The cells have the same DNA but they don’t all use the same sequences. Depending on how they originally developed and where they are in the body, they turn off sequences that aren’t needed for their particular function. A skin cell, for example, doesn’t need the same set of instructions as a neuron which likewise doesn’t need the same set of instructions as bone marrow.
All cells have the same DNA but they don’t all use the same part of it. Think of it like chapters in a book. Let’s say you’re a student in a class and all the students have the same textbook. Now imagine your teacher gives each student an assignment to do a presentation on a different chapter from the book. So everyone has the same book, but each student is only reading one chapter, so their presentations will be different because they’re only about the chapter they read.
It’s basically like that for cells. All of your cells* have a fully copy of your DNA, but they only use the part of your DNA that they need to be that cell. The rest of the parts are turned off. For example, a skin cell doesn’t read the part of your DNA that has instructions for intestinal cells because that part of your DNA is turned off in skin cells.
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