Comparing the genomes of present-day mitochondria or chloroplasts with the genomes of their corresponding bacteria reveals that these organelles do not have many of the genes that are essential for their function. For instance, they lack the many genes that are required for DNA replication. What has happened to these genes?

A) They have been lost during evolution, since the organelles no longer rely on DNA replication.
B) The required genes are kept in the nucleus, but many have evolved by gene transfer from the organelle.
C) These genes have undergone mutations and have changed beyond recognition, but are still present in the organelle.
D) The organelles do not replicate their DNA; they import new DNA from the nucleus.
E) The required genes are on plasmids that are separate from the organelle's genome.