PubMed · 3175654
The source did not provide an abstract. Follow the original record for more information.
Explore connections, maps & timelines
W A Hendrickson. 1988-10-21. Crystallographic citations.. https://doi.org/10.1126/science.3175654
Cite the original work for its findings. Save a collection to share your selection of sources.
KEEP EXPLORING
Using histological and ultrastructural techniques the aims of this study were to investigate whether the mineralization pattern and surface microanatomy of the caries-susceptible fissure enamel were different from those on the caries-inactive lingual surface. The material consisted of 31 unerupted third mandibular molars. The specimens were initially grouped into four categories: (1) without, (2) with initial, (3) with almost completed and (4) with completed root formation. One ground section with fissure-like morphology was selected from each tooth. Using water as a medium the observed birefringence was negative along the lingual and fissure transverses in specimens with almost completed and with completed root formation, while the observed birefringence was positive at different distances in the enamel in sections representing less maturation stages. Qualitative imbibition studies revealed hypomineralized enamel in the lower part of the fissures in specimens representing almost and completed root formation. Imbibed in quinoline, parts of the hypomineralized enamel behaved like a molecular sieve due to the presence of micropores, indicating that the structural arrangement is different from that in the enamel adjacent to this areas. After division of the sections into a lingual and a buccal part, SEM features were described from lower and upper parts of the buccal fissure wall and on lingual enamel in the area corresponding to the bottom part of the fissure. The surface microanatomy varied greatly. Negative developmental irregularities such as fissures and holes were associated with the immature enamel, while matured enamel - particularly fissures - housed many positive developmental irregularities such as enamel caps and protrusions. The crystal size in the mature specimens appeared smaller and more uniform than the crystals from the immature specimens. Apart from the occurrence of hypomineralized enamel in fissures and numerous positive developmental irregularities on the fissure surface, no major differences between fissure and lingual enamel were noticed, neither with respect to mineralization pattern during final stages of tooth development nor to the degree of surface porosity prior to tooth emergence.
BACKGROUND: . DNA-RNA hybrids are substrates for RNase H. This enzyme catalyzes the hydrolysis of the RNA strand in the hybrid form. The polypurine tract (PPT) in human immunodeficiency virus 1 (HIV-1) is a short stretch of purines ( approximately 15 bases) located at the 3'-end of the U3 region of the RNA genome. The PPT has the unique ability to resist digestion by RNase H and serves as a primer for plus-strand DNA synthesis. RESULTS: . The crystal structure of a DNA-RNA hybrid duplex containing a polypurine RNA strand, d(TTCTTBr5CTTC)-r(GAAGAAGAA), has been determined at 1.8 A resolution. The structure was solved by molecular replacement methods and refined to a final R factor of 20.1% (R free 23.7%). The hybrid duplex adopts a standard A-form conformation. All of the sugar rings and glycosidic torsion angles are found in the standard C3'-endo/anti conformation, as seen in A-RNA or A-DNA. The crystal packing is dominated by the DNA strand, where the terminal base pairs of the hybrid abut the neighboring A-DNA sugar-phosphate backbone on the minor groove side. CONCLUSIONS: . The present DNA-RNA hybrid duplex containing a polypurine RNA strand exhibits standard A-form geometry. This observation might suggest that the RNA PPT resists the RNase H activity of HIV reverse transcriptase as a result of its A-form conformation. In addition, there appears to be a correlation between the percentage purine content of the RNA and the DNA backbone conformation.
Explore the source record for details and available documents.