Primer on Mitochondria
1. What are mitochondria and where are they found?
Mitochondria are bean-shaped compartments within cells that supply energy. These compartments, a type of membrane-bound organelle, are found in eukaryotes — organisms whose cells have nuclei, the home of the genome. Multicellular organisms (humans, mice, fish, etc.) as well as some unicellular ones, like yeast, are counted as eukaryotes. Bacteria, though, are not: They are considered prokaryotes for their lack of organelles, including mitochondria and nuclei. Intriguingly, mitochondria vary widely across organisms and even within an organism. Drastic differences can exist in the number of mitochondria per cell, their size and morphology, and even their biochemical capabilities. For example, fatty acids readily broken down by mitochondria in muscle, but not brain tissue. Because of a lack of molecular knowledge about mitochondria and their resident proteins, the basis for such differences is largely unclear.
2. What do mitochondria do? Although mitochondria are perhaps best known for their roles in energy metabolism, they also participate in a plethora of other key biological processes. These include critical functions such as programmed cell death (or “apoptosis”), a normal mechanism through which old or damaged cells can be eliminated. Defects in mitochondria are associated with more than 50 human diseases, ranging from in-born errors of metabolism in infants to neurodegeneration in adults. Moreover, several common diseases, such as cancer and type 2 diabetes, have been associated with mitochondrial dysfunction. Prescription drugs can also disrupt mitochondria. Such drug-induced toxicity is a reason why some drugs are pulled from the market and why some potential drugs fail the clinical trial process.
3. Where do mitochondria come from? Mitochondria, it turns out, have their own tiny genome. And in humans, this mitochondrial DNA is inherited solely from the mother. Such maternal inheritance arises because mitochondria from sperm are lost following fertilization, while those contributed by the egg persist. Because it is maternally inherited, mitochondrial DNA can provide clues about human history, including the most recent common matrilineal ancestor of living humans (so-called “Mitochondrial Eve”.) But there are, in fact, paternal contributions to mitochondria. The parts of the mitochondria that are derived from nuclear genes actually come from both parents (see below). This follows a core principle of human genetics: of the 23 pairs of chromosomes that make up your nuclear genome, roughly half come from Mom and the other half from Dad. Evolutionarily speaking, mitochondria have a very interesting history. They are descendants of an ancient bacterium — a relative of the modern bacterial species, Rickettsia prowazekii — that some 2 billion years ago was enveloped by another cell. That moment marked the beginning of a long and mutually beneficial relationship with eukaryotic cells, known as endosymbiosis. As a result of such “co-habitation”, eukaryotic cells and mitochondria have evolved and adapted to life together, such that now, neither can survive alone.
4. Where do the proteins in mitochondria come from? Because of the organelle’s unusual past, the molecular pieces that make up mitochondria have undergone some shuffling of their own. Mitochondria carry a small circular genome, a vestige of their days as free-living bacteria that has been winnowed during evolution to just a few protein-coding genes. The human mitochondrial genome was decoded in 1981, a full 20 years before the human genome itself was decoded. The organelle’s genome consists of roughly 16,000 chemical units called base pairs, much smaller than the nuclear genome’s 3 billion base pairs. The mitochondrial genome includes just 37 genes: 13 genes that encode proteins and 24 additional non-protein coding genes. The rest of the genes required for a functioning mitochondrion, roughly 1,200 to 1,500 in total, now reside in the nucleus. Identifying these genes from DNA sequence data alone has proven immensely difficult, which is why other large-scale approaches — namely proteomics and computational methods — are required to pinpoint them.
MitoCarta - Protein Catalogue for Mitochondria
CellNEWS - Friday, 11 July 2008
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