It is known that except for DNA a number of different proteins are present
in the chromosomes of eukaryotic cells. The complex of nucleic acids and
proteins is called chromatin. According to data of Hadjiolov et al. (1976)
chromatin isolated from interphase cells after the separation of cytoplasmic and
soluble nuclear components usually contains about 30—40% DNA, 3—5%
RNA and about 50—60% proteins, and according to Lehninger (1976) human
metaphase chromosomes contain roughly 15% DNA, 10% RNA and 75%
protein. These data about the high percentage of protein content in the
chromosomes should not be ignored. DNA-molecules in prokaryotic cells,
viruses including, are also bound or packed with proteins.
The presence of RNA in the composition of chromatin is not
sufficient ground for many researchers to accept it as an integral component of
the chromosomes. It is assumed that RNA is a product of DNA transcription.
This standpoint is contradictory to the opinion of certain authors presuming that
RNA precedes DNA as genetic material (see Lazcano, 1986).
The proteins taking part in the chromatin composition are divided into
two groups: histones and non-histones. The first comprise a group of low
molecular proteins rich in lysine and arginine denoted as H1, H2A, H2B,
H3, H4, H5. The last one (H5) is found in the erythrocytes when they have
nuclei. In the spermatozoa of some fish, amphibia and other animal species
histones are replaced by protamines. The so-called acidic proteins which
are very diverse in composition and possess totally different structures and
properties are classified as comprising the second group. This group is the
least studied one.
image
Figure 2–42. Electron micrograph of native portion of the DNA-molecule (After Stahl, 1966).