Mitochondria contain their own DNA that is distinct from the DNA found in the nucleus. There are just 37 genes found on mitochondrial DNA (mtDNA) though more than e20,000 genes are located on chromosomes in the nucleus.
MtDNA was discovered in the 1960s. Margit Nass and Sylvan Nass saw threads with an electron microscope that were sensitive to DNAses, enzymes that break down DNA. Around the same time, Gottfried Schatz, Ellen Haslbrunner and Hans Tuppy found the biochemical signature of DNA in a pure mitochondrial fraction from yeast.
In humans, mitochondrial DNA is a circular molecule, in contrast to nuclear DNA which is linear. There are two strands of DNA entwined in each circle, an inner light chain and an outer heavy chain. Most of the genes are on the outer strand. Human mtDNA is very small – it contains just 16,569 base pairs!
MtDNA comes in many different shapes and sizes between different species – a lot of evolution has gone on since the first event that fused a mitochondrion and a cell. It can be circular, linear or even in several pieces. Some species have very large amounts of mtDNA but this often contains lots of non-coding regions i.e. regions that don’t make a protein the organism can use.
Despite having 37 genes, only 13 proteins are encoded by mtDNA – all these proteins are involved in oxidative phosphorylation. There are also 22 genes that make tRNAs and 2 genes that make rRNAs that are needed for protein translation. There is also a small stretch of DNA that doesn’t code for any protein. This is involved in regulation of mtDNA replication and contains the start site for replication and for transcription.
All 13 proteins encoded by mtDNA are involved in oxidative phosphorylation.
• Seven genes in mtDNA encode subunits of Complex I – ND1, ND2, ND3, ND4, ND4L, ND5 and ND6.
• Complex II is entirely encoded by genes in the nucleus.
• One subunit of Complex III is encoded by mtDNA, the cytochrome b subunit.
• Three subunits of Complex IV are encoded in mtDNA – the COXI, COXII and COXIII subunits.
• There are two mtDNA genes encoding Complex V, ATPase6 and ATPase8.
The rest of the proteins required for oxidative phosphorylation and other mitochondrial functions are encoded by genes in the nucleus.
It is now thought that mtDNA doesn’t just float around the mitochondrion but is arranged in structures called nucleoids. There may be 3-7 repeats of mtDNA circles in each nucleoid held together by the protein TFAM which is also needed for mtDNA transcription. Proteins involved in mtDNA replication are often found in these structures too e.g. the mtDNA polymerase POLG, the DNA binding protein mtSSB and a helicase called Twinkle! There is likely to be more than one nucleoid in each mitochondrion as each single mitochondrion can have 10-100 mtDNA molecules!
Amazingly mitochondrial DNA can only be inherited from your mother, unlike nuclear DNA which comes from both parents. Any paternal mtDNA found in the sperm is destroyed or lost after fertilization. This means that any mitochondrial diseases carried in the father’s mtDNA cannot be passed onto their offspring!