Which mechanism best describes how two cells with identical DNA can become a muscle cell and a neuron? Use conceptual reasoning about gene regulation.

• A. Muscle cells and neurons have different mitochondrial DNA, which encodes the proteins responsible for the main functional differences between these cell types.
• B. Gene duplication in early embryogenesis creates extra copies of muscle-specific genes in muscle progenitors while neuron progenitors keep copies of neural genes intact.
• C. All cells express all genes equally; specialization is determined only by the nutrients around a cell at any moment and does not involve gene regulation.
• D. Cell-type-specific transcription factors and epigenetic modifications selectively activate and repress different genes, leading to distinct protein expression patterns that produce specialized cell functions.

Answer: (D)