What major organelle are prokaryotes missing that eukaryotes have?

Nucleus

Explanation:

Prokaryotes are the organism that came earlier than eukaryotes on this earth. These prokaryotes do not have a nucleus and other membrane-bound organelles like chloroplast, mitochondria, Golgi apparatus, endoplasmic reticulum, etc.  

So as prokaryotes lack membrane-bound organelle their genetic material is not enclosed in the nucleus therefore their genetic material remains in the cytoplasm of the cell while eukaryotes have membrane-bound nucleus in which their genetic material are present.  

So the nucleus is the major organelle that prokaryotes are missing that eukaryotes have.

hi~~

i would say that hybridization occurs frequently in nature, because there are many types of hybrids that have occurred naturally, such as the mule (male donkey + female horse)