• A K Sharma

      Articles written in Proceedings – Plant Sciences

    • Change in chromosome concept

      A K Sharma

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      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.

    • Chromosome banding in evolutionary plant cytogenetics

      U C Lavania A K Sharma

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      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.

    • Feulgen microspectrophotometric estimation of nuclear DNA of species and varieties of three different genera of Marantaceae

      A K Sharma Sandip Mukhopadhyay

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      Karyological analysis including determination of somatic chromosome number, total chromosome length, volume and karyotype formula andin situ estimation of 4C-nuclear DNA amount were carried out on 14 different species and varieties of the generaCalathea, Maranta andStromanthe. The 4C nuclear DNA amount was estimated through Feulgen microspectrophotometry following single wavelength method and expressed in arbitrary units of relative absorbances. The variation in 4C DNA amounts between the species ofCalathea was not distinct but in two species ofMaranta, a notable variation in nuclear DNA amount was recorded. In addition, the amount of nuclear DNA did not show direct correlation with the total chromosome length and volume. The absence of wide difference in nuclear DNA content at an interspecific level might have some adaptive value.

    • Response to radiation andin vitro growth of two species ofLuzula with non-localised centromere

      Jayanti Sengupta A K Sharma

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      In vitro growth preceded by gamma-radiation was studied in two species with non-localised centromere, namelyLuzula elegans andLuzula pediformis. Seed germination was affected by radiation doses. Cytological investigation revealed dose dependent chromosomal aberration and spindle disturbances. Chromosome break and fragments were present inLuzula elegans but absent inLuzula pediformis. The growth of the callus was affected at higher doses in both the species. The radiosensitivity of the two species ofLuzula with diffuse centromere has been discussed.

    • Ontogenesis ofin vitro shoot bud proliferation inSolanum sarrachoides Sendt.

      N Banerjee S Mukhopadhyay A K Sharma

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      In vitro direct induction of multiple shoot buds ofSolanum sarrachoides Sendt. was obtained from leaf segments using Murashige and Skoog’s basal medium supplemented with 6-γ-γ-(dimethylallylamino) purine (1·5–2·5 mgl−1). Maximum shoot bud proliferation was observed in the medium containing 2 mgl−1 and better growth of plants with well defined roots, leaves, flowers and fruits in the subsequent stages of development. Histological examination at different stages of development confirmed that the multiple buds produced on excised leaf segments arise adventitiously through the formation of numerous tiny protuberances. The complete sequence of events leading to multiple bud formation based on histological studies has been presented.

    • Intraspecific variation of nuclear DNA inCapsicum annuum L.

      Sumona Mukherjee A K Sharma

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      In situ nuclear DNA amount varied significantly between 23 varieties ofCapsicum annuum. Mean values of nuclear DNA showed no correlation with chromosome length. Somatic chromosome number is constant 2n=24 in all the varieties.

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