How does eukaryotic cells develop

Ribosomes in these organelles are similar to those of bacterial ribosomes , and different from eukaryotic ribosomes. Reproduction is by binary fission, not by mitosis. Biochemical pathways and structures show closer relationships to prokaryotes.

Two or more membranes surround these organelles. What Does it all Mean? Summary Eukaryotic cells probably evolved about 2 billion years ago. Their evolution is explained by endosymbiotic theory. Mitochondria and chloroplasts evolved from prokaryotic organisms. Eukaryotic cells would go on to evolve into the diversity of eukaryotes we know today.

Explore More Use the time slider in this resource to answer the questions that follow. When did cells begin to "swallow" other cells? When did respiration develop? The rapid rise in atmospheric oxygen favored which cells? When did eukaryotic cells first form? What distinguished these cells from their predecessors? So the eukaryotic lineage appears to be very ancient, about as ancient as the two prokaryotic lineages. Eukaryotic cells seem structurally far more complex than their prokaryotic counterparts from which they arose , so biologists generally believe that many evolutionary steps must have separated the two.

Nevertheless, the eukaryotic stem on the phylogenetic tree of life spawns many branches before one gets to the split that separates the ancestors of plants from the ancestors of animals, which seems to have happened more than a billion years ago.

There seem to have been many earlier branchings from the eukaryotic stem, all represented by unicellular eukaryotes such as the slime molds, the flagellates, the trichomonads, the diplomonads, the microsporidia, among others. Our current concept of the origin of the eukaryotic cell is in flux, however, and an evolutionary sequence that appears simple when conceptualized on a phylogenetic tree diagram may be far more complex and interesting in reality.

We know that the eukaryotic cell is of ancient origin, but we do not yet know the evolutionary dynamic that underlies its formation. Peter Gogarten in the department of molecular and cell biology at the University of Connecticut at Storrs, gives a broader overview: "The question is the subject of an ongoing and lively controversy. The best guesses for the time when eukaryotes evolved range from just below 2. Work by Gonzalo Vidal of the University of Uppsala in Sweden indicates that single-celled planktonic eukaryotes certainly date back to 1.

The early fossil record is very sparse, however, and small eukaryotic cells present in the fossil record would not necessarily have been positively identified. My colleagues generally agree that the fossil record provides only a most recent estimate for the time when eukaryotes were already abundant; they might have been around a long time before they made it into the fossil record in a recognizable form.

Mitochondria are energy-producing organelles that are thought to have once been a type of free-living alpha-proteobacterium. One of the major features distinguishing prokaryotes from eukaryotes is the presence of mitochondria. Mitochondria arise from the division of existing mitochondria. They may fuse together. They move around inside the cell by interactions with the cytoskeleton. However, mitochondria cannot survive outside the cell. As the amount of oxygen increased in the atmosphere billions of years ago and as successful aerobic prokaryotes evolved, evidence suggests that an ancestral cell with some membrane compartmentalization engulfed a free-living aerobic prokaryote, specifically an alpha-proteobacterium, thereby giving the host cell the ability to use oxygen to release energy stored in nutrients.

Alpha-proteobacteria are a large group of bacteria that includes species symbiotic with plants, disease organisms that can infect humans via ticks, and many free-living species that use light for energy.

Several lines of evidence support the derivation of mitochondria from this endosymbiotic event. Most mitochondria are shaped like alpha-proteobacteria and are surrounded by two membranes, which would result when one membrane-bound organism engulfs another into a vacuole.

The mitochondrial inner membrane involves substantial infoldings called cristae that resemble the textured, outer surface of alpha-proteobacteria.

The matrix and inner membrane are rich with enzymes necessary for aerobic respiration. Micrograph of mammaliam mitochondria : In this transmission electron micrograph of mitochondria in a mammalian lung cell, the cristae, infoldings of the mitochondrial inner membrane, can be seen in cross-section. Mitochondria divide independently by a process that resembles binary fission in prokaryotes. Specifically, mitochondria are not formed de novo by the eukaryotic cell; they reproduce within the cell and are distributed between two cells when cells divide.

Therefore, although these organelles are highly integrated into the eukaryotic cell, they still reproduce as if they are independent organisms within the cell. However, their reproduction is synchronized with the activity and division of the cell.

Mitochondria have their own circular DNA chromosome that is stabilized by attachments to the inner membrane and carries genes similar to genes expressed by alpha-proteobacteria.

Mitochondria also have special ribosomes and transfer RNAs that resemble these components in prokaryotes. These features all support that mitochondria were once free-living prokaryotes.

Mitochondria that carry out aerobic respiration have their own genomes, with genes similar to those in alpha-proteobacteria. However, many of the genes for respiratory proteins are located in the nucleus. When these genes are compared to those of other organisms, they appear to be of alpha-proteobacterial origin. Additionally, in some eukaryotic groups, such genes are found in the mitochondria, whereas in other groups, they are found in the nucleus.

This has been interpreted as evidence that genes have been transferred from the endosymbiont chromosome to the host genome. This loss of genes by the endosymbiont is probably one explanation why mitochondria cannot live without a host. Despite the transfer of genes between mitochondria and the nucleus, mitochondria retain much of their own independent genetic material.

One possible explanation for mitochondria retaining control over some genes is that it may be difficult to transport hydrophobic proteins across the mitochondrial membrane as well as ensure that they are shipped to the correct location, which suggests that these proteins must be produced within the mitochondria.

Another possible explanation is that there are differences in codon usage between the nucleus and mitochondria, making it difficult to be able to fully transfer the genes.

Plastids may derive from cyanobacteria engulfed via endosymbiosis by early eukaryotes, giving cells the ability to conduct photosynthesis. Some groups of eukaryotes are photosynthetic: their cells contain, in addition to the standard eukaryotic organelles, another kind of organelle called a plastid.

There are three type of plastids: chloroplasts, chromoplasts, and leucoplasts. Chloroplasts are plastids that conduct photosynthesis. Chromoplasts are plastids that synthesize and store pigments. Leucoplasts are plastids located in the non-synthetic tissues of a plant e. Like mitochondria, plastids appear to have a primary endosymbiotic origin, but differ in that they derive from cyanobacteria rather than alpha-proteobacteria.

Cyanobacteria are a group of photosynthetic bacteria with all the conventional structures of prokaryotes. Unlike most prokaryotes, however, they have extensive, internal membrane-bound compartments called thylakoids, which contain chlorophyll and are the site of the light-dependent reactions of photosynthesis. In addition to thylakoids, chloroplasts found in eukaryotes have a circular DNA chromosome and ribosomes similar to those of cyanobacteria.

But how did the eukaryotic cell itself evolve? How did a humble bacterium make this evolutionary leap from a simple prokaryotic cell to a more complex eukaryotic cell? The answer seems to be symbiosis — in other words, teamwork. Evidence supports the idea that eukaryotic cells are actually the descendents of separate prokaryotic cells that joined together in a symbiotic union.