Evidence mtDNA has bacterial origins

Many lines of evidence indicate that mitochondria in plants and animals, (and also chloroplasts in plants), evolved from bacteria that were endocytosed into ancestral cells containing a eukaryotic nucleus, forming endosymbionts.

• Circular DNA. Over evolutionary time, most of the bacterial genes encoding components of the present day organelles were transferred to the nucleus. However, mitochondria and chloroplasts in today's eukaryotes retain circular DNA, as do virtually all bacteria.
• Mitochondria contain multiple copies of the mtDNA molecules. Each cell may contain many mitochondria and each mitochondrion may contain many copies of the mtDNA.
• mtDNA exhibits Cytoplasmic Inheritance, i.e. through the viscous contents of the cell rather than via the nuclear DNA. In Yeast cells, a cytoplasmic pattern of inheritance can be shown across generations. In higher plants, the male pollen does not contribute any mtDNA. In mammals and most higher animals, the sperm contributes no (or very few) mitochondria so the mtDNA is maternally transmitted.
• The size of the mtDNA varies greatly between different organisms. Human mtDNA is virtually the smallest, (16,568),  of all the mtDNA molecules that have been sequenced to date. Invertebrate mtDNA is about the same size as human mtDNA, but Yeast mtDNA is almost five times as big at about 78,000. The mtDNA from yeast and other lower eukaryotes encode many of the same gene products as mammalian mtDNA as well as other whose genes are now found in the Nuclear DNA of mammalian cells.
• Differences in the size and coding capacity of mtDNA from different organisms most likely reflects the movement of DNA between mitochondria and the nucleus during evolution. Direct evidence for this comes from the observation that several proteins encoded by mtDNA in some species are encoded by Nuclear DNA in other species.
• mtDNA of animal, yeast and fungal cells, (unlike Nuclear DNA) is very compact, does not contain introns and contains no long non-coding segments. However, the mtDNA of plants, is much larger, and is more variable between species, (watermelon about 330,000 yet musk melon about 2,500,000), mainly due to long non-coding regions and repeated sequences.
• Products of mtDNA remain inside mitochondria. Many proteins required by mitochondria are encoded by Nuclear DNA and imported into the mitochondria from the cytosol. However, as far as is known, no proteins encoded by mtDNA are required outside the mitochondria.
• Mitochondrial ribosomes resemble bacterial ribosomes and differ from cytoplasmic ribosomes in their RNA and protein composition, their size, and their sensitivity to certain antibiotics.
• The genetic code is different in mtDNA than in Nuclear DNA. Three base pairs of DNA form a Codon to specify which particular amino acid is to be used when building a protein. Sixty four possible Codons can then specify each of the 20 amino acids, have some Start and Stop Codons and have some spare. This genetic code is used by all prokaryotic and all eukaryotic cells. However, the genetic code used by animal and fungal mtDNA differs from the standard code. Remarkably, the mtDNA genetic code even differs between species. E.g. the DNA sequence UGA is normally a Stop Codon which specifies the end of a sequence of DNA, thus marking the end of one protein. However, in the mtDNA of animals and fungi, it is read as the amino acid Tryptophan.
• Chloroplast DNA, found in plants is also circular, typically 120,000 to 160,000 base pairs long, and many similar sequences have been found between Chloroplast DNA and Bacterial DNA.

We comply with the HONcode standard for health trust worthy information: Verify here.

Copyright © 2002 - Andy Collinson. All Rights Reserved.

Terms Privacy Funding