The progress made in molecular biology has provided definite evidence that DNA is the genetic material of all cellular organisms. It is mainly localized within the cell nuclei and, in association with histones and acidic proteins, forms the fundamental structure of the chromosomes. The DNA-protein complex, called nucleohistone or chromatin, represents the sum of all existing genes of one cell type and can be considered as a functional genetic unit. All genetic structures contain this fundamental element and only their steric arrangement or multiplicity determine the species-specific differences in chromosomal morphology. The preservation of the three-dimensional structure seems to be the function of histones which, in general, are associated with densely packed and metabolically inactive DNA sequences. The lysine-rich histone I probably has a specific regulatory function since it inhibits the transcription of certain genes. Acidic proteins are mainly to be found in stretched chromosomal structures of high metabolic activity which accentuate the correlation between structure and function of nucleohistones. Approximately 50% of the DNA of the nucleohistone complex is not masked by proteins though only a small percentage of the free nucleotide sequences can be transcribed by RNA polymerase. The "open" structure of nucleohistones finds a plausible explanation in the fact that amino acid sequence analysis of several histones revealed an asymmetric distribution of charged groups and that therefore only these protein fractions can interact with DNA. In Crick's recently published model of the chromosomal organization the unique structure of histones is taken into account and it is assumed that the histone-rich heterochromatin contains multiple but untranscribable control elements. Functional genes are represented by a single stranded DNA duplex within the interbands which utilize a special protein to preserve this unique conformation.
