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F Livolant

Publications and source records attributed to F Livolant.

26 records · Page 2Linked to original sources
PubMed1991

Cholesteric liquid crystalline DNA; a comparative analysis of cryofixation methods.

The ultrastructure of liquid crystalline phases of DNA raises numerous problems because of the structure itself which is fluid and which nature depends on the relative amount of DNA, water and ions. Different cryofixation methods were tested and compared after freeze-fracture of the specimen. A good ultrastructural preservation of the samples can be achieved without addition of any cryoprotectant by quick-freezing against a copper block cooled down to liquid helium temperature. Then, molecular orientations can be followed very accurately and the local disorder around a mean direction which exists in the liquid state is kept in the frozen structure.

Copper↗
PubMed1989

The highly concentrated liquid-crystalline phase of DNA is columnar hexagonal.

The DNA molecule is extremely compacted in bacteria, in cell nuclei, sperm heads and virus capsids. These interactions between DNA molecules are important to our understanding of chromatin condensation. DNA forms multiple liquid-crystalline phases whose nature depends on the polymer concentration, and it has been suggested that the highly concentrated phase of 50-nm DNA molecules is two-dimensionally ordered and smectic-like. We rule out this smectic hypothesis and demonstrate by polarizing microscopy, electron microscopy and X-ray diffraction that this phase is characterized by a columnar longitudinal order and a hexagonal lateral order, with intermolecular distances ranging from 2.8 to 4.0 nm depending on the DNA concentration.

Crystallization↗
PubMed1989

Electron microscopy of liquid crystalline DNA: direct evidence for cholesteric-like organization of DNA in dinoflagellate chromosomes.

Freeze-fracture-etch replicas of concentrated DNA solutions which appeared, by polarized light microscopy, to be in a cholesteric-like liquid crystalline state were examined by high resolution transmission electron microscopy (TEM). Individual DNA molecules were resolvable, and the microscopic morphologies observed for such replicas confirmed the cholesteric organization of DNA molecules in this liquid crystalline state. Furthermore, replica morphologies were strikingly similar to TEM images of dinoflagellate chromosomes in both thin section and freeze-etch replicas, providing strong support for the cholesteric DNA packing model proposed for the organization of DNA in these chromosomes by Bouligand and Livolant.

Animals↗
PubMed1988

Circular dichroism microscopy of compact forms of DNA and chromatin in vivo and in vitro: cholesteric liquid-crystalline phases of DNA and single dinoflagellate nuclei.

Two highly condensed structures of DNA have been analyzed in the circular dichroism (CD) microscope: the cholesteric liquid-crystalline phase of DNA and the nucleus of a dinoflagellate (Prorocentrum micans). In both cases, the DNA shows a helical cholesteric organization, but the helical pitch equals about 2500 nm in the first case and 250 nm in the second one. Since the absorption band of DNA is located at 260 nm, the reflection and absorption bands are well separated in the cholesteric phase of DNA and are overlapping in the dinoflagellate nucleus. However, both structures give a very strong negative CD signal at 265 nm. We show that this very strong signal cannot correspond to a Borrmann effect, i.e., to a superposition of the absorption and reflection bands, but is a differential absorption of left versus right circularly polarized light. This anomalous differential absorption is probably due to a significant scattering of light, inside of the structure, which produces a resonance phenomenon in the absorption band of the chromophore. Therefore, for any helical structure containing a chromophore, the apparent CD can be expressed as CD = [(epsilon L - epsilon R)cl] + (psi L - psi R) + (SL - SR) The first term is true absorption and is located in the absorption band of the chromophore, and the last term is true scattering and is observed at the wavelength corresponding to the helical pitch of the structure. The second term (psi L - psi R) corresponds to the anomalous differential absorption observed in dense superhelical structures of DNA. It superimposes to the first term in the absorption band of the chromophore. psi L - psi R is a measure of the perfection of the helical structure and of the density of chromophores in the material. Intercalative dyes [ethidium bromide and meso-tetrakis(4-N-methylpyridyl)porphine (H2TMpyP-4) and its nickel(II) derivative (NiIITMpyP-4)] were inserted in the dinoflagellate chromatin. The CD signal recorded in their absorption band mimics the signal observed in the absorption band of DNA. In both structures, the negative sign of the CD at 265 nm indicates that the twist occurring between DNA. In both structures, the negative sign of the CD at 265 nm indicates that the twist occurring between DNA molecules is left-handed, and we show that this situation is the most frequently encountered in vivo and vitro.

Animals↗
PubMed1984

Cholesteric organization of DNA in the stallion sperm head.

The fine structure of chromatin in sperm heads was investigated by different microscopic techniques: in vivo examinations in the polarizing microscope, thin sections and freeze-fracture replicas observed by transmission electron microscopy. The freeze-fractured chromatin appears to be formed of superimposed lamellae, each one 330 A thick. These lamellae are parallel to the flattening plane of the sperm head. This situation was already described in other mammal spermatozoa and in particular in the bull and the rabbit. This work presents a new interpretation of this lamellated aspect. The chromatin structure of these spermatozoa is that of a cholesteric liquid crystal. This structure resembles that of a plywood, made of superimposed layers of parallel filaments, but instead of having a right angle between two successive layers, there is a progressive rotation and similar orientation occurs at each 180 degrees rotation. The apparent lamellae result from cleavages due to freeze-fracture between levels of parallel filament orientation. The thickness of lamellae corresponds therefore to the half helicoidal pitch of the cholesteric liquid crystal. This model is consistent with our observations by polarizing microscopy. The lamellation is not visible in thin sections of stallion spermatozoa. There are however biochemical methods to decondense chromatin and we are able to observe this lamellation in sections normal to the flattening plane of sperm heads. The methods used classically to decondense the sperm chromatin lead to extremely varied aspects which are discussed, some of them being closely related to the structure of cholesteric liquid crystals.

Animals↗
PubMed1984

Cholesteric organization of DNA in vivo and in vitro.

In concentrated solutions DNA organizes spontaneously to form the "cholesteric" phase which is one type of liquid crystal. We have reproducibly obtained both continuous cholesteric phases and isolated cholesteric globules in equilibrium with the isotropic phase. A comparison is made between this in vitro cholesteric organization and dinoflagellate chromosomes which present the same organization in vivo. The observed defects are analyzed in the two cases. It appears that the cholesteric organization is due to self-assembly phenomena and that the shape of globules and chromosomes is due both to surface tensions and to the presence of defects.

Chromosomes↗
PubMed1978

Positive and negative birefringence in chromosomes.

By using the optical properties of birefringence of DNA, the arrangement of these molecules has been studied in Dinoflagellate chromosomes and Dipteran polytene chromosomes. These latter are used, here, as a reference material. These observations have been made under a polarizing microscope on intact and stretched chromosomes.--Intact Dinoflagellate chromosomes show a positive birefringence, in contrast with polytene chromosomes bands which are negatively birefringent. From these observations one can deducd the preferential orientation of DNA filaments, in Dinoflagellates, normal to the chromosome axis, and in polytene chromosomes parallel to the same axis.--After stretching, these two kinds of chromosomes are negatively birefringent. In both cases, DNA molecules have been aligned along the stretch axis.--In Dinoflagellate chromosomes the passage from a positive to a negative birefringence is realized without any isotropic stage. The intermediary state presents a biaxial structure.

Animals↗