For convenience, the cell cycle may be divided into two major phases—the interphase (in which the cell is engaged in its cell-specific or tissue- specific activities) and the mitotic phase (in which the nucleus undergoes division).
During mitosis, chromosomes of eukaryotic cells condense, and when stained with basic dyes, the chromatin is easily studied by microscopy.
During the interphase of the cell cycle, most of the chromatin does not exist in the condensed state, and it is difficult, if not impossible, to distinguish individual chromosomes.
However, there are areas of the nucleus called chromo centers that do stain deeply even during interphase. In 1928, E. Heitz identified these chromo centers as portions of chromosomes that remain in the condensed state throughout the cell cycle. Depending on their staining properties, two different types of chromatin may be distinguished in the interphase nucleus (Figs. 20-15 and 20-16). Portions of chromosomes that stain lightly are only partially condensed; this chromatin is termed euchromatin and usually represents most of the chromatin that disperses after mitosis is completed.
In the dark-staining regions, the chromatin remains in the condensed state and is called heterochromatin. Usually there is some condensed chromatin around the nucleolus, called perinucleolar chromatin, and some inside the nucleolus, called intranucleolar chromatin. The perinucleolar and intranucleolar chromatin appear to be connected; together they are referred to as nucleolar chromatin.
Dense clumps of deeply staining chromatin often occur in close contact with the inner membrane of the nuclear envelope and are referred to as condensed peripheral chromatin (Fig. 20-15). Between the peripheral chromatin and the nucleolar chromatin are regions of lightly staining chromatin called dispersed chromatin.
Heterochromatin can be further divided into two types: constitutive heterochromatin and facultative heterochromatin. In constitutive heterochromatin, the DNA is permanently inactive and remains in the condensed state throughout the cell cycle. Facultative heterochromatin is not permanently maintained in the condensed state; instead, it undergoes periodic dispersal and during these times is transcriptionally active.
Heterochromatin is characterized by its especially high content of repetitive DNA sequences and contains very few, if any, structural genes (i.e., genes that encode proteins). However, heterochromatin may be involved in the regulation of gene expression, for when euchromatic genes of known functions are relocated adjacent to heterochromatin, their expression is modified. Euchromatin is believed to contain the structural genes and is expressed when decondensed in the interphase cell.
Chemically, chromatin consists for the most part of DNA and protein. Small quantities of RNA may also be present but the RNA rarely accounts for more than about 5% of the total chromatin present. Most of the protein of chromatin is histone, but “non-histone” proteins are also present. The protein DNA weight ratio varies from 0.8 to 1.3 and averages about 1.1. The ratio varies not only with the species but also among the, tissues of a single organism. Histones are constituents of the chromatin of all eukaryotic organisms except fungi, which therefore resemble prokaryotic organisms in this respect.
1. Chromosome-Chromosome Associations:
It has been known for some time that certain associations occur among the chromosomes arranged on the metaphase plate during nuclear division. There may be size assortment on the spindle such that long chromosomes are on the outside and short chromosomes are on the inside.
Certain chromosomes are known to group together or have arms directed toward one another. G. Hoskins has shown that human chromosomes are linked by inter-chromosomal “connectives.” The connectives are composed of DNA and protein and hold the chromosomes together even when teased from a cell by microsurgical methods. Fibers linking separate chromosomes have been visualized using electron microscopy.
2. Polytene Chromosomes:
In the salivary glands of dipteran flies, of which the genus Drosophila has been most extensively studied, and in certain other tissues as well, the interphase nucleus is characterized by extremely large chromosomes. These so-called giant chromosomes are actually about 1000 parallel and tightly packed copies of the same chromosome. Because of their multiple structures, they are called polytene chromosomes. Along the length of the polytene chromosome, disks or bands of variably staining intensities can be distinguished.
The identification of the genetic content of these bands (and inter-band regions) has played a major part in the genetic analysis of dipteran organisms. Some of the bands of giant chromosomes appear to be swollen or “puffed,” the specific bands exhibiting puffing varying from one tissue to another even within the same organism. It is now well established that the puffed bands of these interphase chromosomes correspond to regions in which the DNA is being actively transcribed into messenger RNA.