Today, I am going to try to give you
an overall view of the so-called second genome of the cell,
the mitochondrial genome, that was discovered in the 1960s.
I will first make a description of the genetic system,
and then I will describe the processes of
replication and transcription of the mitochondrial DNA,
making a special emphasis in the human mitochondrial genome.
In this figure, it is shown,
the role of this genetic system within the mitochondria.
As you can see, the mitochondrial DNA is attached to the mitochondrial inner membrane,
and it is replicated, transcribed,
and the messenger RNA is translated inside the organelle.
The proteins that are codified in this mitochondrial DNA are
assemble with other proteins encoded in the nuclear DNA,
which translated in cytoplasmic ribosome and imported into the mitochondria
to form in this way,
the oxidative phosphorylation system,
the so-called OXPHOS system.
However, for the replication and respiration of this genome,
it is necessary a continuous supply of nuclear encoded proteins,
replication enzymes, et cetera.
As shown in the lower part of the figure,
mitochondrial DNA replication requires
the import of all the enzymes involved in this process.
Also, for mitochondrial DNA transcription,
it is necessary to import an specific RNA polymerase,
and all the transcription factors.
To form the mitochondrial ribosomes,
the organelle have to import all the mitochondrial proteins.
These proteins together with the two ribosomal RNase,
12S, and 16S ribosomal RNase,
encoded in the mitochondrial DNA,
will originate the specific mitochondrial ribosomes.
To translate the mitochondrial messenger RNAs,
the tRNase have first to be charged with the amino acids.
For this, it is necessary to import all the amino acid tRNA synthetases.
For the process of translation it is also required to import all the factors: initiation,
elongation, and termination factors that participates in the synthesis of the proteins.
Altogether, the mitochondria ribosomes translate the messenger RNAs
synthesizing the mitochondrial DNA encoded components subunits of the OXPHOS system.
These proteins are assembled with subunits encoded in the nucleus,
synthesized in the cytoplasm,
and imported into the mitochondria.