A partial answer comes from considering the sequence elements in a typical gene. We'll review the functions of these elements in more detail later. Much of the non-coding DNA in the genome is in the introns within functional genes themselves. Introns are transcribed, and can serve control functions in expression as we will see later. Besides introns, the non-coding DNA includes the spacer sequences between genes, some of which probably serves still unknown control functions.
Structural evidence indicates little DNA change during development .
This theory turned out to be wrong, except for the emphasis placed on the germ line as a special lineage generally set aside early in embryogenesis. Some organisms do undergo chromosome diminution, but probably lose few if any coding sequences. Most organisms don't undergo much if any loss or alteration of DNA sequences, based on a variety of structural evidence. As one example of such evidence, Drosophila polytene chromosomes in different larval tissues show identical banding patterns. Functional evidence for totipotency of differentiated cell nuclei comes from nuclear transplantation experiments in amphibians.