[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"article-detail":3},{"lang":4,"article":5,"alternate":19,"related":24,"latest":33},"en",{"id":6,"slug":7,"title":8,"content":9,"summary":10,"thumbnail":11,"metaDescription":12,"metaKeywords":13,"created":14,"modified":14,"author":15,"authorEn":15,"categoryId":16,"commentCount":17,"thumbnailToContent":18},12674,"mitochondrial-dna-endosymbiosis-hbck","Why Mitochondria Keep Their Own DNA","\u003Cp style=\"margin:0 0 18px;\">In 1967, a 29-year-old biologist submitted a paper. It was 50 pages long and packed with evidence and reasoning. It was rejected.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">She tried another journal. Rejected again. Another, and another — fifteen journals in a row turned the manuscript down.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Her name was Lynn Margulis, publishing under her married name at the time: Lynn Sagan — the then-wife of Carl Sagan, who would later host \u003Ci>Cosmos\u003C\u002Fi>. The paper finally appeared in the \u003Ci>Journal of Theoretical Biology\u003C\u002Fi>.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Then the field laughed at her for nearly twenty years. And every claim she made, piece by piece, turned out to be right.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">\u003Cimg src=\"\u002Fattachment\u002F20261001\u002F98f5769256784c94b52ac1326d95f218.webp\" alt=\"Original title page of Lynn Margulis 1967 paper On the Origin of Mitosing Cells\">\u003C\u002Fp>\n\u003Ch2 style=\"color:#111;font-size:21px;line-height:1.4;margin:28px 0 12px;\">\u003Cstrong>The Evidence That Backed Her Up\u003C\u002Fstrong>\u003C\u002Fh2>\n\u003Cp style=\"margin:0 0 18px;\">The evidence accumulated slowly. Mitochondrial DNA is a completely separate system from nuclear DNA. Nuclear DNA is linear, wrapped around histones, packed into chromosomes. Mitochondrial DNA is a circular loop, bare of histones — structurally identical to a bacterial genome.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Mitochondria carry their own ribosomes, sized 70S, the same as bacteria. The cytoplasm's ribosomes are 80S. Mitochondria reproduce by binary fission — copy the DNA, pinch the cell in two — exactly like bacteria.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">\u003Cimg src=\"\u002Fattachment\u002F20261001\u002Fd905d9b69d8041f4a25d4523269e8141.webp\" alt=\"Linear nuclear DNA wrapped on histones compared with a circular mitochondrial DNA loop\">\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Then gene sequence comparisons delivered the decisive evidence: mitochondria are most closely related to alpha-proteobacteria — a group of bacteria that can respire aerobically.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">So Margulis's argument: an aerobic bacterium was swallowed into the cytoplasm of an anaerobic host cell; the two became mutually dependent, and the first aerobic eukaryotes were born. By the early 1980s, more and more sequence data supported the theory, and endosymbiosis was accepted. Margulis had waited nearly twenty years.\u003C\u002Fp>\n\u003Ch2 style=\"color:#111;font-size:21px;line-height:1.4;margin:28px 0 12px;\">\u003Cstrong>The Genome That Shrank\u003C\u002Fstrong>\u003C\u002Fh2>\n\u003Cp style=\"margin:0 0 18px;\">But here is the puzzle the theory created. If mitochondria were once complete bacteria, their ancestor should have carried a full genome encoding every protein needed for survival. Today, the human mitochondrial genome is only 16,569 base pairs, encoding 37 genes: 13 protein-coding genes, 22 tRNA genes, and 2 rRNA genes.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Where did the rest go?\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Most of it moved into the nucleus. Over the course of symbiotic evolution, mitochondrial genes migrated one by one into the host's nuclear genome; once relocated, the original copies in the mitochondrion were silenced or lost. Some genes vanished entirely — the nucleus already had functionally redundant versions, so keeping a copy made no sense. The mitochondrial genome has been shrinking throughout evolution.