Nucleus- Definition, Structure and Functions Of Nucleus

A specialized structure is present in most cells that are separated from the rest of the cell by a double layer, the nuclear envelope. This membrane is continuous with the endoplasmic reticulum. The nucleus controls and regulates the cell’s activities like cell division and other metabolic activities. It carries the genes which contain hereditary information.

Evolution of Nucleus

Four hypotheses have been proposed to explain the existence of the nucleus.

• The first model is known as the “syntrophic model”. It is hypothesized that the symbiosis originated when ancient archaea, invaded and lived within bacteria similar to modern myxobacteria, eventually forming the early nucleus.

   Which are thought to have developed from a similar endosymbiotic relationship between proto-eukaryotes and aerobic bacteria.  The archaeal origin of the nucleus is supported by observations that archaea and eukarya have similar genes for certain proteins, including histones.

Observations that myxobacteria are motile, can form multicellular complexes, and possess kinases and proteins similar to eukarya, support a bacterial origin for the eukaryotic cell.

• The second model is based on the existence of modern planctomycetes bacteria that possess a nuclear structure with primitive pores and other compartmentalized membrane structures. 
• The most controversial model, known as viral eukaryogenesis, posits that the membrane-bound nucleus originated from the infection of a prokaryote by a virus.
• The exo-membrane hypothesis is the most recent model, it suggests that the nucleus originated from a single ancestral cell that evolved a second exterior cell membrane.

Shape and Appearance of Nucleus

The nucleus is normally around 5-10 μm in diameter in multicellular organisms. It is the largest organelle in the cell which acts as the brain of the cell that controls all the activities happening inside the cell. The smallest nuclei are approximately 1 μm in diameter and are found in yeast cells.

Structure Of Nucleus

The nucleus is composed of various structures.

1. Nuclear envelope

2. Nucleoplasm or nuclear matrix

 3. Chromatin

4. Nucleolus

Nuclear Envelop

The nuclear envelope encloses the nucleus. So it separates nuclear contents from the cytoplasm. It is continuous with the Endoplasmic Reticulum at several points. The nuclear envelope consists of a double membrane. The two membranes are separated by a space of about 20 to 40nm.

A layer of protein is present closely associated with the nuclear side of the inner membrane. It helps to maintain the shape of the nucleus and maintain the organization of the genetic material.

The nuclear envelope is perforated by pores. The diameter of these pores is about 100 nm. The pore complex regulates the movement of macromolecules and particles.

These pores give the nucleus direct contact with the Endoplasmic Reticulum. Nuclear pores are composed of globular and filamentous granules of proteins.

The size of the pores prevents the DNA from leaving. But it allows RNA to move out. the nuclear membrane surrounds the nucleus. It separates the nuclear material from the cytoplasm. The nuclear membrane is composed of two membranes. So it is a nuclear envelope the outer membrane is continuous with the endoplasmic reticulum at certain points. The inner membrane encloses the nuclear contents.

Nuclear pores:

the outer and inner membranes are discontinuing at certain points. So they form pores, called nuclear pores.

The nuclear pores allow the exchange of material between the nucleus and cytoplasm. The number of pores is different in different cells. The undifferentiated cells have numerous pores. These may be about 30,000 per nucleus. On the other hand, differentiated (mature) cells like erythrocytes (RBC)) have only 3 to 4 pores per nucleus. Each pore has a definite structure. It controls the movement of substances, passing through it.

Chromosomes:

 The nucleoplasm is the inner mass of the nucleus. Nucleoplasm contains genetic material.  The chromosomes are thread-like during interphase. Thus the chromosomes appear as a mass of stained material. This stained material is collectively called chromatin.

The chromosomes condense before the division of the cell. So it becomes thick. After condensation of chromatin material, it can be seen as separate structures. The chromosomes contain genes. The genes control all the characteristics of an organism. Eukaryotic species have a characteristic number of chromosomes.

A human cell has a 46-chromosome nucleus. But the sex cells and sperm have only 23 chromosomes in humans. The nucleus contains chromatin material.

This chromatin material is deeply stained with basic dye. This chromatin material is changed into chromosomes during cell division. The inner nuclear membrane contains no ribosomes.

Functions of Nucleus

The nucleus has a very important role to play. As it contains genetic material, it coordinates cell activities like protein synthesis and cell division.

Anatomically the nucleus is made up of several components: nuclear envelope, nuclear lamina, nucleolus, chromosomes, and nucleoplasm are some of these components. All these components work together for the nucleus to accomplish its functions.

These functions are:

•Control of the genetical information of the cell and thus the heredity characteristics of an organism,

• Control of the enzyme and protein synthesis

• Control of cell division and cell growth

• Storage of hereditary material (DNA), RNA, and ribosome

• Regulation of the transcription of the mRNA to protein

• Production of ribosomes

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