Talk about the protofilaments of microtubules.

Each is composed of a dimer of alpha and beta tubulin that are not covalently bonded, so it consists of a succession of alpha and beta monomers, so it has alpha on one end and beta on the other, All protofilaments that make a microtubule are oriented in the same direction, consequently, the entire microtubule is polarized having a plus extremity (beta) and a minus extremity (alpha), polarity is important for polymerization process which mainly occurs on the plus terminal, on the other hand, protofilaments in a bundle are helically aligned because there is a 1nm difference between two protofilament.