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">\u003Cimg src=\"\u002Fattachment\u002F20261001\u002Fb33a0b4b31f444cdb82c12e802e2b726.webp\" alt=\"Gene fragments leaving circular mitochondrial DNA and migrating into nuclear chromosomes\">\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">And genomic studies in 2022 showed the shrinking is still happening today. Wei Wei's team analyzed whole-genome sequencing data from 66,083 humans and found 1,637 nuclear-embedded mitochondrial DNA segments (NUMTs) in total. Over 99 percent of people carry at least one. Most of these segments come from non-coding regions of mitochondrial DNA — transcriptionally and replication-active regions release DNA fragments more readily. When the nuclear genome suffers a double-strand break, free mitochondrial fragments can be filled into the break as part of the repair. In 8,201 parent-offspring trios, the team found three brand-new insertions: neither parent had them, the child did. That works out to roughly one new mitochondrial DNA insertion per 4,000 newborns.\u003C\u002Fp>\n\u003Ch2 style=\"color:#111;font-size:21px;line-height:1.4;margin:28px 0 12px;\">\u003Cstrong>The Thirteen That Refuse to Leave\u003C\u002Fstrong>\u003C\u002Fh2>\n\u003Cp style=\"margin:0 0 18px;\">Which raises the real question: genes can transfer, and they keep transferring — so why do 13 protein-coding genes remain in the mitochondrion, untouched for over a billion years?\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">All 13 encode the oxidative phosphorylation system:\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Complex I: 7 subunits (ND1 through ND6 and ND4L)\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Complex III: 1 (cytochrome b)\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Complex IV: 3 (COX1 through COX3)\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">ATP synthase: 2 (ATP6 and ATP8)\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Every one of them is a protein of the respiratory chain.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">\u003Cimg src=\"\u002Fattachment\u002F20261001\u002Fc97714cbb1bb4b0e93c07e46ec81da70.webp\" alt=\"Respiratory chain complexes I, III, IV and ATP synthase embedded in the inner membrane\">\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Research shows that the genes retained across the most species code for the most hydrophobic proteins, and carry the highest GC content. In 2022, a Bayesian analysis covering 10,328 mitochondrial and 5,176 chloroplast genomes confirmed the pattern: hydrophobicity and GC content are the strongest predictors of which genes stay and which go. And the rule transfers: the prediction rules trained on mitochondrial data predicted chloroplast gene retention just as well. Yet mitochondria and chloroplasts came from two entirely independent endosymbiotic events — an alpha-proteobacterium and a cyanobacterium. Shared retention rules suggest a shared selective pressure.\u003C\u002Fp>\n\u003Ch2 style=\"color:#111;font-size:21px;line-height:1.4;margin:28px 0 12px;\">\u003Cstrong>Why Hydrophobicity Blocks the Move\u003C\u002Fstrong>\u003C\u002Fh2>\n\u003Cp style=\"margin:0 0 18px;\">The 13 retained proteins are all membrane proteins, embedded in the mitochondrial inner membrane, with transmembrane regions dense in water-insoluble amino acids — that is precisely what anchors them in the lipid bilayer.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Move those genes to the nucleus, and their proteins would be synthesized on cytoplasmic ribosomes. Their highly hydrophobic transmembrane segments would be intercepted by the signal recognition particle (SRP) of the endoplasmic reticulum, and the protein would be delivered to the ER membrane — never reaching the mitochondrial inner membrane. The protein goes to the wrong address, and the corresponding respiratory complex cannot be assembled.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">The few species that did successfully transfer individual genes from mitochondria to the nucleus all show the same adaptations: reduced transmembrane hydrophobicity, or transmembrane regions moved to the C-terminus, or the gene split into two separately encoded pieces. Without structural change, the transfer cannot succeed.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">GC content is tied to the mitochondrial environment the same way. The respiratory chain constantly generates superoxide radicals as electrons are passed, so the reactive oxygen concentration inside mitochondria is high. GC base pairs have three hydrogen bonds, AT pairs only two — GC is more stable and less likely to degrade in that oxidizing environment. Genes that survive long-term inside mitochondria need high GC content.