other organs such as bowel and bladder. Mental retar-
dation can also occur. It affects approximately one out
ofevery 1,000 newborns in the United States and is the
most frequent disabling birth defect.
spindle A group of microtubules originating from
the centriole that move chromosomes during cell divi-
sion in eukaryotic cells. Spindles attach at the kineto-
chore—a region on the centromere, the area that joins
chromatids.
spin label A STABLE paramagnetic group that is
attached to a part of another molecular entity whose
microscopic environment is of interest and can be
revealed by the ELECTRON PARAMAGNETIC RESONANCE
spectrum of the spin label. When a simple paramagnet-
ic molecular entity is used in this way without covalent
attachment to the molecular entity of interest, it is fre-
quently referred to as a spin probe.
spin-orbit coupling The interaction of the electron-
spin magnetic moment with the magnetic moment due
to the orbital motion of the electron.
spin probe SeeSPIN LABEL.
spin-spin coupling The interaction between the spin
magnetic moments of different electrons and/or nuclei.
In NMR spectroscopy it gives rise to multiple patterns
and crosspeaks in two-dimensional NMR spectra.
Between electron and nuclear spins, this is termed the
nuclear HYPERFINEinteraction. Between electron spins,
it gives rise to relaxation effects and splitting of the elec-
tron paramagnetic resonance (EPR) spectrum.
See also NUCLEAR MAGNETIC RESONANCE SPEC-
TROSCOPY.
spin trapping In certain solution reactions, a tran-
sient RADICAL will interact with a DIAMAGNETIC
reagent to form amore“persistent” radical. The prod-
uct radical accumulates to a concentration where detec-
tion and, frequently, identification are possible by
ELECTRON PARAMAGNETIC RESONANCE SPECTROSCOPY.
The key reaction is usually one of attachment; the dia-
magnetic reagent is said to be a “spin trap,” and the
persistent product radical is then the “spin adduct.”spiral cleavage An embryonic development charac-
teristic of protostomes. The developing embryo under-
goes cell division (cleavage) from a four-cell embryo to
an eight-cell embryo, with the cells dividing at slight
angles to one another. None of the four cells in one
plane of the eight-cell stage is directly over a cell in the
other plane (oblique to polar axis).
See alsoRADIAL CLEAVAGE.spirochetes Long, slender, coiled (looks like a tele-
phone cord) bacteria that cause disease and are sym-
bionts in the stomachs of ruminants. Spirochetes can be
aerobic or anaerobic, free-living or parasitic. All spiro-
chetes are chemoheterotrophs. Three genera exist, Lep-
tospira (cause icterohemorrhagic fever), Treponema
(cause syphilis), and Borrelia(cause Lyme disease).spleen The spleen is located in the upper left quad-
rant of the abdomen. It has two main functions, acting
as part of the immune system and as a filter. There are
two distinct components of the spleen, the red pulp and
the white pulp. It plays an important role in immune
system activities as part of the lymphatic system.spliceosome A complex of several snRNA molecules
(small nuclear RNA) and proteins that remove introns
(noncoding mRNA) and splice exons (remaining
mRNA sequences).splitter A taxonomist who prefers to create taxo-
nomic categories that are narrowly defined, thus end-
ing up with more genera than a lumper, who prefers to
place closely related genera into a single genus.
See alsoLUMPER.sporangium Atiny closed globe or capsule located
on a sporangiophore in which sporangiospores and312 spindle