Mitochondrial DNA (mtDNA) is present in mitochondria as a circular molecule and in most species codes for 13 or 14 proteins involved in the electron transfer chain, 2 rRNA subunits and 22 tRNA molecules (all necessary for protein synthesis). The number of proteins involved in the electron transfer chain is much larger than 13 or 14, but the others are coded by the nuclear DNA.
In total, the mitochondrion hosts about 3000 proteins, but only about 37 of them are coded on the mitochondrial DNA. Most of the 3000 genes are involved in a variety of processes other than ATP production, such as porphyrin synthesis. Only about 3% of them code for ATP production proteins. This means most of the genetic information coding for the protein makeup of mitochondria is in chromosomal DNA and is involved in processes other than ATP synthesis. This increases the chances that a mutation that will affect a mitochondrion will occur in chromosomal DNA, which is inherited in a Mendelian pattern. Another result is that a chromosomal mutation will affect a specific tissue due to its specific needs, whether those may be high energy requirements or a need for the catabolism or anabolism of a specific neurotransmitter or nucleic acid. Because several copies of the mitochondrial genome are carried by each mitochondrion (2-10 in humans), mitochondrial mutations can be inherited maternally by mtDNA mutations which are present in mitochondria inside the oocyte before fertilization, or (as stated above) through mutations in the chromosomes.
In humans, the heavy strand of mtDNA carries 28 genes and the light strand of mtDNA carries only 9 genes. Eight of the 9 genes on the light strand code for mitochondrial tRNA molecules. Human mtDNA consists of 16,569 nucleotide pairs. The entire molecule is regulated by only one regulatory region which contains the origins of replication of both heavy and light strands. The entire human mitochondrial DNA molecule has been mapped. The rate of mutation in mtDNA is calculated to be about ten times greater than that of nuclear DNA, possibly due to a paucity of DNA repair mechanisms. This high mutation rate leads to a high variation between mitochondria, not only among different species but even within the same species. It is calculated that if two humans are chosen randomly and their mtDNA is tested, they will have an average of between fifty and seventy different nucleotides. This may not seem like much, but when compared to the total number of nucleotides of a human mitochondrial DNA molecule (16,569), as much as .42% of the mtDNA varies between two people.
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In total, the mitochondrion hosts about 3000 proteins, but only about 37 of them are coded on the mitochondrial DNA. Most of the 3000 genes are involved in a variety of processes other than ATP production, such as porphyrin synthesis. Only about 3% of them code for ATP production proteins. This means most of the genetic information coding for the protein makeup of mitochondria is in chromosomal DNA and is involved in processes other than ATP synthesis. This increases the chances that a mutation that will affect a mitochondrion will occur in chromosomal DNA, which is inherited in a Mendelian pattern. Another result is that a chromosomal mutation will affect a specific tissue due to its specific needs, whether those may be high energy requirements or a need for the catabolism or anabolism of a specific neurotransmitter or nucleic acid. Because several copies of the mitochondrial genome are carried by each mitochondrion (2-10 in humans), mitochondrial mutations can be inherited maternally by mtDNA mutations which are present in mitochondria inside the oocyte before fertilization, or (as stated above) through mutations in the chromosomes.
In humans, the heavy strand of mtDNA carries 28 genes and the light strand of mtDNA carries only 9 genes. Eight of the 9 genes on the light strand code for mitochondrial tRNA molecules. Human mtDNA consists of 16,569 nucleotide pairs. The entire molecule is regulated by only one regulatory region which contains the origins of replication of both heavy and light strands. The entire human mitochondrial DNA molecule has been mapped. The rate of mutation in mtDNA is calculated to be about ten times greater than that of nuclear DNA, possibly due to a paucity of DNA repair mechanisms. This high mutation rate leads to a high variation between mitochondria, not only among different species but even within the same species. It is calculated that if two humans are chosen randomly and their mtDNA is tested, they will have an average of between fifty and seventy different nucleotides. This may not seem like much, but when compared to the total number of nucleotides of a human mitochondrial DNA molecule (16,569), as much as .42% of the mtDNA varies between two people.
