Regulatory DNA Seems Largely Responsible for the Differences Between Animal Species
Each gene in a multicellular organism is associated with many thousands of nucleotides of noncoding DNA that contains regulatory elements. These regulatory elements determine when, where, and how strongly the gene is to be expressed, according to the transcription regulators and chromatin structures that are present in the particular cell.
Regulatory DNA defines the gene expression patterns in development.
The genome is the same in a muscle cell as in a skin cell, but different genes are active because these cells express different transcription regulators that bind to gene regulatory elements. For example, transcription regulators in skin
cells recognize a regulatory element in gene 1, leading to its activation, whereas a different set of regulators is present in muscle cells, binding to and activating gene 3. Transcriptional regulators that activate the expression of gene 2 are present in both cell types.
Consequently, a change in the regulatory DNA, even without any change in the coding DNA, can alter the logic of the gene-regulatory network and change the outcome of development. It seems that changes in regulatory DNA are largely responsible for the dramatic differences between one class of animals and another. We can view the protein products of the coding sequences as a conserved kit of common molecular parts, and the regulatory DNA as instructions for assembly: with different instructions, the same kit of parts can be used to make a whole variety of different body structures.