This is the most important organelle of the cell which regulates all its activities. It was discovered by Robert Brown in 1831.
Position:
Usually it is present at the centre of the cell, but in some instances as in mature parenchyma cells the nucleus is peripheral. In Acetabularia, the nucleus occupies various positions, though mainly present at the basal region.
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Shape, Size and Number:
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The shape is variable depending on the cell type. Though the nucleus is generally spherical, ellipsoidal or flattened ones also may be seen in certain cells. The surface is generally smooth but as in leucocytes it may have small infoldings.
The size of the nucleus is also variable depending on the cell; the size of the nucleus being directly proprolional to that of cytoplasm. The following formul or Heywig may be used to determine the size of the nucleus in a particular cell.
When NP = nucleoprotein index, Vn is the volume of the nucleus and Vc is the volume of the cell.
Most of the cells are uninucleate, while some (Paramecium) may be binucleate or multinucleate (Cladophora).
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In certain fungi like Mucor, where there are no septa dividing the plant body into cells, numerous nuclei occur indicating a coenocytic condition.
Ultrastructure of the nucleus:
Ultrastructurally the nucleus has the following parts – 1. Nuclear membrane, 2. Karyolymph or Nuclear sap, 3 Chromonemata, 4 Nucleolus and 5 Endosomes
Nuclear membrane:
In all eukaryotic organisms, a limiting membrane envelops the nucleus. This membrane helps in effective communication between nucleus and cytoplasm. The elements of endoplasmic reticulum contribute to the nuclear envelop during cell division.
Electron microscopic studies have revealed the nuclear membrane to be a double membrane with a number of pores. According to Watson (1959) these pores called nuclear pores are octogonal in shape. The outer nuclear membrane is called endokaryotheca.
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Each membrane is about 75 – 90A thick and they enclose a space between them about 100 – 150A (De Robertis et al 1970).
This space is called the perinuclear space or cisterna. The outer membrane is rough due to the presence of RNA particles. Internally the nuclear membrane is followed by a supporting membrane called fibrous lamina (300A thick).
The nuclear pores are 600A wide and are enclosed by rings called annuli. These together constitute the pore complex. In some instances, the pore complex is covered by a membrane to regulate the permeability.
Nuclear sap (Karyolymph):
The interphase nucleus consists of a homogenous matrix of fluid substances enclosing a network primarily the Karyolymnh is proteinaceous, but also has nucleic acids, enzymes and minerals. It is quite probable that in plants the nuclear sap contributes to the spindles.
Chromonemata:
Enclosed in the Karyolymph and visible in the interphase nucleus are found a number of fibrillar structures constituting a network called chromonemata or chromatin fibrils. These stain positively with basic fuchsin stain.
Some coarse granules are deposited on the chromatin network. These are a called chromocentres and constitute the points of condensation of chromosomes. During cell division the chromatin network breaks up into specific number of chromosomes. Two regions can be identified in the chromatin material. These are heterochromatic region and achromatic region.
The heterochromatic region stains darkly and shows numerous beads like structures called chromomeres. The heterochromatic region has less DNA.
This region is believed to be genetically and metabolically inert. The light staining region of the chromatin is called the euchromatic region. This region contains more of DNA and is supposed to be genetically active.