Keeping it simple, sexual involves gamete asexual does not, sexual involves meiosis and fertilisation asexual does not and sexual produces genetic variation where as asexual does not, offspring are clones
RNA has great capability as a genetic molecule; it once had to carry on hereditary processes on its own. It now seems certain that RNA was the first molecule of heredity, so it evolved all the essential methods for storing and expressing genetic information before DNA came onto the scene.
