Do somatic cells alter their own nucleotide sequence?
One case where human cells alter their DNA occurs in the immune system.  In the early stages of the continuing production of B-cells and T-cells, the developing cell recombines (shuffles) genes in a particular DNA region which codes for what are called Variable, Diverse, and Joining gene segments.  This shuffling is called somatic recombination or V(D)J recombination.  This rearrangement gives the cell a genetic template for a random instance out of a enormous set of possible antigen-recognition proteins used by the immune system to recognize foreign molecules (in order to fight infection).