The funky role of mitochondrial DNA in the eukaryotic cell

Marcella Attimonelli

Department of Biosciences, Biotechnology and Biopharmaceutics
University of Bari - Italy

The eukaryotic cells, differently from prokaryotic cells, contain two genomes.

  1. The nuclear genome, present in the nucleus with a well-defined and organism specific number of chromosomes. Due to the polyploidy feature of the nuclear genome, each chromosome is present in a number of n copies. The human genome is diploid and presents 22 pairs of chromosomes, plus the additional Y and X sexual chromosomes.
  2. The mitochondrial genome, located in the mitochondrion, an organelle network that resides inside the cytoplasm. The number of mostly identical mitochondrial genomes, inside each mitochondrion, and the number of mitochondria per cell is variable and depends upon the energetic needs of the tissue.

Mitochondrion history starts with the endosymbiotic events dated 1,5 billion years ago, when oxygen filled the atmosphere and a proto-bacterium fused with a pre-eukaryotic cell. After the endosymbiotic event, the original bacterial DNA that subsequently evolved in mitochondrial DNA, lost fragments that reached the nucleus and became inserted thereof in a more or less random fashion.

Nowadays the scenario is the following: the mitochondrion still contains a polyploidy genome but this holds only a very low number of coding for protein genes (13 is the mammalian number). However, the number of proteins necessary to fulfil the mitochondrial functions is approximately 1000 and these are ad hoc produced by the nuclear genome and thereby imported in the organelle.

Another point to take into account is the fact that mitochondrial protein synthesis is based on a genetic code that deviates from the so defined “universal code”.

These funky aspects of mitochondrial DNA raise the question why mitochondrial DNA still exists and why mitochondrion persists along the time to keep a genome although small. And in addition, what’s about the different mitochondrial genetic code? Why is it different? And from where originates?

Finally, since the nuclear genome is highly populated with DNA fragments showing high similarity with the mitochondrial DNA counterpart (NumtS - Nuclear mt sequences), the issue of whether the migration of mitochondrial DNA to nuclear DNA started already at the time of the endosymbiotic event or appeared later in the course of evolution is still debated.