Mitochondrial DNA

Lore & Background

Nuclear and mitochondrial DNA are thought to have separate evolutionary origins, with the mtDNA derived from the circular genomes of bacteria engulfed by the ancestors of modern eukaryotic cells. This theory is called the endosymbiotic theory. In the cells of extant organisms, the vast majority of the proteins in the mitochondria (numbering approximately 1500 different types in mammals) are coded by nuclear DNA, but the genes for some, if not most, of them are thought to be of bacterial origin, having been transferred to the eukaryotic nucleus during evolution. The reasons mitochondria have retained some genes are debated. The existence in some species of mitochondrion-derived organelles lacking a genome suggests that complete gene loss is possible, and transferring mitochondrial genes to the nucleus has several advantages. The difficulty of targeting remotely produced hydrophobic protein products to the mitochondrion is one hypothesis for why some genes are retained in mtDNA; colocalisation for redox regulation is another, citing the desirability of localised control over mitochondrial machinery. Recent analysis of a wide range of mtDNA genomes suggests that both these features may dictate mitochondrial gene retention.

Reader's Guide

Across all organisms, there are six main mitochondrial genome types, classified by structure (circular versus linear), size, presence of introns or plasmid like structures, and whether the genetic material is a singular molecule or collection of homogeneous or heterogeneous molecules. Most (bilaterian) animals have a circular mitochondrial genome. With a few exceptions, animals have 37 genes in their mitochondrial DNA: 13 for proteins, 22 for tRNAs, and 2 for rRNAs. Mitochondrial genomes for animals average about 16,000 base pairs in length. The anemone Isarachnanthus nocturnus has the largest mitochondrial genome of any animal at 80,923 bp. The smallest known mitochondrial genome in animals belongs to the comb jelly Vallicula multiformis, which consist of 9,961 bp. In February 2020, a jellyfish-related parasite – Henneguya salminicola – was discovered that lacks a mitochondrial genome but retains structures deemed mitochondrion-related organelles. This is the first multicellular organism known to have this absence of aerobic respiration and live completely free of oxygen dependency. The Armadillidium vulgare has a uniquely large genome when compared to other crustacean average about 30 kB greater allowing it to be a model organism in mtDNA research. There are three different mitochondrial genome types in plants and fungi. Some plant species have enormous mitochondrial genomes, with Silene conica mtDNA containing as many as 11,300,000 base pairs. The genome of the mitochondrion of the cucumber (Cucumis sativus) consists of three circular chromosomes (lengths 1556, 84 and 45 kilobases), which are entirely or largely autonomous with regard to their replication. Protists contain the most diverse mitochondrial genomes, with five different types found in this kingdom. The smallest mitochondrial genome sequenced to date is the 5,967 bp mtDNA of the parasite Plasmodium falciparum.

Did You Know?

• Human mitochondrial DNA was the first significant part of the human genome to be sequenced.
• The anemone Isarachnanthus nocturnus has the largest mitochondrial genome of any animal at 80,923 base pairs.
• In February 2020, a jellyfish-related parasite – Henneguya salminicola – was discovered that lacks a mitochondrial genome and lives completely free of oxygen dependency.
• The smallest mitochondrial genome sequenced to date is the 5,967 bp mtDNA of the parasite Plasmodium falciparum.
• The cucumber (Cucumis sativus) mitochondrial genome consists of three circular chromosomes, lengths 1556, 84 and 45 kilobases.