Mitochondrial DNA definition
Mitochondrial DNA (mtDNA) refers to the DNA that is located inside the mitochondria, an organelle present in eukaryotic cells. MtDNA is double stranded but, unlike nuclear DNA, is circular, not packaged in chromatin, the translation of its codons into proteins does not follow the rules of the universal genetic code and has a different GC content.
Mitochondrial DNA in animals and humans
In animals and humans, mtDNA is small and very compact, containing overlapping genes and only 3% of non-coding DNA, with respect to its origin of replication (D-loop). Mitocondiral genome has about 16500 base pairs (bp) that encode 37 genes necessary for normal mitochondrial function. Of these 37 genes, 13 are responsible for expression of proteins involved in oxidative phosphorylation, with the remaining genes involved in expression of 22 molecules of tRNA and 2 molecules of rRNA. The two chains of mtDNA have different characteristics, being denominated by a different form: heavy chain (H – heavy) that has more guanines, and light chain (L – light) that has more cytosines. In these cells, mtDNA represents only about 1% of total cell’s DNA, however due to its proximity to respiratory chain where reactive oxygen species are produced, it is more susceptible to accumulate mutations than nuclear DNA.
Mitochondrial DNA in yeast
In yeast, mtDNA is large, containing about 80000 bp. Being five times greater than the mtDNA present in animal cells it only encodes about 8 proteins. This is due to the presence of long intron sequences (non-coding DNA). Total mtDNA present in a yeast can reach 18% of total cell’s DNA.
Mitochondrial DNA in plant cells
mtDNA present in plant cells it is quite complex and varies greatly in size, with the smallest genome having about 100000 bp. In these cells, unlike to animal cells, mtDNA accumulates mutations more slowly than nuclear DNA.
Heritability of mitochondrial DNA
In most species, mtDNA’s inheritance mode is solely via maternal progenitor. In animal cells this happens because during sexual reproduction the paternal mitochondria found in spermatozoid’s tail are destroyed by the ovum after fertilization or lost during fertilization.
Replication of mitochondrial DNA and its regulation
mtDNA is stochastically replicated, since there is no regulation over which molecule to replicate, and replication of the two circular chains may not be simultaneous. For this process, is used a DNA polymerase different from that used in nuclear DNA, which is called ‘gamma DNA polymerase’. As regards its transcription, due to the fact that both mtDNA chains only have a promoter region, the mtDNA is transcribed in a polycistronic way, i.e., large mRNA molecules containing several ORFs (open reading frame) give origin to various proteins.
