Nucleo-Cytoplasmic Interrelationship

NUCLEO-CYTOPLASMIC INTERRELATIONSHIP

Before the rediscovery of Mendel’s gene, the evidence for nucleo-cytoplasmic communication as a factor in cell maintenance and development were known. Microsurgery, nucleated halves of various protozoans survived and grew, whereas the enucleated halves of various protozoans degenerated and died were shown by Very warm, Balbiani, and others. In the 1930’s the insertion of nuclei into enucleated amoebae restored pseudopodial activity, feeding behavior and growth was shown. And we came to know that nuclear were essential for the growth and regeneration of the morphologically complicated ciliate Stentor. Hammerling in classical series of experiment last between 1934 and 1954 demonstrated that by the means of interspecific nuclear transplants, those morphological features, notably the shape of caps, were determined by the nucleus. Even after the removal of the nucleus, the cell was able to continue morphogenesis for a time and proposed that the cytoplasm consists of a store of morphogenetic material. Later it is recognized as mRNA molecules which had been produced by the nucleus.

Here we examine the Hammerling’s classical transplantation experiments closely:

Hammerling’s experiment. The body of an alga Acetabularia is about six centimeters long and is differentiated into a foot, a stalk and a cap. The cap is easily regenerated if removed and has a characteristics shape for each species. The single nucleus is situated in the rhizoid portion. Acetabularia crenulate has a cap, the tips of which are pointed with about 31 rays, but Acetabularia Mediterranean has about 81 rays with rounded tips. The remaining portion of the alga has the capacity to regenerate into a whole plant if the cap, stalk or even the nucleated portion of the rhizoid is removed. The nucleated portion always maintains the regeneration capacity but the enucleated part loses this ability after a few decapitations. When the stalk of one species is grafted onto the nucleated rhizoid of the other , an intermediate type of cap is formed. A second cap develops which resembles the cap of species which provides the nucleus, on decapitation. An intermediate type of cap develops when the nuclei of the both species are present in the same cytoplasm. From this experiment, we can conclude that the nucleus are the storehouse for and the control tower of, all hereditary information.

OCCURANCE AND POSITION

In all the eukaryotic cells of the plant and animals, the nucleus is found.The certain eukaryotic cells such as the mature sieve tubes of higher plants and mammalian erythrocytes contain no nucleus. In such cells, nuclei are present during the early stages of development.Since mature mammalian red blood is without any nuclei called red blood “corpuscles”rather than cells(L. corpus=body,especially dead body or corpse).

The prokaryotic cells of the bacteria do not have the true nucleus,i.e.,the single,circular and large DNA molecule remains in direct contact with the cytoplasm.The position or location of the nucleus in a cell is usually the characteristics of the cell type and it is often variable.The nucleus remains located in the centre.But its position may change from time to time according to the metabolic status of the cell.Example,in the embryonic cells the nucleus generally occupies the geometric centre of the cell but as the cells start to differentiate and the rate of the metabolic activities increases,the displacement in the position of the nucleus takes place.Such as the glandular cells the nucleus remains located in the basal portion of the cell.

MORPHOLOGY

Number

The cells contain single nucleus but the number of the nucleus may vary from cell to cell.The number of the nuclei following types of cells has been recognized:

1.Mononucleated cells.Plant and animal cells contain a single nucleus,such cells are called the mononucleate cell.

2.Binucleate cells.The cells which contain two nuclei are called binucleate cells.The cells occur in certain protozoans such as Paramecium and cells of cartilage and liver.

Shape. The shape of the nucleus normally remains related with the shape of the cell,but certain nuclei are almost irregular in shape.The spheroid,cuboid or polyhedral cells(isodiametric cells)contain the spheroid nuclei.The nuclei of the cylindrical,prismatic or fusiform cells are ellipsoid in shape.The cells of the squamous epithelium contain the discoidal nuclei.The leukocytes,certain infusoria glandular cells of some insects and spermatozoa contain the irregular shaped nuclei.Nuclei of cells of silk glands of silk worm have finger – like extensions that greatly increase their surface area.

The shape of the nucleus normally remains related with the shape of the cell,but certain nuclei are almost irregular in shape. The spheroid,cuboid or polyhedral cells(isodiametric cells)contain the spheroid nuclei.The nuclei of the cylindrical,prismatic or fusiform cells are ellipsoid in shape.The cells of the squamous epithelium contain the discoidal nuclei.The leukocytes,certain infusoria glandular cells of some insects and spermatozoa contain the irregular shaped nuclei.Nuclei of cells of silk glands of silk worm have finger – like extensions that greatly increase their surface area.

Size. The nucleus occupies about 10 percent of the total cell volume.Nuclei vary in size from about 3um to 25um in diameter,depending on cell type and contain the diploid set of chromosomes.The size of the nucleus is directly proportional to that of the cytoplasm.R.Hertwig has given the following formula for the reduction of the size of the nucleus of a particular cell.

NP=V_n /V_c - V_n

Where NP is the nucleoprotein index,Vn is the volume of the nucleus and Vc is the volume of the cell.The size of the number of the chromosomes or ploidy.The haploid cell is small sized nuclei then the diploid cells. The polyploid cells contain larger nuclei than diploid cells. The size of the nucleus of a cell depends on the volume of the cell, amount of the DNA and proteins and metabolic phase of the cell.

ISOLATION TECHNIQUES

The number of nuclei is also measured easily in a chamber similar to that used for blood counts.For the isolation of nuclear envelope,nuclei are first of all separated from the rest of the cell.It is normally accomplished by disrupting tissue in homogenizers wherein the clearance such that nuclei are not broken but plasma membrane and endoplasmic reticulum are disrupted.Nuclei can be harvested by differential centrifugation.They lysed by sonication and envelopes separated by density gradient centrifugation.Alternatively,DNAs digestion is extraction with salts releases envelopes which can band on sucrose or cesium chloride gradients.

The chemical organization of nucleus was investigated by two main approaches.The first is essentially biochemical,consist of isolating a large enough number of nuclei to permit analysis by biochemical methods.The second approach an essentially cytologic uses the cytophotometric and radio-autographic methods.A nucleolar fraction is obtained by treating the DNA in a eukaryotic nucleus can determine by microspectrophotometry technique which measures the precision amount of Feulgen material each nucleus contains.

References

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Kotpal, R L. Modern textbook of Zoology. Meerut, India: Rastogi Publication, n.d.

Rastogi, S C. Cell, and Molecular biology. New Delhi: New Age International (P) Limited, 2001.

Verma, P S, and V K Agrawal. cell biology,Genetics,Molecular Biology,Evolution, and Ecology. New Dehli, India: S. Chand and company Ltd., 2012.