During mitosis, the region that holds two sister chromatids together and helps ensure proper separation is the

• A. Centromere
• B. Chromatid
• C. Chromosome
• D. Chromatin

Answer: (A)

The region is the centromere. This constricted part of a chromosome holds the two sister chromatids together and serves as the attachment site for the kinetochore, a protein structure where spindle fibers latch on to pull the chromatids apart during mitosis. The cohesion proteins that keep the sisters linked act at the centromere and are cleaved in anaphase to allow separation, ensuring correct distribution to the daughter cells.

In contrast, a chromatid is one copy of the duplicated chromosome, not the joint region; a chromosome refers to the whole duplicated structure (two chromatids held together at the centromere); and chromatin is the less-condensed form of DNA and proteins before chromosomes condense.