Nucleus contains most of the genes that control the cell. It is the most conspicuous organelle in the eukaryotic cell. Its average diameter is about 5. It performs two major functions:

  1. The information of DNA of nucleus is transcribed into RNA. This RNA is used in the synthesis of proteins by translation. These proteins in the form of enzymes and other proteins determine the specific activities of the cells.
  2. The nucleus stOre genetic information and transfer them during cell division form one cell to the other. They also transfer the characters from one generation to the other.



Nuclear envelope

The nuclear envelope encloses the nucleus. So it separates nuclear contents from the cytoplasm. It is continuous with the ER at number of points.

The nuclear envelope is 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 he 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 ER. Nuclear pores are corn aosed of globular and filamentous granules of proteins. The size of pores prevents the DNA from leaving. But it allows RNto move out.

Chromosomes 0 Pwote in( Hist nri



The nucleoplasm is the inner mass of the nucleus: Nucleoplasm contains chronosomes. The DNA is organized with proteins to form chromosomes. The chomosomes are thread like during interphase. So they cannot be identified as inavidual structures. Thus the chromosomes are appeared as maghof stained material. This stained material is collectively called as eicromatin. The chromosomes condense before the division of cell. So it becomes thick. Now it cart be seen as separate structures. The chromosomes contain genes. These genes control all the character of an animal. Each eukaryotic species has a characteristic number of chromosomes. A human cell has 46 chromosomes in its nucleus. But the sex cells—eggs and sperm have only 23 chromosomes in humans.


The most visible structure within the non dividing nucleus is the nucleolus. The nucleolus is a non-membranous bound structure. The function of the nucleolus is to synthesize ribosomes. Sometimes, there re two or more nucleoli. Their number depends on the species and the cell’s reproductive cycle. Two or three nucleoli are present in most of the cell.

The nucleolus is roughly spherical. It appears as mass of densely stained grar ules under the electron microscope and fibers. Nucleoli are present at nucleolar organizers. These are specialized regions of some chromosomes. They contain multiple copies of genes for ribosome synthesis.

Ribbsomes are preseht in various stages of production. An actively growing cell can produce about 10,000 ribosomes per minute.


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