New applications of laser microbeam irradiation to cell and developmental biology include a new instrument with a tunable wavelength (217- to 800-nanometer) laser microbeam and a wide range of energies and exposure durations (down to 25 × 10 -12 second). Laser microbeams can be used for microirradiation of selected nucleolar genetic regions and for laser microdissection of mitotic and cytoplasmic organelles. They are also used to disrupt the developing neurosensory appendages of the cricket and the imaginal discs of Drosophila .

The role of the kinetochore in chromosome movement was studied by 532-nm wavelength laser microirradiation of mitotic PtK2 cells. When the kinetochore of a single chromatid is irradiated at mitotic prometaphase or metaphase, the whole chromosome moves towards the pole to which the unirradiated kinetochore is oriented, while the remaining chromosomes congregate on the metaphase plate. The chromatids of the irradiated chromosome remain attached to one another until anaphase, at which time they separate by a distance of 1 or 2 p,m and remain parallel to each other, not undergoing any poleward separation . Electron microscopy shows that irradiated chromatids exhibit either no recognizable kinetochore structure or a typical inactive kinetochore in which the tri-layer structure is present but has no microtubules associated with it . Graphical analysis of the movement of the irradiated chromosome shows that the chromosome moves to the pole rapidly with a velocity of -3 ,um/ min . If the chromosome is close to one pole at irradiation, and the kinetochore oriented towards that pole is irradiated, the chromosome moves across the spindle to the opposite pole . The chromosome is slowed down as it traverses the equatorial region, but the velocity in both half-spindles is approximately the same as the anaphase velocity of a single chromatid . Thus a single kinetochore moves twice the normal mass of chromatin (two chromatids) at the same velocity with which it moves a single chromatid, showing that the velocity with which a kinetochore moves is independent, within limits, of the mass associated with it .It is now generally accepted that mitotic spindle formation and chromosome movement involve several structures, including microtubules, centrioles, pericentriolar regions, and the centromere regions of the chromosomes . There have been several investigations into the ultrastructure and chemical composition of the kinetochore, or attachment site of microtubules to the centromeric region (14,(30)(31)(32)35) . Electron microscope studies have revealed a relatively uniform morphology of the kinetochore region in many different eukaryotes (including PtK 2 cells [34]) consisting of a trilaminar structure 0 .3-0 .6 ttm in diameter.Although the kinetochore has been demonstrated unequivocally to be a microtubule-organizing center in vitro (15,22,36,38), very little biochemical information is available on it . However, there are indications that ribonucleoprotein is a component of the kinetochore (9, 32) and it appears to be associated with the inner plate of the trilaminar kinetochore in PtK, cells .One of the earliest studies on the function of the centromere pertains to the mitotic behavior of x-ray-induced chromosome fragments lacking centromeres (11,12). Although these fragments did not join the metaphase plate, there was some sepa-THE JOURNAL OF CELL BIOLOGY " VOLUME 88 MARCH 1981 543-553 ©The Rockefeller University Press " 0021-9525/81/03/0543/11 $1 .00 ration of the chromatids at anaphase. However, they lagged behind the...

We report on micro-Raman spectroscopy studies of porous silicon which show an 'amorphous silicon Raman line at 480 R cm-' from regions that emit visible photoluminescence. A Raman line corresponding to microcrystalline silicon at 510 R cm-' is also observed. X-ray photoelectron spectroscopy data is presented which shows a high silicon-dioxide content in porous silicon consistent with an amorphous silicon phase.

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