The Geometry The Nucleus

Figure 8
MEASURING THE ELECTROMAGNETIC SIGNALS OF THE C ELL CYCLE WITH CD/CIDS
The different stages of the mitotic process give clearly different electrorm gnetic signals when studied with the CD/
CIDS technology. These cell samples were synchronized to ensure that tl e signals of cells in the G, phase, the 5-G 2
phase and the M phase could be studied separately. The three distinct sp< ctra of the cell cycle stages can be seen in
(a). The spectra take into account readings with two different types oflen ; magnification. The raw data forthe three
cell stages as a function of wavelength with a32x magnification are show n in (b).
the polarized light was shifted to the left or right could be
precisely measured in a polarimeter, precise knowledge of
what Pasteur called the "molecular dissymmetry," its geometric
spatial organization, could be precisely adduced.
The modern spectroscopies of CD and CIDS are based
upon sophisticated adaptations of Pasteur's approach. When
a sample is alternately illuminated with successive beams
of first right- and then left-circularly polarized light, the
geometry of the sample will determine how it differentially
absorbs or scatters the light (Figures 6and 7).
When the scientist is looking at the differential rates of
absorption of the left- and right-polarized light the technique
is called circular dichroism. CD spectra have been
used to map the folding patterns of structures ranging from
small "handed" molecules, of the order of nucleic acid
monomers, to very large biological structures like intact
bacterial cells, cell nuclei, and eukaryotic cells in solution.
As the scientific work proceeded, the CD spectra seemed
to have certain unexplained "tails" or signals in a range
where they should not have occurred—namely, in scattering
regions. These scattering contributions appeared in
cases where very large organizations of substance—for example,
eukaryotic cells and cell nuclei—were being studied.
It was then realized that these "differential scattering"
spectra were extremely important. They reflected the
higher-order " landedness" or spiral orientation and folding
of the biolc gical materials being studied.
In short, bic logical materials exhibit a preferential light
scattering for cither left- or right-circularly polarized light
depending not only upon how the material is organized (as
a double helix for example) but also upon the way that it
folds upon itse If in the substance being studied. The differential
scatterir g mode (CIDS) is particularly sensitive to
mapping the hi *her-order "handedness" of the way in which
biological heli :es fold upon themselves, since the technique
is based jpon the preferential light scattering of rightor
left-circular y polarized light.
Using speci; I techniques the scientists obtained several
different samples where the cells were predominantly in
one of the foui stages of the cell cycle. They then measured
398 cells in the CD/CIDS microscope by scanning over the
wavelength wi h a 32-power and a 100-power objective. The
reason for the two different power objectives is that each
captures different amounts of scattered light. In 39 cases,
the same cell was measured with both the 32- and 100-
power objective. In short, the CD/CIDS was measured by
the microscop ! as a function of cell cycle position, on single
Chinese hams! er cells randomly selected from the four different
stages c f the cell cycle: 18 from G„ 5 from S, 2 from
C 2I and 14 f ron M. Dramatic results were obtained showing
21stClNTURY May-June 1988 41