A structural analysis of the role of the nuclear matrix and DNA loops in the organization of the nucleus and chromosome.

The interphase nucleus is characterized by a nuclear matrix structure that forms a residual scaffolding composed of approximately 10% of the total nuclear proteins. The nuclear matrix contains residual elements of the pore-complex and lamina, the nucleolus, and an intranuclear fibrous network that provides the basic shape and structure of the nucleus. In the interphase nucleus this nuclear matrix has been reported to be a central element in the organization of DNA loop domains and to contain fixed sites for DNA replication and transcription. In this study, we have analysed the role of the nuclear matrix and the DNA loop domains in the organization and structure of the number 4 human chromosome. A model is proposed that closely approximates the observed structural dimensions of this chromosome. The model is composed of 30 nm diameter filaments formed from a solenoid of six nucleosomes per turn. This 30 nm solenoid filament is organized as loops of DNA each containing approximately 60 000 base-pairs; each loop is anchored at its base to the nuclear matrix. A radial loop model containing 18 of these loops per turn forms a new unit of chromosome structure termed the miniband. Approximately 106 of these minibands are arranged along a central axis to form the final chromatid. The role of the nuclear matrix in this organization is presented. The accuracy of the proposed model is tested by comparing its features with the known properties of the number 4 human chromosome.