SMC (structural maintenance of chromosomes) protein complexes are an evolutionarily conserved family of motor proteins that hold sister chromatids together and fold genomes throughout the cell cycle by DNA loop extrusion. These complexes play a key role in a variety of functions in the packaging and regulation of chromosomes, and they have been intensely studied in recent years. Despite their importance, the detailed molecular mechanism for DNA loop extrusion by SMC complexes remains unresolved. Here, we describe the roles of SMCs in chromosome biology and particularly review in vitro single-molecule studies that have recently advanced our understanding of SMC proteins. We describe the mechanistic biophysical aspects of loop extrusion that govern genome organization and its consequences.

While the size-dependent optical properties of BSA-stabilized gold nanoclusters are well known, the time-dependent growth mechanism remains to be described. Herein, we systematically compare two synthesis methods with and without ascorbic acid, and show that tuning of BSA-stabilized gold nanoclusters (AuNCs) of different sizes can be performed without the aid of an extrinsic reducing agent and with good reproducibility. We also show that adding ascorbic acid yields larger BSA-stabilized gold nanoparticles (AuNPs), and that AuNPs can only form above a threshold gold precursor concentration. Using computed tomography, we describe how these biomineralized AuNPs show size-dependent X-ray attenuation. Growth of BSA-stabilized AuNCs and AuNPs, over a range of gold precursor concentrations, was followed with steady-state fluorescence and UV-vis spectroscopy for one week, constituting the first study of its kind. Based on our results, we propose a mechanism for BSA-stabilization of AuNCs and AuNPs that can further aid in selective growth of discrete AuNCs and AuNPs.