Every cell in the body — barring rare exceptions — carries an identical copy of the full genome. What makes a liver cell, a neuron, a skin cell and a muscle cell look and behave so differently isn't a difference in DNA sequence: it's which subset of genes each cell type actively transcribes.
A gene locus becomes transcriptionally active when its chromatin opens up (loosens from tightly-packed storage) and cell-type-specific transcription factor proteins bind its promoter, recruiting RNA polymerase to produce mRNA. In a different cell type, that same locus — same DNA sequence, same position on the strand — stays closed and silent, while a different set of loci opens instead.
- Housekeeping genes (green) stay open in every cell type — they run the basic machinery every cell needs.
- Cell-type genes open only when their cell type is selected — chromatin visibly loosens, transcription factors dock, and RNA streams away from the locus.
- Switching cell type never rebuilds the strand — it only changes which loci are "unlocked", exactly as it does inside a real cell during differentiation.