What forms at the waist connecting the sister chromatids?

The centromere is the region of a chromosome where the two sister chromatids are joined together, forming a waist-like constriction. It plays a crucial role in cell division, particularly during the processes of mitosis and meiosis. The centromere ensures that sister chromatids can be properly segregated to daughter cells. This structure contains specific DNA sequences that are recognized by proteins necessary for spindle attachment during cell division, allowing for the equal distribution of genetic material.

In contrast, chromatids refer to the individual strands of a duplicated chromosome, and while they are connected at the centromere, they are not the structure that connects them. Telomeres are protective caps located at the ends of chromosomes that preserve the integrity of genetic data and prevent chromosomes from fusing with each other. Chromosomes themselves are the entire structures made up of DNA and proteins, which include both the centromeres and chromatids. Thus, the unique role of the centromere as the connecting point is critical in the proper segregation of genetic material during cell division, making it the correct choice for this question.