Morphologic Characteristics of Chromosomes | Genetics

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.

Chromonema:

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.

Centromere:

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).

Secondary constrictions:

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.

Nucleolar organiser:

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.

Telomere:

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.

Satellite:

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.

Arm ratio:

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.