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grana (singular granum):
Stacked membrane discs (thylakoids) in chloroplasts that contain
chlorophyll and are the site of the light-trapping reactions
of photosynthesis.
granulocyte:
Category of white blood cell distinguished by conspicuous cytoplasmic
granules. Includes neutrophils, basophils, and eosinophils.
gray crescent:
Band of pale pigmentation that appears in the egg of some species
of amphibian opposite the site of sperm entry following fertilization.
Caused by rotation of the egg cortex and associated pigment
granules. Marks the future dorsal site.
group (functional group):
Set of covalently linked atoms, such as a hydroxyl group ( - OH)
or an amino group ( - NH2), the chemical
behavior of which is well characterized.
growth cone:
Migrating motile tip of a growing nerve cell axon or dendrite.
growth factor:
Extracellular polypeptide signaling molecule that stimulates
a cell to grow or proliferate. Examples are epidermal growth
factor (EGF) and platelet-derived growth factor (PDGF). Most
growth factors have other actions besides the induction of cell
growth or proliferation.
GTP (guanosine 5'-triphosphate):
Major nucleoside triphosphate used in the synthesis of RNA and
in some energy-transfer reactions. Has a special role in microtubule
assembly, protein synthesis, and cell signaling.
GTPase-activating protein (GAP):
Protein that binds to a Ras or Ras-related GTP-binding protein
and inactivates it by stimulating its GTPase activity so that
it hydrolyzes its bound GTP to GDP.
guanine nucleotide releasing protein
(GNRP):
Protein that binds to a Ras or Ras-related GTP-binding protein
and activates it by stimulating it to release its bound GDP
and hind GTP in its place.
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