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Basic note on Nucleus

Nucleus

Table of Contents

WHAT IS NUCLEUS

The nucleus is a double-membraned organelle that contains the genetic material and other instructions required for cellular processes.

According to Belar, Nucleus can be defined as any formation surrounded by cytoplasm from which chromosomes arise during division. It is the major seat of heredity and it controls all the cellular activities.

In Bacteria, Blue green algae, the nuclear material are not bounded, by a nuclear envelope. Organisms in which the nuclear membrane is not bounded by a definite envelop are called prokaryotes. Here, the nuclear material is called nucloid. Organisms with definite nuclii bounded by nuclear envelops are called Eukaryotes.

NUMBER OF NUCLEUS

Cells usually have a single nucleus and said to be mononucleate. Some cells of liver and cartilage have two nuclei are called bi nucleate. Cells with many nuclei are called poly nucleate. The multi nucleate mass of protoplasm is called synctium.

WHAT IS THE SHAPE OF NUCLEUS

The shape of the nucleus varies upon different cell types. It is usually spherical in shape but may be complete irregular as in leucocytes. In spherical, cuboidal or prismatic cells it is eliptical. In squamous cell it is flattened. In insect cells it is branched.

WHAT IS THE POSITION OF NUCLEUS

In adipose tissue cells, the nucleus occupies a peripheral position. In embryonic cells, it occupies a central position but later on many becomes displaced. In glandular cells, it lies in the basal region.

WHAT IS THE SIZE OF NUCLEUS

The smallest nuclei are 0.5 µ in diameter. The largest nuclei can cross over 1mm in length. Nucleus size is the function of chromosome numbers.

WHAT IS THE ULTRA STRUCTURE OF NUCLEUS

The interphase nucleus can be divided into four parts. They are:-

  • Nuclear envelope or karyotheca
  • Nuclear sap or karyolymphs
  • Chromatin
  • Nucleoli

WHAT IS NUCLEAR ENVELOPE

Nuclear envelop is a delicate but well defined barrier that divides the cell into two main envelope in complex double membrane structure. This can be divided as:-

  • Inner and outer membrane:- The inner and outer membranes lie in parallel array at the nuclear periphery. Each membrane is 70-80A° thick. The inner surface of the inner membrane is coated with inner dense laminar. The outer nuclear membrane communicates with the Endoplasmic reticulum at several points. It has ribosomes on other sides. Some areas of outer membrane are without ribosomes and appears to be participating in vesicle formation or blabbing.
  • Perinucleus space:- The two membranes enclose a space between them called perinuclear space which is 150-300A° width. It is probably filled with fixed simpler to that contained in Endoplasmic reticulum in Cisternae.
  • Pores:- The nuclear envelope is perforated in numerous holes called nuclear pores which vary in diameters from 300-1000A°. The two membranes continues at the margin of the pore.
  • Annulus:- According to some the pores are surrounded by a circular structure which was termed as annulus by Collen and Tomlein (1950). Watson (1959) gave the term pore complex to the pore and its associated annular material. The shape of the pores may be circular or octagonal. It has been confined that there exist a thin membrane as diaphragm attached across the pore.

FUNCTION OF NUCLEAR ENVELOPE

The nuclear envelope regulates the passage of ions of small molecules. By the way of pore complex the nuclear envelope regulates the transfer for macro molecules between nucleus and cytoplasm and vice versa.

WHAT IS NUCLEAR SAP

Nuclear sap is also known as karyolymph. Nuclear sap is a transparent semi fluid ground substance that fills the most of the nuclear space. The nucleoplasm is a complex, assortment of chemical substances in which chromatin network remains suspended.

WHAT IS CHROMATIN NETWORK

It consists of Deoxyribonucleic protein fibers embedded in the nucleoplasm. The nucleoplasm consists of DNA and protein which are histones or non histones. Most of the chromatin threads in the center phase nucleus are coiled as condensed, which is called hetero chromatin. It is frequently found near the nuclear envelope and the nucleolus. The uncondensed region of chromatin is termed as Euchromatin.

WHAT IS NUCLEOLUS

Nucleolus was first discovered by Fontana in 1874. It is particularly prominent in cells which have high rate of protein synthesis. For example:- oocytes are secretory cells but it is smaller or absent in cells having little protein synthesis. It is sphenoid body within the nucleus and composed of ribonucleic protein. It is devoid of an envelope. It is found in close association with the nucleolar organizes region of two or more chromosomes.

WHAT IS THE STRUCTURE OF NUCLEOLUS

In the nucleolus 4 chief components have been recognized in nucleolus which are:-

  • The matrix:- It is homogenous and it contains scattered granules and fibrils.
  • Chromatin:- Surrounding the nucleolus like a shell is a peri nucleolar chromatin. The DNA of chromatin serves as a template for RNA synthesis.
  • The fibrils:- The fibrils are 80-100A° in diameter. They contain RNA.

CHEMISTRY OF NUCLEUS

The chemical substances predominant in the nucleus
are the nucleic acids (DNA & RNA) and proteins. It contains:-

  • Basic proteins (Histones)
  • Acid proteins (Non-histones)
  • Nuclear enzymes
  • Nucleic acid
  • Lipids
  • Inorganic substances.

FUNCTIONS OF NUCLEUS

  • The nucleus is the control center of cellular activities. It appears directly to the synthesis of enzymes in the cytoplasm, which in turn regulate cellular functions.
  • It plays a central role in cell division.
  • It determines cell differentiation.

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