With different phases of cell cycle the changes of chromosome structure takes place. At the time of interphase, the chromosomes remain in the form of network fine thread, namely chromatin threads. But during metaphase and Anaphase, the contract and shorten to form distinct rod-like structures. Distinct arm-like chromatid formed of coiled chromonema and knob like protein particles called chromomeres are showed by them. A primary constriction which is known as centromere and a secondary constriction present in some chromosomes, which is associated with nucleolus formation and for this reason it is called nucleolus organiser. A part of chromatid may be depressed off from the secondary constriction which is known as satellite devoid by nucleic acid.
The study of chromosome structure is mainly done from the root tip or the shoot tip of plants which contains the Meristematic tissue or from the pollen mother cells of plants and tissue from sex glands and W.B.C. in animals.
Some chromosome structures we may define under: –
i) Primary or centromeric constriction : the non-stained notched region of a chromosome containing centromere is known as primary or Centromeric constriction.
It appears as a gap in the chromosome, but originally there is no shortage of chromatin here. It is invisible due to its property of non-staining. They contain the centromere, which is variable in position and imparts various shapes to the chromosomes at the time of the anaphasic separation.
ii) Centromere: The centromere is the spherical chromomeric structure at the centre of the primary constriction responsible for the attachment of the chromosome to the spindle fibre. Normally majority of chromosomes have one centromere ; sometimes chromosomes may break to form acntric fragments which are lost during cell division. In the chromosome structure the centromere is the point where the two arms of chromosome meet; it determines the shape of the chromosome. The microtubules of the spindle gets attached to the centromere and the anaphasic separation of the chromosome is initiated.
Classification of chromosome on the basis of centromeric position: -
a) Metacentric: - In this case the centromere occurs at the centre of the chromosome which dvides the chromosome into two equal arms.
b) Sub-metacentric: - In such case the centremereoccursvery close to the centre of the chromosome dividing it into two equal arms.
c) Acrocentric: - Here the centromere occur very close to the terminal end of the chromosome forming one very long and one very short arm.
d) Telocentric: - In this case the centromere is almost terminally situated on a chromosome.
iii) Kinetochore: - The protein disc attached to the centromeric chromomeres to which the spindle microtubules are joined; these are called the kinetochore of chromosome structure. They bind with the microtubule of the spindle fibre.
iv) Secondary Constriction: - Any constriction other than the primary constriction is termed as secondary constriction. They help in the organization of the nucleolus and synthesize m-RNA.
v) Chromatid and Chromonema: It is one of the identical strands of chromosome developing during its replication. At their early stage of condensation the chromatids is called chromonema.
The chromonema is the site where DNA remains in pack.
vi) Chromomere: The chromomere are stainable bead-like structures arranged along the chromosome. In the early prophase of mitosis and meiosis the chromomeres are present. In the chromosome structure they appear as beaded necklace, where the bead like chromomeres are attached to interchromomeric thread. The chromomere are the structural component of chromosome but do not actually represent the genes.
vii) Telomeres: The telomeres are the terminal end of chromosomes. They are formed of heterochromatin and contain repetitive DNA, remains associated with nuclear envelope. The telomeres impart polarity to chromosomes and prevent joining of the chromosome fragments and associated with the production of enzyme telomerease.
viii) Satellite: The satellite is the terminal round knob-like heterochromatin body beyond the secondary constriction of a chromosome. The shape and size of it is constant in a chromosome and its diameter is same as the chromatid. By a thin chromatin filament the satellite is connected to the chromatid. There is no specific function of a satellite but it is just a morphological feature of chromosome structure.