This suggests that inhibitory acridine molecules can occupy the sites on the RNA polymerase that are specific for binding the nucleoside triphosphate substrate or the bases of the DNA, when these become accessible during the copying process.

Acridines↗
PubMed2002

The role of double-strand break-induced allelic homologous recombination in somatic plant cells.

During meiosis, homologous recombination occurs between allelic sequences. To evaluate the biological significance of such a pathway in somatic cells, we used transgenic tobacco plants with a restriction site for the rare cutting endonuclease I-SceI within a negative selectable marker gene. These plants were crossed with two tobacco lines containing, in allelic position, either a deletion or an insertion within the marker gene that rendered both marker gene and restriction site inactive. After the double-strand break induction, we selected for repair events resulting in a loss of marker gene function. This loss was mostly due to deletions. We were also able to detect double strand break-induced allelic recombination in which the break was repaired by a faithful copying process from the homologue carrying the shortened transgene. The estimated frequency indicates that homologous recombination in somatic cells between allelic sites appears to occur at the same order of magnitude as between ectopic sites, and is thus far too infrequent to act as major repair pathway. As somatic changes can be transferred to the germ line, the prevalence of intrachromatid rearrangements over allelic recombination might be an indirect prerequisite for the enhanced genome plasticity postulated for plants.

Alleles↗
PubMed1996

Two different but related mechanisms are used in plants for the repair of genomic double-strand breaks by homologous recombination.

Genomic double-strand breaks (DSBs) are key intermediates in recombination reactions of living organisms. We studied the repair of genomic DSBs by homologous sequences in plants. Tobacco plants containing a site for the highly specific restriction enzyme I-Sce I were cotransformed with Agrobacterium strains carrying sequences homologous to the transgene locus and, separately, containing the gene coding for the enzyme. We show that the induction of a DSB can increase the frequency of homologous recombination at a specific locus by up to two orders of magnitude. Analysis of the recombination products demonstrates that a DSB can be repaired via homologous recombination by at least two different but related pathways. In the major pathway, homologies on both sides of the DSB are used, analogous to the conservative DSB repair model originally proposed for meiotic recombination in yeast. Homologous recombination of the minor pathway is restricted to one side of the DSB as described by the nonconservative one-sided invasion model. The sequence of the recombination partners was absolutely conserved in two cases, whereas in a third case, a deletion of 14 bp had occurred, probably due to DNA polymerase slippage during the copy process. The induction of DSB breaks to enhance homologous recombination can be applied for a variety of approaches of plant genome manipulation.

Base Sequence↗
PubMed1975

Enzymatic multiplication of a chemically synthesized DNA fragment.

A synthetic DNA fragment of 19 residues was enlarged by the enzymatic addition of deoxyadenylate residues to its 3'-end with calf thymus terminal deoxynucleotidyl transferase. The 3'-terminus of this elongated DNA strand was blocked with 2', 3'-dideoxyadenylate to prevent hydrolysis by the 3'-exonuclease function of E. coli DNA polymerase I. This elongated and 3'-blocked fragment was annealed to an oligomeric primer and used as a template for the synthesis of a complementary copy of the synthetic 19-mer. The product of such a repair synthesis was separated by gel filtration and analyzed by nearest neighbor techniques. All template strands were copied with complete repair in over 90% of the chains. Facile recovery of the elongated template by virtue of its size permitted repetition of the copy process, thus allowing accumulation of the desired strand.

Animals↗
PubMed1993

Rapid direct reversal photography.

This is a description of a method which enables fast, direct-reversal photography of radiological images for slides and was designed to retain: (i) continued use of an existing small darkroom; (ii) use of Kodak Rapid Process copy film; and (iii) use of standard X-ray processors. By adaptation of a commercial photographic flash power pack, a flash head and a reflector it is possible to achieve a somewhat expensive but very successful system which enables the very rapid production of good quality image slides for lectures.

Audiovisual Aids↗
PubMed2002

Hologram recording in polyvinyl alcohol/acrylamide photopolymers by means of pulsed laser exposure.

The variation in transmittance produced when a photopolymer is irradiated with a pulsed laser is analyzed and experimental results obtained when diffraction gratings are stored using pulsed exposure are presented. In either case, the influence of the energy of the irradiation pulse, the number of pulses, and the pulse repetition rate were studied. The photopolymer used was an acrylamide/polyvinyl alcohol dry film with a yellow eosin-thiethanol-amine mixture as a photoinitiator system. The recording of the gratings was performed by use of a holographic copying process. The samples were exposed and holograms recorded with a collimated beam from a frequency-doubled Nd:YAG (532 nm) Q-switched laser. Our initial results show that it is possible to obtain diffraction gratings with a diffraction efficiency of 60% and a refractive index modulation up to 2.8 x 10(-3). The energetic sensitivities achieved are close to those obtained with the same material and continuous irradiation without a preprocessing of the gratings.

