Practice question

Nuclear envelope is perforated by two thousand to four thousand pores per mammalian nucleus, each one hundred twenty nanometers diameter channel composed of nucleoporins forming selective barrier separating nuclear and cytoplasmic compartments. Central channel filled with phenylalanine-glycine repeat containing disordered domains forms hydrogel allowing passive diffusion of ions and molecules under about forty kDa, but macromolecules bearing nuclear localization signals require binding to importin alpha-beta heterodimers and transit via transient interaction with FG repeats powered by Ran-GTP gradient high in nucleus generated by RCC1 exchange factor. Export uses CRM1 recognizing nuclear export signals. This regulated transport controls transcription factor entry, mRNA export, ribosomal subunit export and prevents ectopic translation of unspliced RNAs in cytoplasm. ATP synthesis via chemiosmotic coupling occurs in mitochondria, RNA splicing in nuclear speckles, chromosome replication at replication forks during S phase. Transport capacity reaches one thousand molecules per pore per second, loss of gradient leads to mislocalization and developmental defects.