Tag: Synthesis  ·  Subject: Science & Technology  ·  Format: Perspective

Your cells carry the descendants of a bacterium

Nearly every cell in your body contains small compartments that help turn energy from food into ATP, a molecule cells use to power much of their work.¹ They are called mitochondria (singular: mitochondrion).

Mature red blood cells are the exception.²

The ancestor of mitochondria was a bacterium, an organism of its own.³

Roughly two billion years ago, another single-celled organism, the host cell, ended up with that bacterium inside it.

The host cell was an ancestor of every animal, plant and fungus alive today, including you.³

The date is an estimate. Published figures vary widely. “Roughly two billion years” is a summary, not a measurement.⁴

This piece follows one thread: what became of the bacterium after the merger.

A bacterium ended up inside the host cell

One proposed explanation is that the host cell swallowed the bacterium.^3,5 In this account, the bacterium was not digested. It kept living inside, and when the host cell divided, its descendants passed into the new cells.

This is disputed. Poole and Gribaldo, in a 2014 review, argue that swallowing is the only route that fits current data.⁵ Martin and colleagues, in a 2017 review, argue that a host cell without mitochondria may not have had the energy to feed on prey this way. They propose that the bacterium got inside another way.⁶ That argument rests on energy calculations. Neither version was observed directly, and the question is open.

What is not in doubt is the outcome: the descendants of the two lineages became part of the same cell.

Two clues show its ancestor was an organism of its own

A mitochondrion has DNA of its own (DNA is the molecule that holds a cell’s instructions).^3,8 This mitochondrial DNA is kept apart from the nuclear DNA, which sits in the nucleus (the compartment that holds most of a cell’s DNA).

The stronger clue is in the genes (stretches of DNA that hold instructions).

Comparisons of mitochondrial genes with those of living organisms trace their ancestry to bacteria related to the alphaproteobacteria, a large group still alive today.^3,7

If the ancestor of mitochondria was an organism of its own, why do mitochondria now depend on the cell they sit in?

It was rebuilt, not just kept

Human mitochondria kept only a few genes of their own. Thirteen of them are instructions for making proteins (the molecules that do a cell’s work).⁸

The bacterial ancestor of mitochondria had thousands of genes.⁹

Some of the missing genes moved into the host cell’s own DNA. Many were lost.^10,11

A moved gene now sits in nuclear DNA. Your cell reads it, makes the protein, and sends it into the mitochondrion.

In humans, about ninety-nine in every hundred of a mitochondrion’s proteins are made by your cell and sent in.¹²

Even the protein that copies mitochondrial DNA is made by your cell.¹³

So a mitochondrion is more than its bacterial ancestor kept inside a cell. Over time it was rebuilt, mostly from instructions held in the nucleus.

The merger changed both sides. The host cell came to rely on the mitochondrion for energy. The mitochondrion came to rely on proteins made from nuclear genes.

Whose cell is it?

Ask which part of a mitochondrion is yours and which is borrowed. There is no clean line.

In humans, mitochondrial DNA is normally inherited from the mother.¹⁴ Nearly all of a mitochondrion’s proteins are built from your nuclear genes.

One compartment runs on two sets of instructions.

Thoughtwyre synthesis. This is our reading, not a reported finding. Many of the bacterium’s genes were lost or moved into the host cell’s own DNA.^10,11 In some cases, where the host cell could already perform a function, the bacterium’s own copy of a gene may have become redundant (unnecessary). Genes that are no longer needed can be lost.^10,15 A moved gene rarely goes back.^9,10 Together, these losses and moves may have made a return to independent life harder. No decision to stay was needed.

Mitochondria descend from bacteria that were organisms of their own. Over time they were rebuilt, until your cells supply nearly all of their proteins.

The oldest merger in your body is older than your body.

Glossary

Mitochondrion (plural: mitochondria) — A small compartment inside a cell that helps turn energy from food into ATP.

ATP — A molecule that cells use as a ready supply of energy to power much of their work.

Bacterium — A single-celled organism with no nucleus.

Alphaproteobacteria — A large and varied group of bacteria, many still living today.

Cell — The basic unit of living things. Your body has trillions.

Host cell — The single-celled organism that came to hold the bacterium that became mitochondria.

DNA — The long molecule that carries a cell’s instructions.

Gene — A stretch of DNA that holds the instructions for one protein, or for another working molecule.

