The paternal and the maternal pronuclei move towards each other with the help of microtubules, which begin to be formed immediately after impregnation, i.e., by the penetration of the spermatozoon. They grow in a star-like pattern out of the paternal centrosome directly beside the forming paternal pronucleus (= formation of an aster made of dozens of microtubules). The microtubular proteins themselves arise from the cytoplasma of the oocyte.
While the microtubules of the aster pull the pronuclei together in the center of the oocyte, the synthesis of the DNA is taking place in the pronuclei. This duplication takes roughly 12 hours. The pronuclei grow in size in this time.