Mitochondrial diseases.

• Heteroplasmy. Heteroplasmy refers to the presence of different types of mitochondrial DNA in a cell or individual. Mitochondria multiply by dividing independently of the cell cycle, and in the copying process errors can occur that accumulate over time. This variation in mitochondrial DNA is known as heteroplasmy.
• Mitochondrial inheritance. Mitochondria are inherited primarily through the mother, and therefore, mitochondrial DNA (mtDNA) is inherited exclusively from the mother, unlike nuclear DNA, which is inherited from both parents. Mutations or variants in mitochondrial DNA are passed from mother to children, but only daughters can pass these mutations on to their offspring. Male children do not pass on their mitochondrial DNA to their offspring.
• Mitochondrial genetic code. The mitochondrial genetic code is the set of rules that determines how the information contained in mitochondrial DNA (mtDNA) is translated into proteins within the mitochondria, and is similar but not identical to the universal genetic code used in the rest of the cell, which is found in nuclear DNA. In the mitochondrial genetic code, some of the amino acid codings are different, meaning that certain codons in the mRNA translate into different amino acids compared to the nuclear genetic code. Some of the most notable differences in the mitochondrial genetic code are:
  • The AGA codon in the nuclear code codes for the amino acid arginine, while in the mitochondrial code it codes for serine.
  • The AGG codon in the nuclear code codes for the amino acid arginine, while in the mitochondrial code it also codes for serine.
  • The UGA codon in the nuclear code is usually a stop codon (termination codon), but in the mitochondrial code it codes for the amino acid tryptophan.

• Mitochondrial chromosome. The term "mitochondrial chromosome" refers to the genetic material contained in mitochondria, but it is important to note that mitochondria do not have chromosome structures similar to chromosomes in the nucleus. Here are some key features of mitochondrial DNA:
  • Shape and Structure: Mitochondrial DNA usually appears in the shape of a closed circle, in contrast to the linear chromosomes in the nucleus.
  • Genes: Mitochondrial DNA contains a total of 37 genes.
  • Transcription and Translation: Mitochondria are capable of transcribing and translating their own DNA, which allows them to synthesize the proteins necessary for their function. However, most proteins required for mitochondrial function are encoded by the nucleus and then imported into the mitochondria.

The 37 genes of the Cambridge Reference Sequence for human mitochondrial DNA and their locations[30]