Journal Article↗
PubMed1993

[Preparation of bonded porcelain inlays and onlays using a milling-coping system].

The Celay-system offers an interesting alternative to existing systems for esthetic posterior restorations with ceramics. The basic idea of the concept is to prepare, either directly on the patient or indirectly on a plaster model, a pro-inlay using a light-curable precision material. The modelling also covers the occlusal surface. This pro-inlay is then used with the help of a milling center to machine a precise replica out of a ceramic block. The copying process is carried out via three-dimensional mechanical scanning and milling using eight axes of freedom. In this way additional corrections on the ceramic within the mouth can almost completely be avoided. The system is designed to be used as a direct chairside method by the dentist or as an indirect method by the dental technician. It is a promising but unproved restoration, more research and long-term clinical evaluation is recommended.

Adhesives↗
PubMed1996

Construction of a self-complementary nucleoside from deoxyguanosine.

The 8-hydroxyguanine:adenine mispairing scheme that spontaneously occurs in vivo through oxidative metabolism of DNA was edited to obtain a pair closely fitting the Watson-Crick geometry in which 2 purine bases of identical structure but oppositely rotated in the syn and anti configurations are hydrogen-bonded. The structure thus designed, 2-amino-8-hydroxypurine, was synthesized as a DNA precursor from deoxyguanosine in 8 steps by oxidation at carbon 8 of guanine followed by reduction at carbon 6. Polydeoxynucleotides embodying this self-complementary base are expected to undergo direct copying processes through polymerase catalysis.

Base Composition↗
PubMed2006

Processes of copy-number change in human DNA: the dynamics of {alpha}-globin gene deletion.

Ectopic recombination between locally repeated DNA sequences is of fundamental importance in the evolution of gene families, generating copy-number variation in human DNA and often leading to pathological rearrangements. Despite its importance, little is known about the dynamics and processes of these unequal crossovers and the degree to which meiotic recombination plays a role in instability. We address this issue by using as a highly informative system the duplicated alpha-globin genes in which ectopic recombination can lead to gene deletions, often very prevalent in populations affected by malaria, as well as reduplications. Here we show that spontaneous deletions can be accessed directly in genomic DNA by using single-DNA-molecule methods. These deletions proved to be remarkably common in both blood and sperm. Somatic deletions arise by a strictly intrachromosomal pathway of homologous exchange that also operates in the germ line and can generate mutational mosaicism, whereas sperm deletions frequently involve recombinational interactions between homologous chromosomes that most likely occur at meiosis. Ectopic recombination frequencies show surprisingly little requirement for long, identical homology blocks shared by paralogous sequences, and exchanges can occur even between short regions of sequence identity. Finally, direct knowledge of germ-line deletion rates can give insights into the fitness of individuals with these alpha-globin gene deletions, providing a new approach to investigating historical levels of selection operating in human populations.

DNA↗
PubMed2006

Evaluation of new image processing conditions for digital mammograms from Fuji computed radiography.

PURPOSE: A new processing parameter (T-type parameter) setting for gray scale was developed to improve the image quality of digital mammograms. To clarify the usefulness of this parameter setting, we have evaluated the image quality of digital mammograms (hard copy images) processed with this parameter, and compared it with S/F mammography. MATERIAL AND METHODS: Mammograms were made under the same radiographic conditions by the S/F and FCR systems (type 1, S/F; type 2 and 3, FCR with new T-type parameters; type 4 and 5, FCR with conventional parameters). A total of 49 images from 10 cases was selected for evaluation testing. Evaluation items were the contrast visibility of mammary glands and adipose tissues together with their granularity and sharpness. Eleven medical doctors participated in evaluating the images. RESULTS: The FCR hard copy images processed with the T-type parameter settings were significantly preferred over the conventional S/F images for the contrast visibility of mammary glands and adipose tissue. As for the other items (except for granularity), the FCR hard copy images processed with the T-type parameter settings were subjectively evaluated as slightly better than or equal to the S/F images. In contrast, the conventional S/F images were significantly preferred over the FCR hard copy images processed with the conventional parameter settings. CONCLUSION: The image quality of FCR hard copy images processed with the T-type parameter settings was preferred over that of conventional S/F images as evaluated by medical doctors who specialized in mammography interpretation.

Female↗