Final answer:
The constricted area where the two strands of a chromosome are joined is known as the centromere, which is crucial for the proper alignment and separation of sister chromatids during cell division.
Step-by-step explanation:
The two strands of a chromosome are joined at a constricted area called the centromere. This area becomes particularly important during cell division as it is the location where sister chromatids, which are two identical copies of the same chromosome linked together after DNA has replicated, are connected. The proteins known as cohesin proteins help maintain the integrity of this structure, allowing the sister chromatids to be aligned and joined at the centromere. Each chromatid attaches to spindle microtubules via a protein complex at the centromere known as the kinetochore during mitosis.