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When a egg is initially fertilized, the resulting cell and its immediate progeny (after a very few rounds of cell division) can initially become any type of cell in that organism. For example, it can become a brain cell, blood cell, bone cell - whatever.
Then as it goes through successive rounds of cell division, it's developmental scope (the range of possible cells it can become) gets limited. At the molecular level, certain genes are "turned off" or not transcribed - usually this involves packing the DNA up tightly, where it is not possible for the enzymes which transcribe the DNA to reach it. It also involves covalent modification of the DNA itself - metylation, acetylation, phenomena like that.
This is referred to as "cell differentiation."
... its simply a phenomena happens to (generally the daughter-cell after division) ... It takes a new form and new job (after variation in its DNA), like lymphocyte B becomes Plasmocyte production of antibodies ...