Biologically speaking, where does the “self” reside?

As a youth, faithful in some sort of soul, I was not interested in neurological studies as I believed in an inevitable afterlife. Naturally, this intellectual limitation committed atrocities to my understanding of science. Now that I have abandoned my beliefs in the mystical and see possibility of immortality in nanotechnology and medicine, I wish to know what constitutes the “self” and how this can be sustained as one sustains his body.
The future of technology will offer two novel methods to counter the aging process; nanotechnology will intervene at the cellular level and other sciences will be able to create organs from one’s DNA. Of course, as the brain ages, one would need to replace it- would one die in the process? Would he “experience” the thoughts of the subsequent human? Is there a way of preventing the aging process within organs? Any associated comments?