Export of Mitochondrial and Chloroplast Genes

Organelle genomes, such as those in both mitochondria and chloroplasts, are smaller than those of their prokaryotic ancestors. This is because, during evolution, the majority of their genes were exported to the nucleus while many others were lost before developing into a mitochondrial or chloroplast genome. 

These exported genes are known as nuclear integrants of organellar DNA.  Specifically, the genes from the mitochondria are nuclear integrants of mitochondrial DNA, and those from the chloroplast are nuclear integrants of plastid DNA. 

One theory of why cells may transfer the genes from mitochondria and chloroplasts to the nucleus is that the electron transfer reactions in mitochondria and chloroplasts generate mutation-causing free radicals. The export of these genes reduces exposure to free radicals and the likelihood of harmful mutations.

Additionally, the nucleus has a more effective DNA repair system than either mitochondria or chloroplasts.

As mitochondrial and chloroplast DNA are inherited from a single parent only, they cannot undergo sexual recombination. However, once the genes are incorporated into nuclear DNA, genes from both parents are inherited. 

Sexual recombination allows rearrangement of genes from both parents which can prevent the accumulation of undesired mutations and can improve adaptation to the surrounding environment. 

Nuclear DNA’s transcription and translation machinery are different from those of mitochondria and chloroplasts; therefore, the exported genes must undergo several modifications to function properly. 

These changes include the insertion of new DNA sequences for a promoter and a terminator required for proper mRNA and protein production. A targeting sequence is also added to direct the protein product to the mitochondria or chloroplast. 

Most exported genes retain their original function in the mitochondria and chloroplast.  However, in some cases, it has led to the development of genes with new functions.