Plus and minus ends

In solution a,b-tubulin dimers can only form a polymer when above a threshold concentration, known as the critical concentration (Cc). The critical concentration for addition of a-,b-tubulin dimers at the plus end is lower than addition at the minus end so microtubules tend to grow at the plus end. Experimental evidence that tubulin is preferentially added onto the plus end is seen by adding tubulin to a bundle of microtubules from a flagellar nucleus. This shows that tubulin is mostly lost and gained from the same end (the plus end).