How do cells in a multicellular organism become specialized?

Cells becomes specialized through cell differentiation

Explanation:

Cell differentiation is the process by which the cell in a multi cellular organism gets differentiated.  

Basically the function of a cell is determined by DNA. During cell division identical cells are produced with varied genetic identity and this "variation in genetic identity" causes different cells to perform different function. The DNA of a cell with specific genetic area is expressed based on the functionality of the cell irrespective of the expression of  entire DNA areas.

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