Mitochondrial Genome

Scientists have known since the early 1960s that the nucleus is not the only location for DNA in a eukaryotic cell. The mitochondria (and the chloroplasts in plant cells) harbor their own small genome. The genes found on the circular 16,569 base-pair piece of mitochondrial DNA (mtDNA) in human cells code for thirteen proteins, two ribosomal RNAs (rRNA), and twenty-two transfer RNAs (tRNAs), all of which are essential for the production of ATP by the mitochondria. Each individual organelle contains several copies of the mitochondrial genome. Although they comprise only Structure of a mitochondrion. a small portion of the proteins found in the mitochondrion, all thirteen proteins encoded by the mtDNA are essential, because they are necessary for oxidative phosphorylation and the production of cellular ATP. All of the remaining mitochondrial components are encoded by nuclear genes and are imported into the organelle.

The mitochondrial genome in mammals is extremely compact, with essentially no introns and very little DNA sequence between genes. Each of the protein and rRNA genes is immediately flanked by tRNA genes. Initial transcription of mtDNA produces large RNA molecules that are then processed into smaller units to generate mature tRNAs, rRNAs, and mRNAs.

The two mtDNA strands in the circular molecule can be separated based on their density (due to their differing nucleotide compositions), and are thus designated as the heavy strand (H-strand) and the light strand (L-strand). Both strands are transcribed completely, making two long RNA molecules. Since the two strands are complementary (not identical), they do not each code for the same genes. Instead, the H-strand transcript codes for most of the proteins and tRNAs, while the L-strand codes for most of the rRNAs. Ninety percent of the L-strand does not code for useful products, and is degraded after it is transcribed. The processed L-strand transcript also functions as the starting point for replication of the mitochondrial chromosome.