The PWS optical instrument is built on a commercial inverted microscope (Leica, Buffalo Grove, IL, DMIRB) using a Hamamatsu Image-EM CCD camera C9100-13 coupled to an LCTF (CRi Woburn, MA) to do hyperspectral imaging. Spectrally resolved images are collected between 500 and 700 nm with 2-nm steps. Broadband illumination is provided by an Xcite-120 light-emitting diode lamp (Excelitas, Waltham, MA). For live-cell measurements, cells were imaged live and maintained under physiological conditions (5% CO2 and 37°C) via a stage top incubator (In Vivo Scientific, Salem, SC; Stage Top Systems). As described in Results, PWS measures the spectral SD of internally optical scattering originating from nuclear chromatin, which is related to variations in the refractive index distribution (Σ). Those variations in the refractive index distribution are characterized by the mass scaling or chromatin packing scaling, D. Therefore, D was calculated from maps of Σ. A detailed description of the relationship between Σ and D is provided in section S3 (18, 21, 29).

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