Medical Genetics 1. chromatin Chromatin is that portion of the cell nucleus which contains all of the DNA of the nucleus in animal cells.(A small amount of special dNa is also found in the mitochondria of the cell cytoplasm outside of the cell nucleusMedical Genetics 1. chromatin Chromatin is that portion of the cell nucleus which contains all of the DNA of the nucleus in animal cells. (A small amount of special DNA is also found in the mitochondria of the cell cytoplasm outside of the cell nucleus.)