Change in chromosome concept
Evidences showing that chromosomes in eukaryota, with their complex organization, exhibit dynamism in structure and behaviour for controlling differentiation, variability and reproduction have been reviewed. An outline has been given of the stepwise evolution of the functionally differentiated chromosome segments and their chemical make up including protein components and sequence complexity of DNA. The outline includes the origin of chromosomal control of metabolism in extranuclear organelles.
During organogenesis, the chromosomes, maintaining the basic genetic skeleton, undergo variation in structure and chemical components, thus exhibiting dynamism. Addition, loss and inactivation of heterochromatic segments have been correlated with adaptation and alteration in chromosome size. Data on the possible role of repeated sequences in control and integration have been presented.
Endomitotic replication of the chromosome, in place of normal mitosis, during differentiation is an example of dynamic behaviour, adapted to meet the need of uninterrupted transcription during differentiation without increasing the number of cells. The predetermined symmetrical growth of the organism is thus maintained.
In several asexually reproducing species, dynamism is manifested in genetically controlled unusual behaviour of chromosomes, where the somatic tissue often represents a chromosome mosaic. It has been adapted to meet the need of generating variability and genotypes through bud mutation in the absence of effective sexual reproduction.
Data in favour of the concept of dynamism indicate that evolution of eukaryotic chromosome has involved progressive complexity of chromosome structure on the one hand, and flexibility in its behaviour and structure, maintaining the basic genetic make up, on the other. It has enabled the chromosomes to exert supreme control on all aspects of metabolism which are sequential and phasic in higher organisms.