The nuclear genome encodes the remaining mitochondrial proteins, which are then transported into the mitochondria. The genomes of these organelles have become far smaller than those of their free-living predecessors. This is mostly due to the widespread transfer of genes from prokaryote progenitors to the nuclear genome, followed by their elimination from organelle genomes.
The nuclear genome now holds the instructions for making mitochondrial proteins, which are then transported into mitochondria. The genomes of these organelles have shrunk significantly compared to their free-living ancestors. This reduction is largely due to the transfer of genes from prokaryotic ancestors to the nuclear genome and subsequent removal from organelle genomes.