Ex

  Biology (NTSE/Olympiad)  

1. The cell

Nucleus, Structure & Working

Nucleus :
• It is the most important part of cell which control all the activities of cell.
Structure of Nucleus :
• The nucleus has a double layered covering called nuclear membrane.
• The nuclear membrane has pores inside the nucleus to its outside, that is, to the cytoplasm.
• The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
• Chromosomes are composed of DNA and protein.
• Functional segments of DNA are called genes.
• In a cell which is not dividing, this DNA is present as part of chromatin material.
Function of nucleus :
• It play a important role in cellular reproduction.
• DNA contain the information necessary for constructing & organizing cells.
• Nucleoid - In some organisms nuclear region of cell may be poorly defined due to the absence of nuclear membrane. Such an undefined nuclear region called nucleoid.
Note :
Prokaryotic cell- Cell which do not have well defined nuclear region . called prokaryotic cells. Pro - Primitive
Karyon - nucleus
Eukaryotic cells - Cells which have well defined nuclear region, called eukaryotic cells.
• Along with nucleus membrane, prokaryotic cells lack most of cell organells.

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