A carbocation can electrophilically attack a double bond, the result being the elongation of the hydrocarbon in a polymerization reaction. A variation of this process occurs in the biosynthesis of the isoprene derived lipids, the terpenes and steroids. In an early stage in the process isopentyl pyrophosphate combines with the allylic carbocation derivative of its isomer (dimethylallyl pyrophosphate). Two C5 units have combined to form a C10 unit. In the pathway to squalene, a close precursor to cholesterol, the addition of another C5 unit by the same reaction process forms a C15 unit, which can then combine with another C15 unit (by a different reaction process) to form C30 squalene.












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