A partial answer comes from considering the sequence elements in a typical gene. You should be familiar with all of these elements and their functions from your previous course work. We'll review some of them in more detail as we go along. Much of the non-coding DNA in the genome is in the introns within functional genes. 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. Functional evidence for totipotency of differentiated cell nuclei comes from nuclear transplantation experiments. Mice, sheep, and cows have all been cloned by this technique, which raises thorny ethical questions about the possible application of cloning to humans.