October 25, 2007

Mitochondria: The Veiled Lady of The Cell

Mitochondria photoThey are so enigmatic they can be easily termed the 'veiled lady'(ladies). You might argue that ladies, in general, are by nature, quintessential enigma themselves. Mitochondria, then ( plural for mitochondrion) are even more elusive.

Cells need power to operate. They have some tiny organelles, lined by a double layer of membranes called mitochondria. These sausage shaped organelles produce power in the form of an energy rich compound, the ATP or adenosine tri phosphate, by a process called oxidative phosphorylation. It then supplies ATP to the rest of the cells. Thus ATP might be thought of as 'portable batteries'. As in power producing factories, mitochondria also produce 'pollution' in the form of highly dangerous free radicals. These free radicals damage the organelles themselves in addition to other parts of the cells. Free radical injury also leads to aging and other disease states. The double layered membranes prevent them from diffusing out to the cytosol. In addition, mitochondria play a vital role in apoptosis (programmed cell death).

The history of the mitochondria is even more interesting. Billions of years ago, eukaryotic cells (cells containing nucleus and cytoskeleton) engulfed aerobic bacteria to survive. Aerobic bacteria, by definition, use oxygen (air) and generate energy. By engulfing and incorporating them, cells could have independent energy producing units in them. Thus mitochondria are actually endosymbionts (chloroplast is another example: endosymbiotic theory of Lynn Margulis). This explains why mitochondria have separate genomes. Apart from the nucleus, DNA can be found only in the mitochondria.

A cell can contain few hundreds to thousands of mitochondria, depending the energy requirements of that particular cell/tissue. For example, the heart, kidney, muscle, nervous tissues are voracious energy spenders, and hence have more mitochondria. As expected, mitochondrial diseases affect them most.

Mitochondria has a double stranded circular DNA (mtDNA). They are capable of transcription (copying of the DNA code in a complementary manner, much like the 'negative' of a photo film), and translation (execution of the software like code into real chemical ingredients-proteins). It is 16,569 bp long (base pairs; adenine, guanine, cytosine and thymine are the four nitrogen containing bases, and since the DNA is double stranded->pairs). This DNA codes for 13 proteins, 2 rRNAs (ribosomal Ribo Nucleic Acid) and 22 t RNAs (transfer RNA). Most (99%) of mitochondrial proteins are produced by the nuclear DNA (nDNA) though.

Evolutionarily, we inherited the mitochondria from bacterias, eons ago. But whom did I inherit them from? My mother! Sperms contribute little mitochondria (if any): mitochondria come exclusively from mother. Thus in contrast to Mendelian genetics, where genes from both parents determine the genotype; mitochondrial genetics is entirely different. My mitochondrial DNA was inherited from my mother, who inherited her own mitochondrial DNA from her mother. Thus if you backtrack up the family tree, you will find the mitochondrial eve, the proverbial first lady ( or a cohort of ladies, to be more precise).
So, in essence, mitochondrial genetics is population genetics. They are widely used in anthropological, demographical and forensic studies. But not all mitochondrial genes are same, mutations occur very frequently and theres no paternal gene to compensate for them. Unmutated mitochondrial genes can coexist with mutated ones, in a state called heteroplasmy (as opposed to homoplasmy where all genes are the same). Mutations cost dear, since mitochondrial DNA lacks the repair mechanism of their nDNA counterpart. In addition they lack the protective histone covering.

As is expected, tissues having high energy demands suffer. Leber's hereditary optic neuropathy(LHON), cardiopathy, myopathy (ragged red fibres: surplus mitochondria give them these distinctive appearance on histology), Luft's syndrome (a very rare condition where the person sweats a lot to dissipate the extra heat generated by mitochondria gone wild, in addition to raised core body temperature and polyphagia and other abnormalities), MELAS (mitochondrial encephalomyopathy, lactic acidosis, stroke) and many others.
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