What is chromatin?

The length of a strand of human DNA is much longer than you’d think! In fact if it was laid out in a straight line it would be several metres in length. The size of a nucleus is approximately 6 micrometers in diameter so you can imagine that DNA needs to be packaged very well so that it can fit. The proteins that DNA is packaged with are called Histones. Histones are small spherical structures which the DNA can wind around to make it more compact in the nucleus. This combination of DNA and histones is referred to as chromatin. 
 Nucleosomes 
 The basic structural unit of chromatin is referred to as the nucleosome. A nucleosome consists of a core of eight histone proteins and the DNA that wraps around them. Many nucleosomes join together connected by the continuous DNA strand giving the typical “beads on a string” appearance we associate with chromatin. These nucleosomes are dynamic structures which can be altered to affect the degree to which the DNA code can be read and proteins produced. This is a key concept in epigenetics. 
 The cell cycle 
 The appearance of DNA changes based on where the cell is in the cell cycle. In interphase, DNA exists as chromatin. However during mitosis, non-histone proteins are recruited which allow the chromatin to condense further. This results in the formation of chromosomes.