Forms of Chromatin (With Diagram)

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.