The morphologic characteristics of chromosomes are best studied during metaphase and anaphase of cell division. The chromosome was supposed to be covered by a membranous Pellicle. Electron microscope studies later showed that there is no definite membranous pellicle surrounding the chromosomes.
A matrix of some sort, possibly protein, lipid, may be present. Other structures present in the chromosome include the centromere, secondary constrictions, nucleolar organisers, telomeres and satellites.
Embedded in the matrix of each chromosome are two identical, spirally coiled threads, the chromonemata. The two chromonema are so highly coiled together that they appear as a single thread of about 800Å thickness.
Each chromonemata consists of about 8 microfibrils, each of which is 60- 100Å formed of double helix of DNA. A DNA strand with protein sheath is about 30-40Å but without protein it is 20Å thick. The adjacent strands of DNA are separated by a distance of about 25Å.
The shape of chromosomes is determined by the primary constriction located at the point where the arms of a chromosome meet. Within the constrictions there is a clear zone containing a small granule, or spherule. This clear region is the so- called centromere or kinetocore. It is functionally related to the chromosomal movements that occur during mitosis.
Usually each chromosome has only one centromere (monocentric); however, there may be two (dicentric) or more (polycentric), or the centromeres may be diffused, e.g. as in Ascaris megalocephala and in Hemipteran bug (Insects).
In addition to the primary constriction or centromere the arms of the chromosome may show one or more secondary constrictions (called secondary constrictions II). These constrictions are different from nucleolar organiser (called secondary constrictions I), although some cytologists also refer to the nucleolar organiser as the secondary constriction.
They are distinguished from the primary constriction by the absence of marked angular deviations of the chromosomal segments.
Sometimes it has been seen that secondary constriction is associated with the nucleolus during interphase and is found to participate in the formation of nucleolus. It is, therefore, known as Nucleolar organiser and represents about 0.3% of the total amount of nuclear DNA. It, therefore, has several gene loci which are believed to be concerned with the formation of 28s and 18s ribosomal RNA.
This term applies to each of the extremities of a chromosome. If chromosomes are fractured by X-rays, the resulting segments may fuse again. They will not, however, fuse with the telomere. It appears that the telomere has a polarity that prevents other segments from joining with it.
In the Ascarid worms the status of telomeres is rather uncertain, since interstitial portions of the germ-line chromosomes became terminal, after fragmentation in the somatic nuclei.
If the secondary constriction is sub-terminal or near one end of the chromosome, the small part at the terminal and beyond the constriction appears as a knob and is known as satellite. It is customary to designate as SAT-chromosomes those having a satellite.
The ratio of the length of the long arm to the short arm of a chromosome is characteristic for each single chromosome. In acrocentric chromosomes the arm ratio is high. Metacentric and sub-metacentric chromosomes have a low arm ratio. In the humans, metacentric chromosomes have arm ratios ranging from 1.07 to 1.95, submetacentrics from 2.05 to 3.75 and acrocentrics from 5.00 to 11.94.