A K Sharma
Articles written in Proceedings – Plant Sciences
Volume 87 Issue 8 August 1978 pp 161-190
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.
Volume 92 Issue 1 February 1983 pp 51-79
The introduction of chromosome banding techniques for linear differentiation of chromosomes have allowed the identification of the heterochromatic segments on the chromosomes. These heterochromatic segments are primarily composed of repetitive DNA, which are discernible in the form of dark staining regions by Giemsa C band staining or exhibit enhanced or reduced fluorescent bands by Q banding techniques depending upon the particular type of DNA repetition. The analyses of banding patterns have allowed in plants, the identification of chromosomes or parts of chromosomes, which have been utilized for inter- and intra-species comparisons. Based on the information of banding patterns, amount and distribution of heterochromatic segments, coupled with karyotypic features and morphological similarities; the probable phylogenetic relationships in various plant taxa from Gymnosperms, Angiosperms (both dicots and monocots) have been suggested. The information on heterochromatin recognition have also been utilized in suggesting probable ancestry of polyploids and the trend of evolution in varietal differentiation and speciation. Analysing the data, a probable phylogenetic significance and the direction of change in heterochromatin evolution in plants is suggested.
Volume 93 Issue 3 July 1984 pp 337-347
Karyological analysis including determination of somatic chromosome number, total chromosome length, volume and karyotype formula and
Volume 98 Issue 6 December 1988 pp 489-493
Volume 99 Issue 4 August 1989 pp 307-312
Volume 100 Issue 1 February 1990 pp 1-6