Microtubule dynamic instability involves the existence, within a population of microtubules, of sub‐populations of growing and shrinking microtubules which interconvert apparently at random. We consider the scope and limitation of experimental observations of individual microtubules by video enhanced dark‐field microscopy.This unique experimental phenomenon has been rationalized by the presence of a ‘‘cap’’ of tubulin‐GTP which can stabilize the growing state. We have modelled this process quantitatively by numerical simulation and illustrate the basic principles by computer graphics.The inherent &agr;‐&bgr; asymmetry of the microtubule lattice determines that the relationship between the addition reaction of tubulin‐GTP and the related hydrolysis of a polymer tubulin‐GTP is different at the two ends of the microtubule. In the single layer, Lateral Cap model for microtubule dynamic instability, a plausible mechanism has been proposed for the dynamic properties at the ‘‘active’’ (presumed &bgr;‐out) end in which the tubulin‐GTP which is hydrolyzed is related longitudinally to the binding site by the 13‐start protofilament helix. [1,2].We now show a similar but distinct mechanism could hold for the ‘‘inactive’’ (presumed &agr;‐out) end of the microtubule. Lateral hydrolysis rules (related to 5‐ or 8‐ start helical contacts) predict that the &agr;‐end could in fact be less dynamic and cooperative in terms of reduced amplitudes of growth and shrinking. This would make a distinctive contribution to the J(c) plot of microtubule growth versus [tubulin‐GTP]. These predictions are thus amenable to experimental verification.This approach illustrates how the helical lattice symmetry of the microtubule polymer can confer unique dynamic characteristics, which derive from the heterodimeric structure and guanine nucleotide binding properties of the component protein tubulin. It also provides a basis for the interpretation of the interactions of microtubules with anti‐mitotic drugs used in chemotherapy.