The chromosome shape is observed at anaphase. The gross structure of the mitotic chromosome is usually a rodlike body with one constriction at the centromere, called the primary constriction. The position of primary construction determines the chromosome shape. In general, plants have longer chromosomes than animals and species having lower chromosome number have longer chromosomes. In some chromosomes further constriction can be seen which may include pinching off a small chromosomal section called the satellite. These secondary constrictions are often associated with regions where the nucleolus is formed or attached. When a mitotic chromosome is stretched out, as in prophase, the knob like chromosome regions along its length show distinct sizes and occupy specific positions. This gives each nonhomologous chromosome a morphologically distinct appearance.
Chromosome morphology reveals a coiled filament throughout the length of a chromosome. This filament is called chromonema. The chromonemata from the gene bearing portions of the chromosomes. The chromonemata are embedded in the achromatic substance known as matrix. Matrix is enclosed in a sheath or pellicle. Both matrix and sheath are nongenetic materials and appear only at metaphase when the nucleolus disappears. It is believed that nucleolar material and matrix are interchangeable. It may be noted that a chromatin is half chromosome longitudinally, two chromatids being connected at the centromere. A chromosome operates as a distinct structural unit. Each molecule that forms a linear chromosome must contain a centromere, two telomeres, and replication origins. Three types of specialised nucleotide sequences in the DNA control these basic functions.
Most chromosomes possess a special region which has some role in organisation of developing spindle at pre metaphase of mitosis that separate daughter chromosomes at primary constriction or centromere. They are permanent and autonomous segment of chromosomes. It usually appears as a non staining region or constriction at cell division. In most species, each chromosome has a single centromere in a fixed position where the two sister chromatids of a chromosomes are joined or held together during mitotic phase. The centromere appears as a narrow area in the chromosome known as the primary constriction. During cell division, microtubules of the spindle fibres attached to kinetochore of centromeres. At anaphase centromeres are the first part of chromosomes to move towards opposite poles. Acentric fragments of chromosomes do not become attached to the spindle. They float about freely in the cytoplasm at mitosis and do not go to either pole.
Centromere serves as an important landmark in the identification of different chromosomes of a species. Chromosomes are divided into four classes according to the position of their centromeres they are fallowing
1) Metacentric: The centromere is located in the middle of the chromosome.
2) Submetacentric: The chromosome is located on one side of the central point.
3) Acrocentric: Centromeres are located near to one end of the chromosome; they are also called sub telocentric chromosome.
4) Telocentric: The centromere is located at one end of the chromosome; such centromeres are called terminal centromere.
In some species, each chromosomes has more than one centromere. Such chromosomes are called polycentric chromosomes.
Telomere: Telomere is located at one end of a chromosome. Telomere is composed of six bases TTAGGG repeated thousands of times and maintain the structural integrity of the chromosome. They do not normally fuse or unite with telomere of other chromosomes. During cell division, when new DNA is produced, telomere loses nearly 100 bases. It means that the new DNA does not contain about 100 bases in its telomere. With cell division the length of telomere decreases. By seeing the length of telomere one can know how many times the cell divided.
