Packing DNA into chromosomes

Abstract

Electron microscopy of HeLa metaphase nucleoids (i.e. whole metaphase cells exposed to 2 M salt and non-ionic detergent) spread by the Kleinschmidt technique, reveals a variety of protein-depleted structures (spreads) derived from chromosomes. Spreads vary in size and shape. At one extreme are oval structures with one or more cores surrounded by a network of supercoiled fibres. These fibres are probably arranged as loops and we estimate that 600–1000 may emerge from a single, large core region. At the other extreme are chromosome-shaped spreads with an elongated core which takes the form of a multifibred axis. At intervals groups of lateral fibres appear to emerge from each axis to produce the network. Spreads intermediate between these extremes occur in which axial fibres can be resolved in only part of the elongated core. Similar structures are observed in chromosomes deproteinized and spread after isolation by a procedure which preserves high molecular weight DNA. The appearance of chromosomes isolated by the Wray-Stubblefield hexylene glycol procedure agrees in general with previous findings of others, except that in some more extended spreads axial fibres are visible. We believe our observations are consistent with the idea that the chromonema of each metaphase chromatid contains regions of multistranded DNA. We do not propose, however, that the chromatid is functionally multineme, but rather that axial fibre folding either within or between chromomere regions contributes to packing of DNA in the metaphase chromosome. These regions of constraint re also postulated as the locations of emergence of lateral loops. In spread preparations the axial fibres are seen clearly only when chromatids have been elongated beyond the contracted metaphase length. Elongation would be produced both by relaxation of chromosomal coils (gyres) and by extension of the chromonema upon deproteinization and spreading. Whereas in deproteinized nucleoids the long axis is liable to elongate, fragment or collapse, the chromomena of Wray-Stubblefield chromosomes is locked at the metaphase (gyred) length and axial fibres are generally not visible. We propose that the assembly of the complex DNA axis of the metaphase chromosome from its extended interphase counterpart plays a major part in increasing the DNA packing ratio in the mitotic cell.