Eukaryotic DNA are enormous in length, and therefore must be tightly packed in order to be manageable (Figure 1). DNA is wrapped around 8 protein called histones, forming bead-like units called nucleosomes. Nucleosomes form a string-like structure called chromatin. Depending on the phase of the cell cycle, chromatin condensation changes from a low density packaging state to be highly condensed into short and thick structure known as chromosomes.

One X shaped, blue chromosome with a small section at its tip encircled. Zoom in of that section shows yellow string wrapping three loops composed of a string of balls, each ball wrapped twice with the yellow string. A zoom in of the yellow string shows a view of the DNA double helix.

Figure 1: States of DNA packing in eukaryotic cells. DNA packaged in a highly condensed chromosome (top), a loosely arranged string of nucleosomes (middle) and as a linear DNA strand (bottom).