Nuclear energy
No nuclear fission occurs in the body.
stars.... stars are made of hydrogen, helium, and nuclear fusion
chain reactions
A critical assembly of fissile material
Fusion ocurs in the stars including our sun. I don't know of fission occurring naturally anywhere, but there is evidence it once did in a few places on earth a long time ago. At that time there would have been more U-235 which has now reduced by decay. It may occur elsewhere in the universe in planets like earth, but we have no way of knowing this. Natural nuclear fission: Oklo - please see the related link, below
Nuclear Fusion
Currently, it can't. May be it could be in future.
The term fission generally means to split into parts. When we talk about splitting the atom, we generally apply the term nuclear fission to the process. Nuclear fission, the breaking apart of an atomic nucleus, can occur spontaneously, or it can occur as the result of the bombarding of a nucleus with radiation. Nuclear fission is covered in another related question, and it is linked below.
You get nuclear fission in:nuclear fission reactorsatomic fission bombs
The heat and the light in stars is the same thermal nuclear fission that our Sun (a star) produces.
Nuclear fission splits an atom of one element into two atoms of two different elements. This is not usually called transmutation. However, fission is accompanied by subsequent steps, which usually include transmutation.