CELL PICTURES: Mitochondrial DNA (mtDNA)

mtDNA image #1: Sectioned Mitochondrion with Mitochondrial DNA shown spooling out from organelle; this image is 500 pixels across the original is 4000 x 4000 pixels.

Mitochondria are small organelles concerned with energy production in the cell. Cells with high energy requirements have large numbers of mitochondria. Mitochondria are double membraned organelles with the inner membrane thrown into a series of folds (cristae) that cross the interior of the mitochondrion like bulkheads in the hull of a ship. The spaces between these cristae is filled with the mitochondrial matrix.

Mitochondria contain their own genetic material in the form of circular DNA (shown in close up as a coloured twisted ring). The mitochondrial DNA lies in the matrix (a small piece of DNA can be seen in the upper right of the mitochondrion). This "genome" is reminiscent of the circular bacterial DNA genome. Indeed, mitochondria are thought to be derived from bacteria that were taken into cells in the distant past. Like permanent parasites, they now replicate in parallel with their "host" cells. However, mitochondria are now crucial to cellular functioning and so are more properly regarded as symbionts. When cells divide, their mitocondria divide as well and pass on their DNA to their daughter mitochondria. This is a non-sexual process and so the mitochondrial DNA does not get rearranged. However it does suffer from mutations and so drifts in its sequence across the generations. During sexual reproduction, the egg cell retains its mitochondria and with it the mother's mitochondrial DNA. This maternal mitochondrial DNA allows us to track back family trees down the maternal line. mtDNA is therefore important in phylogeny and in forensics.

mtDNA image #2: Mitochondrion (in green) with emphasised Mitochondrial DNA shown in foreground; this image is 500 pixels across the original is 4096 x 4096 pixels.

mtDNA image #3: Mitochondrion (in blue) with Mitochondrial DNA (thin red circle) shown to scale; this image is 500 pixels across the original is 4096 x 4096 pixels.

Human mitochondrial DNA comprises 16,569 base pairs (Sequence published in 1981 in NATURE: Sequence and organization of the human mitochondrial genome) which equates to a total length of about 5.6 microns or somewhat over eleven times the width of an average mitochondrion (mitochondria are about 0.5 microns or 500 nm across). In this image, the feint red circle therefore illustrates the relative size of the human mitochondrial DNA molecule to a mitochondrion. In the mitocondrion, this circular DNA genome would be scrunched down and barely visible. There are generally several copies of the genome per mitochondrion.