If I understand correctly, long lived and highly metabolically active cells like neurons still replace most, if not all, of their atoms during their lifetime, since metabolism and environmental radiation cause chemical damage which needs to be repaired in order to keep the cell functional.
There's actually been some cool studies on DNA extracted from brain tissue from people born before the first nuclear tests...
DNA is turned over for the most part during DNA replication. Neurons are terminally differentiated and do not divide, and thus their DNA is more or less frozen in place even as other molecules turn over (barring small repair events). People born before the first nuclear tests have neurons bearing a different carbon isotope ratio in their DNA than people born afterwards, and this was used in a study to determine the rate over a huma...
http://www.technologyreview.com/view/541311/the-false-science-of-cryonics/
Michael Hendricks is a neuroscientist and assistant professor of biology at McGill University.