Organ specificity of template activity of mammalian chromatin

Thomou, Helen (1970) Organ specificity of template activity of mammalian chromatin. PhD thesis, University of Glasgow.

Abstract

Cell differentiation is correlated with, the differential activity of genes in a given cell (and by implication in a functionally defined mixture of cell types, i.e. a tissue). One of the topics of major Importance in differentiation is concerned with the proportion of the genome which is active in RNA synthesis, i.e. the relative amount of genomic information which brings about the morphological changes characteristic of adult tissues. The template specificity of isolated chromatins in supporting RNA synthesis t/as studied in order to clarify the relationship between genomic expression and organ specificity. The molecular hybridization technique was used for RNAs transcribed vitro from several mouse chromatins. This technique provides a sensitive method for distinguishing between various populations of RNA molecules. It was found first that different populations of RNA molecules examined are not only distinguishable but also show large differences among them and second, that only a part of the genome was active in transcription. By competition experiments it was found that the RNAs produced by chromatin in a cell-free RNA system were similar to RNAs isolated from the whole homologous tissue. This provides evidence that the template specificity associated with chromosomes in vivo is retained by isolated chromatin. A particular feature of this study is the high degree of reproducibility of the hybridisation values for each tissue and the dissimilarities displayed by the different tissues. When DNA from mouse kidney was used as template in a cell free system and the RNA produced was annealed, then the amount of hybridisation was almost three times greater than the hybridization which occurred when chromatin was used as template. From these results it appears that many of the genes in animal somatic cells, which are considered as being in a completely differentiated state, are in fact inactive in supporting RNA synthesis, the inactivation pattern of the genome in such cells being distinctive for each tissue. The mechanisms which control cell division in mammalian organs are not understood. Various experimental models have been used to study this problem. The cases under investigation include in vivo models, i.e. induction by folic acid of RNA synthesis in mouse kidneys and the appearance of new species of RNA in mouse kidneys after the ligature of one ureter. Studies have been made on the effect of folio acid administration in stimulating changes in the vitro RNA synthesis such that new kinds of RNA molecules appear in mouse kidney at specific times after the treatment. The ability of kidney chromatin in supporting RNA synthesis has been studied in untreated (control) and treated animals. It was found that folic acid changes profoundly the template activity of the isolated chromatin from mouse kidneys, RNA transcribed from chromation at early stages hybridizes with twice as much DNA as RNA transcribed from normal chromatin. By competition experiments it was confirmed that the cell free system produces the same RNA species as are found vivo which indicates that the integrity of the chromatin is maintained during isolation. Furthermore, the experiments showed that new kinds of RNA molecules are transcribed from kidney chromatin after the early hours of folate. The transcriptional activity of the genome eventually falls to the control level after two days. Another method for stimulating the appearance of new species of RNA in mouse kidneys is to ligate one ureter. The template activity of chromatin from the ligated kidney has been investigated from aero hours until 48 hours. The maximum hybridization efficiency (twice the control) is exhibited at 36 hours while RNA transcribed from chromatin isolated at early stages hybridizes with as much DNA as RNA transcribed from normal chromatin. The purpose of these experiments has been to compare the template activity of chromatin isolated at various times after the ligature of one ureter with those obtained after folic acid treatment in mice and to determine if the same control mechanism operates in both phenomena. The relationship between these two systems is discussed.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: John Paul
• Keywords: Developmental biology
• Date of Award: 1970
• Depositing User: Enlighten Team
• Unique ID: glathesis:1970-73158
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jun 2019 08:56
• Last Modified: 14 Jun 2019 08:56
• URI: https://theses.gla.ac.uk/id/eprint/73158