Ultrastructure of interphase chromosomes

Resinless section electron microscopy of HeLa cell mitotic architecture

The use of resinless sections extends embedment-free electron microscopy to the cytoskeleton of thick specimens. Here we examine HeLa cells rounded at mitosis. Extraction of mitotic HeLa cells with Triton X-100 removes lipids and soluble proteins, leaving the cytoskeletal framework and spindle apparatus. After fixation, the samples are embedded and sectioned, and the temporary embedding resin is removed for direct visualization in the electron microscope. The micrographs show that the cytoskeletal framework, chromosomes, spindle, and centrioles form an interconnected entity. The pericentriolar region, indistinct in conventional micrographs, appears composed of distinct fibers interconnecting the spindle microtubules and centriole. The resinless sections also reveal characteristic lacunae at late anaphase/early telophase. These probably result from reformation of the interphase cytoskeleton lagging reassembly of the nucleus.

The human chromosome. Electron microscopic observations on chromatin fiber organization

Human lymphocytes were grown in short-term tissue culture and were arrested in metaphase with Colcemid. Their chromosomes were prepared by the Langmuir trough-critical point drying technique and were examined under the electron microscope. In addition, some chromosomes were digested with trypsin, Pronase, or DNase. The chromosomes consist entirely of tightly packed, 240 +/- 50-A chromatin fibers. Trypsin and Pronase treatments induce relaxation of fiber packing and reveal certain underlying fiber arrangements. Furthermore, trypsin treatment demonstrates that the chromatin fiber has a 25-50 A trypsin-resistant core surrounded by a trypsin-sensitive sheath. DNase digestion suggests that this core contains DNA.

Chromosome structure--a model based on DNA replication

A model of chromosome structure is proposed which assumes: (i) that DNA replication is accomplished via right-hand (RH) rotation; and (ii), that where replicating DNA segments arc very long RH-rotation will not proceed with absolute freedom. It is expected that inhibition of rotation in daughter molecules will lead to the formation of left-hand-individual (LH-I) coiling systems in the two daughter molecules. It is also expected that inhibition of rotation in the parental molecule will cause the LH-I coiled daughters to be held together in a right-hand-relational (RH-R) association. The interaction between LH-I and RH-R coiling is expected to cause separation of the daughter molecules. Multistranded DNA-containing structures are expected to show, in addition to the LH-I coiling heirarchies formed within individual strands, an RH-R coiling heirarchy formed by the complex as a whole. An LH-I coiling heirarchy was looked for, and found, in Cleveland's drawings of flagellate chromosomes. Evidence for the existence of RH-R coiling was also found. Results of electron-microscope studies on chromosome structure were briefly examined, as were the structures of lampbrush chromosomes, salivary chromosomes and "normal" chromosomes. These studies provided additional, though less direct, evidence in favor of the replication hypothesis and the predictions developed from it.