There are various quantitative karyotypic ratios to observe the karyotype variations and precise description of the karyotype such as relative length, centromeric index, total form percent, disprsin index, disparity index, coefficient of variation, volume of chromosomes, value of relative chromatin and so on. Arm ratio 1 being the most symmetrical and 4 is the most asymmetrical. The degree of asymmetry of chromosome complement depends on the four arm ratios (1 to 4) and the size of the smallest and largest chromosome and three different proportions of the metacentric chromosomes (ABC) of a given chromosome complement. There may be 12 different types of karyotype categories depending on increasing asymmetry in chromosome complement. For example, a group of species resemble each other in the number, size and form of their chromosomes. Karyotype highlights closely or distantly related species based on the similarity or dissimilarity of the karyotypes. Karyotype may be defined as the study of chromosome morphology of a chromosome complement in the form of size, shape, position of primary constriction or centromere, secondary constriction, satellite, definite individuality of the somatic chromosomes and any other additional features. It could be considered as a valuable chromosome marker for distinguishing chromosome of related species and races. For example, a very distinct such type of chromosome knob could be observed in maize ( Zea mays) at pachytene stage of meiosis I. less or more than the diameter of chromosome arm. Knobs are considered to be a spherical bodies or regions with spherical in shape and sometimes diameter of these spherical bodies is equal in width to chromosome arm, but the size may vary i.e. The relative distribution of the chromocentres on the chromosome structure, sometimes considered to be of significant evolutionary value. Some of the Chromocentres could be resolved into large number of strings of chromomeres which are much larger in size as compared to the chromomeres found in the distal region of the chromosmes arms during the mid-prophase. Chromocentres are the regions with varying size near the centromere in the proximal regions of chromosome arms. Chromomeres are regular features of all prophase chromosomes but their number, size, distribution and arrangements are specific for a particular species at a particular stage of development. Heterochromatic regions could be easily recognized on chromosome structure in the form of chromomeres, chromocentres and knobs. easily changed from euchromatin to heterochromatin and vice-versa. Facultative heterochromatins are not permanently conserved or condensed and in unstable form i.e. Most probably, thick and condensed state of the constitutive heterochromatin, replicates late in S-phase with reduced frequency of genetic recombination. It consists of multiple repeats of DNA sequences with quite less density of genes in this region which are transcriptionally inactive. not changed from heterochromatin to euchromatin and vice-versa. Constitutive heterochromatins are permanently conserved or condensed and in stable form i.e. Heterochromatin could be of two type’s constitutive and facultative heterochromatin. On the other hand, heterochromatin is slightly opposite to the euchromatin with dark stained region, tightly packed, genetically inactive, not involved in the active transcription, thick appearance with more RNA content than DNA. Euchromatin is a part of chromatin which takes less stain, loosely packed, genetically active, involved in active transcription, dispersed appearance with more DNA content than RNA. Chromatin is divided into euchromatin and heterochromatin. Chromatin from interphase (loose mixture of DNA, RNA Protein) to thick mitotic structure (tightly packed or compressed mixture of DNA, RNA Protein) packed through Nucleosome model of DNA packing. Chromatin is a mixture of DNA, RNA and proteins could be easily visible during the interphase and prophase of the cell division cycle.
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