De novo synthesis of n-3 and n-6 PUFAs. n-3 PUFAs are derived from ALA. ALA in the liver is desaturated to stearidonic acid and then elongated to eicosatetraenoic acid, which is then desaturated to EPA. DHA is synthesized from EPA in a two-step reaction sequentially catalyzed by elongase and desaturase. The biosynthesis of n-6 PUFAs starts with the conversion of linoleic acid to DGLA via desaturation and elongation reactions. DGLA is then further desaturated to arachidonic acid. AA: Arachidonic Acid, ALA: α-linolenic acid, DHA: docosahexaenoic acid, DGLA: dihomo-gamma-linoleic acid, EPA: eicosapentaenoic acid, GLA: gamma-linolenic acid, LA: linoleic acid, n-3: omega-3 polyunsaturated fatty acids, n-6: omega-6 polyunsaturated fatty acids, PUFAs: polyunsaturated fatty acids.