Protein — A molecule that does a cell’s work: building, moving, copying, powering.

Nucleus — The compartment in most cells that holds most of the DNA. “Nuclear DNA” and “nuclear genes” are the DNA and genes kept there.

Redundant — Not needed, because something else already does the same job.

Sources

1. Cooper G.M. The Cell: A Molecular Approach, 2nd edition, chapter “Mitochondria”. Sinauer Associates, 2000. (NCBI Bookshelf)
   Supports: how mitochondria help turn food energy into ATP.
2. Moras M., Lefevre S.D., Ostuni M.A. From erythroblasts to mature red blood cells: organelle clearance in mammals. Frontiers in Physiology, 2017.
   Supports: mature red blood cells lose their organelles, mitochondria included.
3. Gray M.W., Burger G., Lang B.F. Mitochondrial evolution. Science 283(5407):1476–1481, 1999.
   Supports: mitochondria descend from bacteria; they keep DNA of their own.
4. Wang S., Luo H. Dating Alphaproteobacteria evolution with eukaryotic fossils. Nature Communications 12, 2021. doi 10.1038/s41467-021-23645-4.
   Supports: dates the origin of Alphaproteobacteria at about 1.9 billion years ago, using eukaryote fossils; context for the “roughly two billion years” estimate, which is approximate and varies between studies.
5. Poole A.M., Gribaldo S. Eukaryotic origins: how and when was the mitochondrion acquired? Cold Spring Harbor Perspectives in Biology 6(12):a015990, 2014.
   Supports: the view that swallowing (phagocytosis) is the only route that fits current data.
6. Martin W.F., Tielens A.G.M., Mentel M., Garg S.G., Gould S.B. The physiology of phagocytosis in the context of mitochondrial origin. Microbiology and Molecular Biology Reviews 81(3):e00008-17, 2017.
   Supports: the view that swallowing was probably not the route; the energy-calculation argument.
7. Martijn J. et al. Deep mitochondrial origin outside the sampled alphaproteobacteria. Nature 557:101–105, 2018 (abstract).
   Supports: mitochondria trace to bacteria related to the alphaproteobacteria.
8. Anderson S. et al. Sequence and organization of the human mitochondrial genome. Nature 290:457–465, 1981.
   Supports: human mitochondrial DNA is a small genome of 37 genes, 13 of them for proteins.
9. Butenko A., Lukeš J., Speijer D., Wideman J.G. Mitochondrial genomes revisited: why do different lineages retain different genes? BMC Biology 22:15, 2024.
   Supports: the ancestor had thousands of genes; mitochondrial genomes keep between 1 and about 70; gene loss came in bursts; a moved gene rarely goes back.
10. Kelly S. The economics of organellar gene loss and endosymbiotic gene transfer. Genome Biology 22:345, 2021.
    Supports: many genes were lost or moved to the host; unneeded genes can be lost; transfer is hard to reverse.
11. Lang B.F. et al. An ancestral mitochondrial DNA resembling a eubacterial genome in miniature. Nature 387:493–497, 1997.
    Supports: a living relative keeps 97 genes in its mitochondrial DNA, so other mitochondrial genomes have shed genes.
12. Rath S. et al. MitoCarta3.0: an updated mitochondrial proteome now with sub-organelle localization and pathway annotations. Nucleic Acids Research, 2021.
    Supports: about 99 in every 100 mitochondrial proteins are made from nuclear genes; 13 come from mitochondrial DNA.
13. Copeland W.C. Defects in mitochondrial DNA replication and human disease. Critical Reviews in Biochemistry and Molecular Biology, 2012.
    Supports: the enzyme that copies mitochondrial DNA is made from a nuclear gene.
14. Luo S. et al. Biparental inheritance of mitochondrial DNA in humans. PNAS, 2018. Disputed by Balciuniene J., Balciunas D. A nuclear mtDNA concatemer (Mega-NUMT) could mimic paternal inheritance of the mitochondrial genome. Frontiers in Genetics 10:518, 2019 (background for “normally”).
    Supports: mitochondrial DNA is normally maternal; a 2018 report of paternal inheritance is disputed.
15. Moran N.A., Mira A. The process of genome shrinkage in the obligate symbiont Buchnera aphidicola. Genome Biology 2(12):research0054, 2001 (a bacterial symbiont of aphids; cited for the general principle that unneeded genes are lost).
    Supports: genes that are no longer needed are lost in symbionts.