The biomolecular condensate nucleolus has a significant role in synthesis of ribosomal RNA components and represents the base of ribosome biogenesis in cells. During cell differentiation the demands for proteins are changing. Embryonic stem cells (ESCs) known for their developmental plasticity and high division rate show tendency to greater amount of proteins compare to somatic cells. Equivalently to protein amount quantity of functional ribosomes also rises through increased number of actively transcribed ribosomal genes or rDNA in nucleolus. Shortage of studies regarding dynamics of precursors of ribosomal RNA molecules (pre-rRNA) in dividing ESCs prevents a wholesome understanding and assessment of their role in relation to cell’s pluripotency. In the process of applying various methods for labeling target pre-rRNA molecules in mouse ESC the hybridization technique stood out as the most suitable one. With it we discovered never seen before condensates within mitotic chromosomes and were able to prove their spatial overlap with mitotic transcription sites of nascent RNAs. Conducting proximitiy labeling technique HyPro we got insight into interaction hubs of RNAs in mitotic condensates and identified multiple snoRNAs involved in process of maturating pre-rRNAs. Spatial entanglement of precursors of ribosomal RNAs and snoRNAs, outstanding snord3b-ps2, suggests new function of ribosomal genes in dividng ESCs and shakes the foundation on which meaning of mitotic pre-rRNAs was build.
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