mitochondria
Meaning of mitochondria: Muscle and nerve fail first when these break down, because those tissues burn the most energy and store the least.
Definition of mitochondria
Mitochondria are compartments inside almost every human cell that convert nutrients and oxygen into a chemical fuel the cell can spend. They sit in the cytoplasm, the jelly that fills a cell around its nucleus, and each one carries a small circular set of its own DNA, separate from the DNA held in the nucleus. The singular is mitochondrion.
What do mitochondria actually do?
Their best known job is energy. Fuel drawn from food is broken down in stages, and the final stage happens on the folded inner membrane of the mitochondrion, where a chain of proteins passes electrons along and uses the energy released to make adenosine triphosphate, the molecule cells spend on everything they do. Oxygen accepts the electrons at the end of that chain, which is the reason breathing matters at the level of a single cell.
Energy is not the whole story. Mitochondria buffer calcium, take part in building steroid hormones and part of the hemoglobin molecule, and hold proteins that, when released, trigger programmed cell death. A cell decides whether to die largely through signals passing across the mitochondrial membrane. They are also numerous rather than solitary: a heart muscle cell may contain thousands, while a mature red blood cell, which sits in the cytoplasm category of exceptions, carries none at all.
Why is mitochondrial DNA different?
| Mitochondrial DNA | Nuclear DNA | |
|---|---|---|
| Shape | Small circle | Long linear chromosomes |
| Copies per cell | Many, often hundreds | Two of each chromosome |
| Inheritance | From the mother only | From both parents |
| Genes carried | A few dozen, all tied to energy production | Roughly twenty thousand |
Because a cell holds many copies, a mutation is usually present in some copies and absent from others, a mixture called heteroplasmy. The proportion carrying the fault helps determine whether anything goes wrong, which is why the same inherited change can produce serious illness in one family member and almost nothing in another.
What happens when they fail?
Tissues that never stop working feel it first. Faults in mitochondrial function, whether from mitochondrial DNA or from the many nuclear genes that also supply mitochondrial parts, tend to show up in muscle, in nerve tissue, in the brain, the eye, the inner ear and the heart. The resulting disorders vary widely in age of onset and in which organs are involved, which makes them notoriously hard to recognize from symptoms alone.
Used in a sentence
Genetic testing identified a mutation within the patient's mitochondrial DNA.
Good to know
Mitochondrial DNA passes from a mother to all her children and from no father to any child, a pattern distinct from the inheritance of nuclear genes; many mitochondrial disorders nonetheless arise from nuclear genes that supply mitochondrial components.
Word origin
Where “mitochondria” comes from
Formed from Greek mitos, meaning thread, and khondrion, meaning small granule.