Abstract

The nucleolus is a large membraneless organelle located in the nucleus of eukaryotic cells. It is a multicomponent structure primarily made of RNA and proteins and serves as the site of ribosome biogenesis. It has been extensively studied as a prototype of a membraneless organelle whose assembly is driven by phase separation, but factors affecting the dynamics of nucleolar assembly are not well understood. Experiments show that the size of the nucleolus scales with the size of the nucleus, which decreases through reductive divisions in the early C. elegans embryo. These observations are consistent with the idea that a finite pool of constituent nucleolar proteins sets the ultimate size of assembling nucleoli. Here, we extend this limiting pool model to account for assembly dynamics and consider the role of nascent ribosomal RNA synthesis in nucleolar assembly. We show that experimental measurements are inconsistent with a model of passive assembly to an equilibrium, phase-separated state, but are well described by a model in which the transcription of ribosomal RNA actively drives nucleolar assembly. We find that our model of active transcription-driven assembly captures the rapid kinetics observed in early embryos at different developmental stages and for different RNAi perturbations. Significantly, our model predicts a scaling of the time to assembly with the volume of the nucleus to the one-third power, which is confirmed by experimental data. Additionally, we analyze the fluctuations in the size of the nucleolus and find that the active model captures the rapid decay in the autocorrelation function of the experimental data. Our study highlights the importance of active processes such as transcription in the assembly of membraneless organelles and points to these processes as providing control over the placement and timing of their assembly in the cell.

Publication Date

6-2024

Document Type

Thesis

Student Type

Graduate

Degree Name

Physics (MS)

Department, Program, or Center

Physics and Astronomy, School of

College

College of Science

Advisor

George Thurston

Advisor/Committee Member

Pratik Dholabhai

Advisor/Committee Member

Moumita Das

Campus

RIT – Main Campus

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