The aerobe now became a mitochondrion . Perhaps the genes which the anaerobe took would not fit into a single chromosome , or perhaps for some other reason, but anyway its genes were split over several chromosomes. These chromosomes were wrapped up in a membrane in the middle of the cell to make the nucleus . Notice we used the same word to describe the central part of an atom, because the word means center. All cells more advanced than bacteria are like this.
Eukaryotes are much larger and more complex than bacteria, and have many more genes.
Eukaryotes appeared about 1.5 billion years ago. Almost all the plant and animal cells you can see around you are eukaryotic. They contain structures called organelles .