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">So: hydrophobicity prevents proteins from being correctly shipped from the cytoplasm to the inner membrane, and high GC content keeps DNA stable in the mitochondrial oxidative environment. A billion-plus years of screening — everything that could transfer, already has.\u003C\u002Fp>\n\u003Ch2 style=\"color:#111;font-size:21px;line-height:1.4;margin:28px 0 12px;\">\u003Cstrong>The Second Reason: Speed\u003C\u002Fstrong>\u003C\u002Fh2>\n\u003Cp style=\"margin:0 0 18px;\">But \"cannot transfer\" is only half the story. Even if transfer were technically possible, mitochondria need these genes on site.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">The respiratory chain works by passing electrons between complexes and pumping protons across the membrane, building a membrane potential that drives ATP synthase. If a complex subunit is damaged, or electron transfer efficiency drops, the membrane potential changes — and the mitochondrion must rapidly adjust expression of the corresponding proteins.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">If all genes sat in the nucleus, the response path would be long: signal from mitochondrion to nucleus, nuclear transcription, mRNA translation, protein shipped back, assembled into the respiratory chain. If the gene is local, a change in membrane potential can directly trigger local gene expression — far faster.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">That is the core of the CoRR hypothesis: co-location for redox regulation.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">\u003Cimg src=\"\u002Fattachment\u002F20261001\u002F986eddf989e54ef48ebf7c9010ebd2c0.webp\" alt=\"Mitochondrion regulating local gene expression from membrane potential changes\">\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">And it makes sense at the level of the whole cell. A eukaryotic cell can hold anywhere from a few hundred to several thousand mitochondria, each regulating its own gene expression according to its own energy state. If every mitochondrial gene sat in the nucleus, the nucleus would have to coordinate hundreds of independent mitochondrial states at once. Each mitochondrion carrying its own genome, regulating its own respiratory chain — far more efficient.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">That is why it keeps its own DNA.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Want to see what a bacterium — the ancestor that became the mitochondrion — actually looks like? \u003Cstrong>WWAI\u003C\u002Fstrong> includes an \u003Cstrong>Escherichia coli specimen\u003C\u002Fstrong> where gram-negative short rods are clearly visible. Just search \"WWAI\" in your app store and download it today.\u003C\u002Fp>\n\u003Cdiv class=\"dp-template-card\" style=\"border-radius:8px;box-shadow:0 2px 8px rgba(0,0,0,0.1);margin:10px 0;max-width:100%;overflow:hidden;width:100%;\">\n \u003Ca style=\"display:block;text-decoration:none;\" href=\"https:\u002F\u002Fyun-hub.chat\u002Flink\u002F?app=wwai&clickid=stellarx&dplink=specimenid%3D1046\" target=\"_blank\">\u003Cimg class=\"image_resized\" style=\"display:block;height:auto;max-width:100%;width:100%;\" src=\"\u002Fattachment\u002F20260824\u002F5ba5dd3c38404fe785f43c44423830de.png\" alt=\"5ba5dd3c38404fe785f43c44423830de\">\n  \u003Cbutton style=\"align-items:center;background-color:#1f983e;border-radius:0 0 25px 25px;border-style:none;color:#ffffff;cursor:pointer;display:flex;font-family:Times New Roman;font-size:19px;height:40px;justify-content:center;padding:0;width:100%;\">EXPLORE NOW\u003C\u002Fbutton>\u003C\u002Fa>\n\u003C\u002Fdiv>\n\u003Cp style=\"margin:0 0 18px;\">References:\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">[1] Lane N, Martin W. The energetics of genome complexity. Nature, 2010, 467(7318): 929-934.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">[2] Wei W, Schon KR, Elgar G, et al. Nuclear-embedded mitochondrial DNA sequences in 66,083 human genomes. Nature, 2022, 611(7934): 105-114.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">[3] Johnston IG, Williams BP. Evolutionary inference across eukaryotes identifies specific pressures favoring mitochondrial gene retention. Cell Systems, 2016, 2(2): 101-111.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">[4] Giannakis K, Arrowsmith SJ, Richards L, et al. Evolutionary inference across eukaryotes identifies universal features shaping organelle gene retention. Cell Systems, 2022, 13(11): 874-884.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">[5] Björkholm P, Harish A, Hagström E, et al. Mitochondrial genomes are retained by selective constraints on protein targeting. PNAS, 2015, 112(33): 10154-10161.