There are two hypotheses about the origin of mitochondria, namely the endosymbiosis theory and the non-endosymbiosis theory.
Internal symbiosis theory
The theory believes that mitochondria originated from mitochondrial ancestors that were swallowed by another cell-the original mitochondria-a gram-negative bacterium capable of tricarboxylic acid cycle and electron transfer. This aerobic bacteria is a branch under the phylum Proteobacteria and is closely related to Rickettsia. After the original mitochondria are swallowed, they are not digested, but form a symbiotic relationship with the host cell-the host can get more nutrients from the host, and the host can use the energy produced by the host-this relationship increases the cell's Competitiveness, so that it can adapt to more living environments. In the long-term mutually beneficial symbiosis that benefited both the host and the host, the original mitochondria gradually evolved into mitochondria [7], which successfully coupled glycolysis in the host cell with the tricarboxylic acid cycle and oxidative phosphorylation in the original mitochondria. Some studies believe that this symbiotic relationship occurred approximately 1.7 billion years ago, almost the same as the period of evolutionary divergence to produce eukaryotes and archaea. However, the order of appearance of mitochondria and eukaryotic nuclei is still controversial.
Evidence that has been found to support the endosymbiosis theory includes:
Genetic information transfer: Recent studies in molecular biology and bioinformatics have found that there is genetic information in the nucleus of eukaryotic cells that may belong to respiratory bacteria or cyanobacteria, indicating that most of the original respiratory bacteria and cyanobacteria have a long common genome. A shift to the nucleus occurred during evolution.
The mitochondrial genome has obvious similarities with the bacterial genome: 1) Mitochondria have their own DNA, which is similar in shape to the circular DNA of bacteria without histone binding; 2) Base ratio, nucleotide sequence, gene structure characteristics, etc. Similar, without 5mC; 3) Mitochondria have their own DNA polymerase and RNA polymerase, which can replicate and transcribe independently; 4) The sedimentation coefficients of its mRNA and rRNA are similar to those of bacteria.
Mitochondria have an independent and complete protein synthesis system: Compared with the protein synthesis system of eukaryotic cells, most of the characteristics of mitochondrial protein synthesis are more similar to those of the bacterial protein synthesis system, including 1) Protein synthesis starts from N-formylmethionine , And eukaryotic cells start from methionine; 2) mitochondrial ribosomes are smaller than the 80S ribosomes of eukaryotes; 3) 5S rRNA exists in mitochondria, chloroplasts, and prokaryotes, while many eukaryotic ribosomes exist 5.8SrRNA; 4) Protein synthesis factors in mitochondria have the recognition specificity of prokaryotic ribosomes, but cannot recognize cytoplasmic ribosomes; 5) Mitochondrial mRNA and mitochondrial ribosomes form polysomes; 6) Protein synthesis on mitochondria and chloroplasts Can be inhibited by chloramphenicol and tetracycline, but cycloheximide, which inhibits protein synthesis in eukaryotes, has no inhibitory effect on them; 7) Mitochondrial RNA polymerase can be inhibited by prokaryotic RNA polymerase inhibitor rifamycin Inhibited, but not inhibited by the eukaryotic RNA polymerase inhibitor actinomycin D, etc.
Mitochondrial division is similar to bacteria: mitochondria and chloroplasts divide and multiply in a constricted manner, similar to bacteria.
Membrane characteristics of mitochondria: The outer mitochondrial membrane is similar to the inner membrane of eukaryotic cells, and the inner mitochondrial membrane is similar to the plasma membrane of bacteria; the protein/lipid ratio of the inner mitochondrial membrane is much larger than that of the outer membrane, which is similar to that of bacteria.
Other characteristics: The phospholipid composition, respiratory type and the primary structure of Cyt c of mitochondria are very close to those of Paracoccus denitrificans or purple non-sulfur photosynthetic bacteria, suggesting that the ancestor of mitochondria may be one of these two bacteria.
Genetic code comparison: The genetic code of mitochondria is more similar to the genetic code of Proteobacteria;
Disadvantages:
From an evolutionary perspective, how to explain that the metabolically dominant symbiont transfers a large amount of genetic information to the host cell instead?
Can't explain how the nucleus evolved, that is, how did prokaryotic cells evolve into eukaryotic cells?
There are introns in the genomes of mitochondria and chloroplasts, but there are no introns in the genomes of eubacterial prokaryotes. If the theory of endosymbiosis origin is agreed, where do the introns in the genomes of mitochondria and chloroplasts occur?
Non-endo-symbiosis theory
The theory of non-endo-symbiosis is also called "the theory of cell division chemistry". It is believed that the occurrence of mitochondria is evolved from the membrane structure in the biomembrane system such as the cell membrane or the endoplasmic reticulum membrane. There are several models of non-endo-symbiosis theory. The mainstream model believes that in the initial stage of cell evolution, the duplication of the prokaryotic cell genome is not accompanied by typical amitosis, but the cell membrane near the nucleus invades to form a double membrane. One of the genomes is surrounded, isolated, and cell division occurs. The undivided daughter cells slowly evolve into highly specialized cell structures such as nucleus, mitochondria and chloroplasts.
Disadvantages:
Little experimental evidence
Can't explain why mitochondria, chloroplasts and bacteria have so many similarities in DNA molecular structure and protein synthesis performance
It is also difficult to explain the source of mitochondria and chloroplast DNA enzymes, RNA enzymes and ribosomes.
Can the nucleus of eukaryotic cells originate from the nuclear area of bacteria?
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