More than 1.5 billion years ago, a momentous event occurred when two tiny primitive cells became one. Perhaps more than any other event, other than the origin of life itself, this merger fundamentally changed the course of evolution on Earth.
One cell eventually found its way inside the other cell, evolving into a structure that schoolchildren learn to call the “powerhouse of the cell”: the mitochondria. This new structure provided significant energetic advantages to the host, which was a prerequisite for the later evolution of complex multicellular organisms.
But that’s only part of the story.of mitochondria It is not the only important structure within complex eukaryotic cells. There is a nucleus covered with a membrane, genome. There is an entire system of endomembranes, including the endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, and vacuoles, that are essential for producing, transporting, and recycling proteins and other cargo in and around the cell.
Where did all these structures come from? This is a very difficult question to tackle because the events are lost in the distant past and there are few traces of evolutionary clues. Researchers have proposed various hypotheses, but cell biologists are using several new tools and techniques to investigate the beginnings of this complex structure and shed some light on its possible origins. It was only recently that it was created.
Microbial fusion
The idea that eukaryotes arose from the union of two cells dates back more than 100 years, but it was not accepted until the 1960s, when late evolutionary biologist Lynne Margulis articulated the endosymbiosis theory, and it became less well-known. It didn’t even turn out. Mitochondria likely originate from a type of microorganism known as Alphaproteobacteria, Margulis said, a diverse group that now includes bacteria that cause scrub typhus and bacteria important in plant genetic engineering. include.
Nothing was known about the nature of the original host cell. Scientists claimed that there are various membrane structures inside, which are already quite complex. Such cells would have been able to ingest objects by swallowing them. This is a complex and energetically expensive eukaryotic function called phagocytosis. That may be how mitochondria first entered the host.
But this idea, called the “late mitochondrial” hypothesis, does not explain why or how host cells became so complex in the first place.
2016, Evolutionary Biologist bill martincell biologist Sven Gould and bioinformatician Shriram Gargfrom the University of Düsseldorf in Germany have proposed a very different model, known as the “mitochondrial early” hypothesis. They argued that because today’s primitive cells lack internal membrane structures, it is highly unlikely that cells from more than 1.5 billion years ago had internal membrane structures.
Instead, scientists believe that the endomembrane system (a collection of parts found inside today’s complex cells) was developed shortly after Alphaproteobacteria colonized a type of relatively simple host cell called an Archaea. They reasoned that it may have evolved.A membrane structure would have arisen from bubbles, or vesicles, released by mitochondrial ancestors.
Gould, Garg, and Martin et al. argue that because free-living bacteria constantly release vesicles for a variety of reasons, it is reasonable to assume that they will continue to release vesicles even when trapped inside a host. It has been pointed out that this is true.
Eventually, these vesicles would become specialized for the functions that membrane structures perform in eukaryotic cells today. They can even fuse with host cell membranes, helping to explain why eukaryotic plasma membranes contain lipids with bacterial characteristics.
Vesicles may have served an important early function, biochemists say dave spyger PhD from the University of Amsterdam. The new endosymbiont would have produced large amounts of toxic chemicals called reactive oxygen species by oxidizing fatty acids and burning them for energy. “They destroy everything and are especially toxic inside cells,” Speiger says. Isolating them in vesicles would have helped protect cells from harm. he says.
Gould, Garg and Martin added that another problem posed by the new guest could have been solved by creating a membrane barrier. When the Alphaproteobacteria arrived, some of their DNA would mix with the archaeal host’s genome, disrupting important genes. To fix this, we need to evolve the machinery to string these foreign bodies (today known as introns) together. messenger RNA: copy of a geneTherefore, these protein manufacturing procedures are not garbled.
But that created yet another problem. Ribosomes, the protein-manufacturing machinery, work very fast, combining several amino acids per second. In contrast, the cellular intron removal system is slow, cutting out approximately one intron per minute. Therefore, if the cell cannot keep the mRNA away from the ribosomes until it is properly processed, the cell will produce a large amount of meaningless and useless material. protein.
The membrane surrounding the nucleus provided the answer. Acting as a spatial barrier, it allows mRNA splicing to finish in the nucleus before intron-free mRNA is translated in the cell’s internal fluid, the cytosol. “This is the selection pressure behind nuclear origins,” Martin says. To form it, vesicles secreted by the endosymbiont flatten and wrap around the genome, creating a barrier that prevents ribosome entry but allows small molecules to pass freely through. You can
