Why do the cells have more chromosomes? How can they be useful if they're so different from normal human cells?
Usually (always?) tumor cell are not only immortal, they also have problems with the chromosome copy process. The extra chromosomes are copies of the normal chromosomes. The number varies in every cell line to cell line.
So these cells are different, but most of their molecules are exactly like the molecules of a normal person. So they are useful to study most of the properties of humans cells.
Some people think that these cells are a new specie, a specie that lives in laboratories. These cells go from culture plates to culture plates at the minimal distraction. If someday the scientific want to erase all of them, it will be very difficult because a lot of other "cell cultures" are really HeLa. There are some examples of "transmissible cancer" that does something similar in the wild, in animals instead of culture plates. The cells in these tumors are more related to the cells in other tumors of the some kind, that to the cell in the actual animal host. Examples:
* Tasmanian Devil: http://en.wikipedia.org/wiki/Devil_facial_tumour_disease
* Dogs: http://en.wikipedia.org/wiki/Canine_transmissible_venereal_t...