Eukaryote chromosomes are composed or "packaged" by strands of negatively-charged DNA wrapping themselves around positively-charged globular proteins called histones. The histones form clusters called nucleosomes that clump together to form chromatin fibers in the nucleoplasm or cytoplasm inside the nuclear membrane. When the DNA in a chromatin strand is being used during early protein synthesis, it is loosely packed and called euchromatin. When it is not being used, it is coiled into a tight package called heterochromatin.
Prior to mitosis (the division of the eukaryote nucleus) and cytokinesis (division of the cytoplasm), the DNA in the nucleus is replicated and condensed into mitotic chromosomes that become visible under the light microscope. Each replicated strand is called a chromatid while still attached to its copy by a central point called the centromere. When the copies are separated during mitosis, each chromatid becomes an independent chromosome.