Final answer:
The nucleosome core is composed of an octamer of histone proteins including two each of H2A, H2B, H3, and H4, not H4a as mentioned in the question.
Step-by-step explanation:
The nucleosome is the fundamental unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound in sequence around eight histone proteins. These proteins include two copies each of histones H2A, H2B, H3, and H4, forming an octamer. Therefore, the statement in the question seems to contain a typographical error, as histone H4 is the correct term, not H4a. The nucleosome, which appears as "beads on a string" when viewed under an electron microscope, plays a vital role in folding DNA into higher order structures. This compact form enables DNA to fit within the nucleus and also plays a role in gene regulation.
Furthermore, identical copies of chromatin that are held together by the protein cohesin at the centromere are called sister chromatids, not histones or nucleosomes as implied in a conceptual mix-up in the question.