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">[6] Allen JF. Control of gene expression by redox potential and the requirement for chloroplast and mitochondrial genomes. Journal of Theoretical Biology, 1993, 165(4): 609-631.\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 18px;\">Figure 1 is the original title page of Margulis (1967); all other illustrations are AI-generated.\u003C\u002Fp>\n\u003Cp style=\"color:#111;margin:28px 0 8px;\">\u003Cstrong>Related reading\u003C\u002Fstrong>\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 20px;\">\u003Ca style=\"color:#111;text-decoration:none;\" href=\"https:\u002F\u002Fstellarx.tech\u002Farticle\u002Ffood-coma-rice-slump-mzrb\">\u003Cimg class=\"image_resized\" style=\"border-radius:8px;display:block;height:auto;margin-bottom:6px;max-width:520px;width:100%;\" src=\"https:\u002F\u002Fcdn.banyunjuhe.com\u002Fattachment\u002F20260929\u002F1464a50bbff14215a2565949348dd3f9.webp\" alt=\"Food Coma: Why Heavy Rice Meals Drain Your Energy\">\u003C\u002Fa>\u003Cbr>\u003Ca style=\"color:#0b6cb0;text-decoration:none;\" href=\"https:\u002F\u002Fstellarx.tech\u002Farticle\u002Ffood-coma-rice-slump-mzrb\">\u003Cstrong>Food Coma: Why Heavy Rice Meals Drain Your Energy\u003C\u002Fstrong>\u003C\u002Fa>\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 20px;\">\u003Ca style=\"color:#111;text-decoration:none;\" href=\"https:\u002F\u002Fstellarx.tech\u002Farticle\u002Fafternoon-slump-second-wind\">\u003Cimg class=\"image_resized\" style=\"border-radius:8px;display:block;height:auto;margin-bottom:6px;max-width:520px;width:100%;\" src=\"https:\u002F\u002Fcdn.banyunjuhe.com\u002Fattachment\u002F20260929\u002F2073711f32524a08a70b404490e7b2f3.webp\" alt=\"Why You're Sleepy at 3 PM but Wide Awake at 7 PM\">\u003C\u002Fa>\u003Cbr>\u003Ca style=\"color:#0b6cb0;text-decoration:none;\" href=\"https:\u002F\u002Fstellarx.tech\u002Farticle\u002Fafternoon-slump-second-wind\">\u003Cstrong>Why You're Sleepy at 3 PM but Wide Awake at 7 PM\u003C\u002Fstrong>\u003C\u002Fa>\u003C\u002Fp>\n\u003Cp style=\"margin:0 0 20px;\">\u003Ca style=\"color:#111;text-decoration:none;\" href=\"https:\u002F\u002Fstellarx.tech\u002Farticle\u002Flobe-finned-fish-human-evolution\">\u003Cimg class=\"image_resized\" style=\"border-radius:8px;display:block;height:auto;margin-bottom:6px;max-width:520px;width:100%;\" src=\"https:\u002F\u002Fcdn.banyunjuhe.com\u002Fattachment\u002F20260929\u002F439279ee01a442458b5244cab45d59f9.webp\" alt=\"Humans Are Lobe-Finned Fish: The Evolution Proof\">\u003C\u002Fa>\u003Cbr>\u003Ca style=\"color:#0b6cb0;text-decoration:none;\" href=\"https:\u002F\u002Fstellarx.tech\u002Farticle\u002Flobe-finned-fish-human-evolution\">\u003Cstrong>Humans Are Lobe-Finned Fish: The Evolution Proof\u003C\u002Fstrong>\u003C\u002Fa>\u003C\u002Fp>","","\u002Fattachment\u002F20261001\u002Ff3cd7f3b06cf4354bc7cb2d3490dd163.webp","Mitochondria were once free-living bacteria; most of their genes moved into the nucleus long ago. The 13 that stayed are too hydrophobic to ship back.","stellarx, wwai, mitochondrial DNA, endosymbiosis, hydrophobicity, CoRR hypothesis","2026-10-01 15:46:35","Science Guide Wwai",77,0,false,{"id":20,"slug":21,"title":22,"lang":23},12665,"hong-kong-milk-tea-vs-bubble-tea","港式奶茶為什麼這麼苦？「絲襪奶茶」和珍珠奶茶到底有什麼不同？","zh",{"prev":25,"next":29},{"id":26,"slug":27,"title":28,"categoryId":16},12675,"constipation-colon-motility-rdnp","Why Constipation Happens: A Colon Story",{"id":30,"slug":31,"title":32,"categoryId":16},12673,"brain-fog-fascia-stretch-vpnc","Brain Fog Fix: The Stretch That Clears Your Head",[34,39,44,49],{"id":35,"slug":36,"title":37,"thumbnail":38,"categoryId":16},12685,"food-adulteration-history-bwxa","Ancient Food Was Never Pure: Urine, Lye, and Fake Fish","\u002Fattachment\u002F20261001\u002F03bbfa285e6545d6b3144a2fb3e790f9.webp",{"id":40,"slug":41,"title":42,"thumbnail":43,"categoryId":16},12684,"glowing-algae-story-zptc","When Lockdown Laziness Grew Glowing Algae","\u002Fattachment\u002F20261001\u002F2915a99ca6dd44bb99d4131bc2f639b5.webp",{"id":45,"slug":46,"title":47,"thumbnail":48,"categoryId":16},12683,"alpha-gal-syndrome-gmht","Why a Tick Bite Can Make You Allergic to Red Meat","\u002Fattachment\u002F20261001\u002Faa24a6f7e36b4b2fb5b8a0f6c3bfd1d8.webp",{"id":50,"slug":51,"title":52,"thumbnail":53,"categoryId":16},12682,"centipede-leg-count-comparison-lqzv","Who Has More Legs: House Centipede or Centipede?","\u002Fattachment\u002F20261001\u002Fb139d5c1b20648f39e4eca5fb29ac3f2.webp"]