Among all the organelles in an animal cell, only mitochondria have their own independent genomes. Animal mitochondrial DNA is a double-stranded, closed-circular molecule with around 20,000 base pairs. Mitochondrial DNA is unique in that one of its two strands, the heavy, or H, -strand is guanine rich, whereas the complementary strand is cytosine rich and called the light, or L, -strand. Compared to nuclear DNA, mitochondrial DNA has a very low percentage of non-coding regions and is marked by the complete absence of introns. Also, their genes are very closely spaced and some of them even have overlapping regions. D-loop is the most important regulatory non-coding region of mitochondrial DNA, which also contains the origin of the replication for the H-strand. Mitochondrial genetic code differs from nuclear DNA code with respect to a few codons. For example, codon UGA, AUA, and AGA/AGG codes for STOP codon, isoleucine, and arginine, respectively, in nuclear DNA while the same codons codes for tryptophan, methionine and STOP codon, respectively, in animal mitochondrial DNA.
Replication of nuclear DNA is coordinated with the cell cycle and must be finished before cell division occurs. Another characteristic feature of the mitochondrial genome is its relaxed DNA replication, where unlike nuclear DNA, replication is independent of the cell cycle and can go on in daughter cells even after cell division.
In mammals, mitochondrial DNA gets inherited only from the mother’s oocyte as the mitochondria present in the sperm are selectively degraded by a ubiquitin-mediated pathway in the zygote. Mutations in mitochondrial genes can result in diseases such as Leber’s hereditary optic neuropathy or Leigh syndrome; therefore, if the mother carries such mutations, her offspring can inherit these diseases. Recently, new therapies such as mitochondrial replacement can allow the birth of an unaffected child to an affected mother. The nucleus of the mother’s oocyte is transferred to an enucleated oocyte of a healthy donor with normal mitochondria before fertilization. This technique has led to the birth of the so-called “three-parent baby,” who did not inherit the mother’s mitochondrial disease.
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