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We have seen an increasing body of evidence that rRNA plays a key functional role, as well as a structural role, in protein synthesis. Although a typical eukaryotic cell produces about 10,000 different RNA species, rRNA comprises about 80% of the total RNA to meet the cellular need to produce more than a million new ribosomes per generation. Typically a eukaryotic cell retains several hundred rDNA repeat unit clustered in various chromosomes. In oogenesis, these genes are selectively replicated several thousand times to form extrachromosomal circular DNA. Ribosomal RNA gene transcription rates are known to be regulated by activated RNA polymerase I, undermethylation of rDNA repeat unit and by certain hormones, etc.. Also great progress has been made to delineate rDNA pro-motor, enhancer sequence and transcriptional factors for RNA polymerase I. Quite species-specific transcription of rRNA has been attributed to the transcriptional factors. Active research is being progressed to understand the regulation of the rRNA expression in cancerous, aging, nutritionally unballanced and diseased cell. It is obvious that probes targeting the rRNA can considerably increase the sensitivity of the test for detection, and identification of microorganism. In the future much thrust will be put for-ward to understand the structure-function relationship of the rRNA molecule in detail.
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