More about mitochondria
DNA is packaged up in the nucleus of all cells. However, did you know that DNA is also found in mitochondria? This mitochondrial DNA is being used for DNA barcoding.
What are mitochondria?
Mitochondria are membrane-enclosed organelles found in most eukaryotic cells. They have their own DNA and replicate independently.
Mitochondrion
Diagram of a mitochondrion showing the inner and outer membranes, and the folded cristae.
Mitochondria are often referred to as the ‘power station’ of cells. They’re the home of cellular respiration and generate energy essential for cells to keep going.
Explore this further in the article Mitochondria – cell powerhouses to discover more about the important role they play in releasing energy from food.
The mitochondrial genome
The entire human mitochondrial genome was sequenced in 1981. It is a circular DNA molecule containing 37 genes and is 16,568 base pairs in length.
Why do mitochondria have their own DNA?
Mitochondria used to be independent organisms and probably evolved to work together with other cells in a symbiotic relationship. Over time, they’ve evolved to depend completely on their hosts.
What’s special about the mitochondrial genome?
The mitochondrial genome has some rather special characteristics:
It’s maternally inherited.
It’s apparently free from recombination.
It lacks introns.
It consists primarily of coding regions.
It evolves rapidly.
Its genome organisation is generally conserved.
CO1 – a gene from mitochondria
Cytochrome c oxidase subunit 1 (CO1) is one of the genes in the mitochondrial genome. It codes for a large transmembrane protein. This protein is highly conserved across species that use mitochondria to generate energy.
Mitochondria and DNA barcoding
A region of the CO1 gene region is approved by the Consortium for the Barcode of Life (CBOL) for assembling DNA barcodes. This is the only gene sequence currently approved for DNA barcoding, thought it’s reasonable to expect that other gene sequences will be approved over time.
For further information, see the article The ideal barcoding gene.
For DNA barcoding projects, mitochondrial DNA has several advantages over nuclear DNA:
Multiple copies – there are multiple copies of the mitochondrial genes in each cell. In comparison, there are only two copies of nuclear genes. Multiple copies of mitochondrial genes make it easier to obtain DNA for PCR and sequencing.
No introns – all of the mitochondrial DNA codes for proteins. There are few non-coding sequences, called introns. This makes mitochondrial genes shorter and easier to work with.
Sequence variation – mitochondrial genes are 5–10 times more variable between species than nuclear genes, but show a very low (often only 1–2%) amount of variation within the same species.
Useful link
Etch A Cell ran the Powerhouse Hunt project, in which users were asked to draw around each mitochondria seen in images. The goal was to significantly improve understanding of mitochondrial biology and, by assisting with the process of manual segmentation, this helped train computers to automatically segment. The results can be see here. You could use the images in your class.
