Are sister chromatids are held together by a structure called synapse?

Sister chromatids are held together by a structure called centromere, not synapse. The centromere is a specialized region of the chromosome that serves as the attachment point for spindle fibers during cell division.

Synapse, on the other hand, is a specialized junction or point of contact between two neurons (nerve cells) that allows for the transmission of electrical and chemical signals from one neuron to another.