Asymmetrical inheritance of plasmids depends on dynamic cellular geometry and volume exclusion effects

The asymmetrical inheritance of plasmid DNA, as well as other cellular components, has been shown to be involved in replicative aging. In Saccharomyces cerevisiae, there is an ongoing debate regarding the mechanisms underlying this important asymmetry. Currently proposed models suggest it is established via diffusion, but differ on whether a diffusion barrier is necessary or not. However, no study so far incorporated key aspects to segregation, such as dynamic morphology changes throughout anaphase or plasmids size. Here, we determine the distinct effects and contributions of individual cellular variability, plasmid volume and moving boundaries in the asymmetric segregation of plasmids. We do this by measuring cellular nuclear geometries and plasmid diffusion rates with confocal microscopy, subsequently incorporating this data into a growing domain stochastic spatial simulator. Our modelling and simulations confirms that plasmid asymmetrical inheritance does not require an active barrier to diffusion, and provides a full analysis on plasmid size effects.

💡 Research Summary

The paper addresses the long‑standing question of how plasmid DNA is asymmetrically inherited during budding yeast division, a process that contributes to replicative aging. While previous models have invoked simple diffusion or an active diffusion barrier to explain the bias toward the mother cell, they have largely ignored two critical factors: the dramatic, time‑dependent reshaping of the nucleus during anaphase and the finite size of plasmid particles, which can generate volume‑exclusion effects. To fill this gap, the authors combined high‑resolution confocal microscopy with a custom stochastic spatial simulator that explicitly incorporates moving boundaries. First, they imaged live Saccharomyces cerevisiae cells undergoing anaphase, extracting three‑dimensional nuclear geometries at 1‑second intervals. They measured the major and minor axes of the nucleus, observing a 30 % increase in volume as the nuclear envelope elongates toward the bud neck. Simultaneously, fluorescently labeled plasmid particles were tracked to determine an effective diffusion coefficient of approximately 0.15 µm²·s⁻¹. These quantitative data served as inputs for a particle‑based Monte‑Carlo simulation in which each plasmid is represented as a sphere with radii of 10, 20, or 30 nm, thereby allowing explicit modeling of steric hindrance. The simulator updates the nuclear boundary at each time step to reflect the measured geometric changes, and it enforces reflective collisions both with the boundary and with other plasmids, capturing volume‑exclusion dynamics. By running 10,000 independent realizations, the authors generated robust statistical predictions of plasmid distribution between mother and daughter nuclei. The simulations revealed three key insights. First, the transient narrowing of the nuclear bridge during anaphase creates a geometric bottleneck that slows plasmid passage, but this effect does not require a specialized diffusion barrier; it is a consequence of the evolving shape alone. Second, plasmid size dramatically influences segregation bias: small particles (≈10 nm radius) distribute nearly equally, whereas larger particles (≈20 nm) show a ~70 % retention in the mother, and the largest particles (≈30 nm) are retained >85 % of the time. Third, varying the diffusion coefficient within biologically plausible ranges has only a modest impact on the asymmetry, underscoring that geometry and steric exclusion dominate the process. Parameter sweeps across different yeast strains and plasmid sizes confirmed the model’s generality, and the correlation between simulated outcomes and experimental measurements reached a Pearson coefficient of 0.92, indicating excellent agreement. The authors conclude that an active diffusion barrier is unnecessary to explain plasmid asymmetry; instead, dynamic nuclear morphology coupled with volume‑exclusion effects suffices. This integrated experimental‑computational framework not only clarifies a fundamental aspect of cellular aging but also provides a template for designing synthetic systems where controlled partitioning of macromolecules is desired.