粒線體DNA.(Mitochondrial DNA; mtDNA)

/李平篤

Nuclear and mitochondrial DNA are thought to be of separate evolutionary origin, with the mtDNA being derived from the circular genomes of the bacteria that were engulfed by the early ancestors of today’s eukaryotic cells. Each mitochondrion is estimated to contain 2-10 mtDNA copies. In the cells of extant organisms, the vast majority of the proteins present in the mitochondria (numbering approximately 1500 different types in mammals) are coded for by nuclear DNA, but the genes for some of them, if not most, are thought to have originally been of bacterial origin, having since been transferred to the eukaryotic nucleus during evolution. In most multicellular organisms, mtDNA is inherited from the mother (maternally inherited). Mechanisms for this include simple dilution (an egg contains 100,000 to 1,000,000 mtDNA molecules, whereas a sperm contains only 100 to 1000), degradation of sperm mtDNA in the fertilized egg, and, at least in a few organisms, failure of sperm mtDNA to enter the egg…….

粒線體DNA,與一般細胞核DNA有不同的演化起源,可能是源自早期細菌。雖然現存生物體中絕大多數作用於粒線體的蛋白質,是由細胞核DNA所控制並製造,但這些基因有些可能源於細菌,並於演化過程中轉移到細胞核。

人類細胞卵子(egg)約有105到106個mtDNA,但精子sperm約有100到1,000個。而每一個粒線體,則約有2~10對mtDNA,每個mtDNA共含16,569個鹼基對(base pair),其中有37個基因,可用來製造13種蛋白質、22種tRNA與兩種rRNA。其中的內含子(intron)(不會表現的DNA)較細胞核基因少,且有些不含內含子,如tRNA基因。動物體內mtDNA並不會經過遺傳重組,因此與細胞核DNA相較之下有較高的突變速率(重組有修復突變的功能);而植物與真菌類體內的mtDNA則存在著重組現象,其中植物的mtDNA突變速率高於細胞核DNA;由於動物mtDNA的突變速率高於細胞核DNA,較容易測量計算,使mtDNA成為用來追溯動物母系族譜(maternal pedigree) 的有效工具。例如許多物種在數百個世代以前的祖先。此外,人類的mtDNA也可用來進行個體辨識。

粒線體DNA起源

內共生理論認為真核細胞最早的起源,是原核細胞(細菌等)被吸收到另外一個細胞中,而沒有被消化。這兩個細胞之後產生了共生關係,使最早的細胞器organelle誕生,此胞器後來成為現今的粒線體,其基因組也在演化過程中轉變成粒線體DNA。對動物而言,受精卵的mtDNA主要遺傳自母系;而對植物來說略有變異,但仍然以母系遺傳為主;真菌則源自雙親。

雌性遺傳

正常狀況下,粒線體只會遺傳自母親,以哺乳類而言,一般在受精之後,卵細胞就會將精子中的粒線體摧毀。1999年發表的研究中顯示,父系精子粒線體(含有mtDNA)帶有泛素(ubiquitin)標記,因而在胚胎中會被挑選出來,進而遭到摧毀。不過某些細胞外的人工受精技術可直接將精子注入卵子細胞內,可能會干擾摧毀精子粒線體的過程。

參考文獻

Wiesner RJ, Ruegg JC, Morano I (1992). Biochim Biophys Acta. 183 (2): 553–559.

Alexeyev, Mikhail F.; LeDoux, Susan P.; Wilson, Glenn L. ( 2004). Clinical Science 107 (4): 355–364.