Chromatin and chromosomes are both structures of DNA, but chromosomes are condensed chromatin. DNA exists as chromatin a majority of the time so that the DNA is accessible to proteins for transcription and proteins can be made during the process of translation. Chromatin is condensed into chromosomes during mitosis to ensure that replicated genetic information is divided equally between the two resulting daughter cells.
Chromatin is DNA coiled into units called nucleosomes. DNA is made accessible to transcription proteins through enzymatic modification and chromatin remodeling. Chromosomes are even more tightly coiled than chromatin and are condensed into supercoils. In order for DNA to be condensed into chromatin and chromatin to be condensed into chromosomes, proteins called histones coil the DNA. The histones provide the energy necessary for compaction and serve as structures for DNA to coil around. The energy provided by positively charged histones results from electrostatic interactions with negatively charged DNA.
DNA must be condensed as chromatin and even more so as chromosomes because all of the DNA of a singular diploid human cell that has not been coiled is estimated to be about 2 meters long. In comparison, the typical eukaryotic cell is 10 to 100 nanometers in diameter.