Parts of the Chromosome centromere heterochromatin (darkly staining region) euchromatin (lightly staining region) sister chromatids 36 Figure 2. Parts of the Chromosome Mustrated by: Rosa Mia L. Pontillo parm qarm Parts of the Chromosome 1. Chromatids- two identical halves of a replicated chromosome after, the Synthesis phase or the S phase of the cell cycle. 2. Centromere- the attachment points of the two chromatids of a chromosome. It is also described as the constriction pat which divides the chromosome into two sections, or "arms." 3. Short arm- or p arm - upper arms of the chromosome which is usually shorter. 4. Qarm - lower arms of the chromosome which is usually longer.,​