cross-linking glycan   One of a heterogeneous group of branched polysaccharides that help to cross-link cellulose microfibrils into a complex network. Has a long linear backbone of one sugar type (glucose, xylose, or mannose) with short side chains of other sugars.
cross-presentation   A process in which extracellular proteins taken up by specialized dendritic cells can give rise to peptides that can be presented by class I MHC proteins to cytotoxic  T cells.
crRNAs   Small noncoding RNAs (≈30 nucleotides) that are the effectors of CRISPR-mediated immunity in bacteria.
cryoelectron microscopy   Technique for examining a thin film of an aqueous suspension of biological material that has been frozen rapidly enough to create vitreous ice. The specimen is then kept frozen and transferred to the electron microscope. Image contrast is low, but is generated solely by the macromolecular structures present.
cryptochrome   Plant flavoprotein sensitive to blue light. Structurally related to blue-light-sensitive enzymes called photolyases (involved in the repair of ultraviolet-induced DNA damage) but do not have a role in DNA repair. Also found in animals, where they have an important role in circadian clocks.
Cubitus interruptus (Ci) Latent transcription regulator that mediates the effects of Hedgehog.
cyclic AMP (cAMP) Nucleotide that is generated from ATP by adenylyl cyclase in response to various extracellular signals. It acts as a small intracellular signaling molecule, mainly by activating cAMP-dependent protein kinase (PKA). It is hydrolyzed to AMP by a phosphodiesterase. (Figure 15–25) cyclic AMP phosphodiesterase Specific enzyme that rapidly and continuously destroys cyclic AMP, forming 5′-AMP. (Figure 15–25).
cyclic AMP-dependent protein kinase (protein kinase A, PKA) Enzyme that phosphorylates target proteins in response to a rise in intracellular cyclic AMP. (Figure 15–26) cyclic GMP (cGMP) Nucleotide that is generated from GTP by guanylyl cyclase in response to various extracellular signals.
cyclic GMP phosphodiesterase Specific enzyme that rapidly hydrolyzes and degrades cyclic GMP.
cyclin   Protein that periodically rises and falls in concentration in step with the eukaryotic cell cycle. Cyclins activate crucial protein kinases (called cyclin-dependent protein kinases, or Cdks) and thereby help control progression from one stage of the cell cycle to the next.
cyclin-dependent kinase (Cdk) Protein kinase that has to be complexed with a cyclin protein in order to act. Different cyclin–Cdk complexes trigger different steps in the cell-division cycle by phosphorylating specific target proteins. (Figure 17–10) cyclin–Cdk complex   Protein complex formed periodically during the eukaryotic cell cycle as the level of a particular cyclin increases. A cyclin-dependent kinase (Cdk) then becomes partially activated. (Figures 17–10 and 17–11, and  Table 17–1, p. 969) cytochrome   Colored heme-containing protein that transfers electrons during respiration and photosynthesis.