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Biomedical subjects

J H TAYLOR

Publications and source records attributed to J H TAYLOR.

At least 19 recordsLinked to original sources
PubMed1960

Asynchronous duplication of chromosomes in cultured cells of Chinese hamster.

Chromosome duplication (DNA synthesis) was studied in cultured cells of Chinese hamsters by means of autoradiography following thymidine-H(3) incorporation. The technique used was to expose an asynchronously dividing population of rapidly growing cells for a 10 minute interval to a medium with thymidine-H(3). Cells were then transferred to a medium with excess unlabeled thymidine. The population was sampled at intervals thereafter and studies made of the frequency of labeled interphases and division figures, and the patterns of labeling of specific chromosomes. The average generation time during these experiments was about 14 hours. DNA synthesis occurred during an interval of about 6 hours and stopped 2 to 3 hours before metaphase. After metaphase the chromosomes usually begin duplication again within 5 to 6 hours. Grain counting, to estimate the amount of tritium incorporated after a short contact with thymidine-H(3) and at intervals after transfer to a medium with excess unlabeled thymidine, indicated that the intracellular pool of labeled precursors was diluted within less than a minute so that further labeling would not be detected. The chromosomes labeled during the contact period retained their precise pattern of labeling through another duplication cycle and no turnover of DNA or loss of tritium was detectable. Five or 6 chromosomes of the complement have segments typically late in duplication. Two of these are the X and Y chromosomes. The long arm of the X chromosome and the whole Y chromosome are duplicated in the last half of the interval of DNA synthesis. The short arm of the X chromosome in a male strain is duplicated in the first half of the interval. In another strain (female), one X chromosome had the same timing, but the other one was all duplicated in the last half of the period of DNA synthesis. The DNA in the short arms of 2 medium sized chromosomes, as well as most of the DNA in 1 or 2 of the smallest chromosomes of the complement was replicated late. The study has led to the hypothesis that various chromosomes or parts of chromosomes have a genetically controlled sequence in duplication which may have some functional significance.

Animals↗
PubMed1959

The metabolism of chromosomal ribonucleic acid in Drosophila salivary glands and its relation to synthesis of desoxyribonucleic acid.

Incorporation of adenine-8-C(14) into chromosomal nucleic acids of Drosophila salivary glands has been observed with the autoradiographic technique. RNA-C(14) and DNA-C(14) were detected as the fractions extractable by ribonuclease digestion and resistant to ribonuclease, respectively. Extractions with desoxyribonuclease and acids were also used to identify the nucleic acids. Time-course curves were determined from grain counts. RNA-C(14) concentration reached a maximum in 2 hours, and decreased after the 4th hour. DNA-C(14) concentration reached its maximum within 8 hours, and showed no decreases during a 48-hour experiment. In the latter part of the period of observation, morphological differentiation of the gland occurred, the decrease in RNA-C(14) became very rapid, and a large increase in DNA-C(14) was observed. Marked decrease in RNA-C(14) and increase in DNA-C(14) were detectable in a few hours when isotope was administered shortly before visible differentiation of the gland. Measurements of nuclear size indicated no significant decreases in RNA-C(14) amount prior to the period of differentiation. During this later period, a large decrease in RNA-C(14) amount occurs suddenly, and the same amount of C(14) is added simultaneously to the DNA fraction, as expected if RNA-C(14) is utilized in the synthesis of DNA.

Animals↗
PubMed1958

Evidence for two metabolically distinct types of ribonucleic acid in chromatin and nucleoli.

Patterns of radioisotope incorporation are useful characteristics in describing cellular RNA fractions, and have indicated a distinctive "nuclear" RNA. In order to characterize the RNA fractions of the two nuclear components, nucleoli and chromatin, and to determine thereby the precise localization of the RNA typical of isolated nuclei, time-courses of P(32) incorporation into nucleolar, chromosomal, and cytoplasmic RNA of Drosophila salivary glands have been determined from autoradiograms. Two experiments are reported which cover 12 and 18 hour periods, including an initial 2 hour feeding on P(32). Concentrations of RNA-P(32) (identified by ribonuclease digestion) were determined by grain counts. After 1 hour only the nucleolar RNA is labelled. Activity is detectible in chromosomal and cytoplasmic RNA after the 2nd hour. The nucleolar fraction reaches its maximum activity shortly after transfer of the larvae to non-radioactive food, the other fractions several hours later. Maximum activities persist in the chromosomal and cytoplasmic fractions; nucleolar activity decreases after the 9th hour. The observed differences in times at which incorporation begins and maximum activities are reached, and in maintenance of maximum activities indicate that chromosomal and nucleolar RNA are distinct fractions. The metabolic characteristics which have been ascribed to "nuclear" RNA apply only to the nucleolar fraction.

Cell Nucleolus↗