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substrate:
Molecule on which an enzyme acts.
substratum:
Solid surface to which a cell adheres.
subunit:
Component of a multicomponent complex - for example, one protein
component of a protein complex.
sulfhydryl (thiol, - SH):
Chemical group containing sulfur and hydrogen found in the amino
acid cysteine and other molecules. Two sulfhydryls can join to
produce a disulfide bond.
supercoiled DNA :
Region of DNA in which the double helix is further twisted on
itself.
symbiosis:
Intimate association between two organisms of different species
from which both derive a long-term selective advantage.
symport:
Form of co-transport in which a membrane carrier protein transports
two solute species across the membrane in the same direction.
(See also co-transport.)
synapse:
Communicating cell-cell junction that allows signals to pass from
a nerve cell to another cell. In a chemical synapse the signal
is carried by a diffusible neurotransmitter; in an electrical
synapse a direct connection is made between the cytoplasms of
the two cells via gap junctions.
synaptonemal complex:
Structure that holds paired chromosomes together during prophase
I of meiosis and that promotes genetic recombination.
syncytium:
Mass of cytoplasm containing many nuclei enclosed by a single
plasma membrane. Typically the result either of cell fusion or
of a series of incomplete division cycles in which the nuclei
divide but the cell does not.
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