Extranuclear Inheritance

Mitochondria and chloroplasts are self-replicating organelles: They can arise only by division of preexisting mitochondria or chloroplasts. DNA molecules are replicated and divided up among the daughter organelles after division. As a result, organelle genes show hereditary continuity from cell to cell and from parent to offspring, as do the genes in the nucleus. In Multiple copies of the mitochondrial genome exist in each organelle, which may bear different alleles. Replication and partitioning can create mitochondria with only one allele type. fact, the ancestry of mtDNA and cpDNA can be traced back to intracellular symbionts (termed "endosymbionts").

Early in evolutionary history, an ancestor of the eukaryotes ingested an ancestor of the aerobic α-proteobacteria. This bacterium avoided digestion and became a permanent resident of the host cell, dividing within it and providing it with energy from aerobic metabolism. Gradually, over millions of years, the endosymbionts transferred most of their genes to the host nucleus, becoming completely dependent on the host cells. The host cells, in turn, came to depend on the symbiont for aerobic energy production.

Much later, an ancestor of the modern green algae and plants ingested a cyanobacterium capable of photosynthesis. Gradually this endosymbiont also lost most of its genes to the nucleus and became dependent on the host cell, while providing the host with energy from photosynthesis. The resulting organelles are self-replicating, like the original symbiont.

Because mitochondria and chloroplasts originated as endosymbiotic bacteria, their genomes differ from the nuclear genome in several important Tracked mutations in extranuclear genes, as in the chloroplasts of the plant cell shown here can be used to study the evolution of a species. ways. First, all of the organelle genes are located on a single, circular DNA molecule. Second, the genes are virtually contiguous, with little or no intergenic DNA. Third, the gene coding sequences are continuous. In other words, there are no (noncoding) introns separating gene-coding sequences. Also, each organelle has many copies of the DNA molecule, and each cell usually has more than one organelle.

Plant cells commonly have from two to several hundred chloroplasts, while animal cells often have hundreds of mitochondria. As a result, each cell has hundreds or thousands of mtDNA or cpDNA molecules, and hence of each mitochondrial or chloroplast gene. In effect, each cell contains a small population of organelle genes. This is in contrast to the nucleus where, with few exceptions, there are only two copies of each chromosome and gene (or one copy in haploid cells).