Descriptive of a mechanism of polymerization. The head of a biosynthetic monomer is defined as the chemically activated site (i.e. the carbonyl groups of amino acyl-tRNA or fatty acyl-CoA esters, the hemiacetal oxygen of a UDP-monosaccharide, the a-phosphate of a nucleoside triphosphate, the alkyl oxygen of isopentenyl pyrophosphate), and the tail as the other site on a monomer that will be joined to an adjacent moiety. Headward elongation occurs when a monomer condenses by displacement of the activating group (tRNA, CoA, pyrophosphate) of the growing polymer (protein, fatty acid or terpene biosynthesis). Conversely, tailward elongation occurs when the condensation removes the activating group of the newly added monomer (nucleic acid or polysaccharide biosynthesis).