Because this is a biography, the ending is a bit of a wrap-up. Skloot describes what's happened to some of the important people in the book and describes the conclusion of her work on the book.
We also get a poignant scene between Deborah and Skloot that took place sometime during their research together, in which Deborah addresses the issue of immortality. It's a poetic ending to a work that had to deal with many unpleasant and scientific issues, and makes the ending much more hopeful and satisfying.