Chromatin is DNA packed together by proteins called histones, and it is found in the nucleus of a cell. Chromatin is packed even further to form a chromosome. Chromosomes carry genetic information in the form of genes.
DNA is one of the longest molecules. Unpacked DNA occupies a much larger volume than when it is packed, and the space available for the DNA in a cell is much smaller than the space that it would occupy when unpacked. DNA packing is essential to gene regulation in living organisms. With the help of ions and other molecules, DNA can condense in order to cope with volume constraints.
The first step in DNA condensation in a eukaryotic cell is the DNA wrapping around histones to create chromatin. Nucleosomes are formed when the DNA continues to curl and wraps around histone cores, which are groups of eight histones.
The DNA continues to compact until it forms a chromosome, which resembles the letter X. Either side of the X is called a chromatid, and both chromatids are held together in the center by a centromere.
DNA condensation differs slightly in prokarotic cells. Bacteria and viral DNA is packaged with the help of polyamines as well as other proteins.