What is the portion of the DNA molecule that holds the pair of chromatids together called?

The portion of the DNA molecule that holds the pair of chromatids together is known as the centromere. This specialized region of the chromosome is crucial during cell division, as it acts as the attachment point for the spindle fibers that pull the chromatids apart into daughter cells. During the cell cycle, particularly in metaphase and anaphase of mitosis, the centromere ensures that sister chromatids remain connected until the appropriate time for separation.

The other terms reflect different aspects of genetic structure. For instance, a chromatid refers to one half of the duplicated chromosome and exists only after DNA replication has occurred. A chromosome is a complete structure of DNA and its associated proteins, consisting of two sister chromatids connected by a centromere. Chromatin is the complex of DNA and protein found in the nucleus, existing in a less condensed form during interphase, before it condenses into distinct chromosomes during cell division. Thus, the centromere specifically serves the role of connecting chromatids, which is why it is the correct choice.