Origin of the Eukaryotes by Symbiosis

Unlike the cells of the more primitive prokaryotes, eukaryotic cells are divided into compartments by membranes (Fig. 19.01). The most important of these is the nucleus, where the chromosomes reside. Eukaryotes are defined by the possession of a nucleus that typically contains several linear chromosomes. Higher eukaryotes are normally diploid and have pairs of homologous chromosomes, although this is not always the case with the less advanced eukaryotes.

In addition to a nucleus, almost all eukaryotic cells (animal, plant or fungus) contain mitochondria. Plant cells contain chloroplasts as well as mitochondria. Both of these organelles provide the majority of energy for all cellular processes. These organelles contain their own genomes and encode at least a few of their own proteins. The organelle genome is prokaryotic in nature. It consists of a circular DNA molecule that is not bound by histones. Mitochondria and chloroplasts synthesize their own ribosomes, which are more closely related to those of bacteria than to those of the eukaryotic cytoplasm. Both mitochondria and chloroplasts are roughly the same size and shape as bacterial cells, and the organelles grow and divide in the same manner as bacteria (Fig. 19.02).When a eukaryotic cell divides, each daughter cell inherits some of its parent's mitochondria and chloroplasts. If either organelle is lost, it cannot be reconstructed because the nucleus does not have all of the genetic information needed to synthesize the entire organelle.

FIGURE 19.02 Chloroplasts arise by Division

Transmission electron micrograph of a dividing chloroplast in a bean seedling. Chloroplasts and mitochondria divide independently of the eukaryotic cell in which they reside. The organelles divide by binary fission in a manner reminiscent of prokaryotic cells. The plastid shown here is technically an "etioplast", a precursor chloroplast that has not yet developed any green pigment. From: Biochemistry and Molecular Biology of Plants by Buchanan, Gruissem and Jones, 2000, American Society of Plant Physiologists.

Mitochondria are derived from ancestral bacteria that specialized in respiration whereas chloroplasts are descended from ancestral photosynthetic bacteria.

The symbiotic theory proposes that the complex eukaryotic cell arose by a series of symbiotic events in which organisms of different lineages merged. The cells of higher organisms are thus not individuals but symbiotic associations. The word "symbiosis" is from the Greek sym, meaning together, and bios, meaning life. The nuclear genes of a eukaryotic cell are sometimes referred to as derived from the "urkaryote." The urkary-ote is the hypothetical ancestor that provided the genetic information found in the present day eukaryotic nucleus.

According to the symbiotic theory, mitochondria are descended from bacteria that were trapped long ago by the ancestors of modern eukaryotic cells. These bacteria received shelter and nutrients, and in return, devoted themselves to generating energy by respiration. During the eons following their capture, these bacteria became narrowly specialized for energy production, lost the ability to survive on their own, and evolved into mitochondria. The term endosymbiosis is sometimes used to indicate those symbiotic associations where one partner is physically inside the other (endo is from the Greek for inside), as in the present case.

Plant cells contain chloroplasts that perform photosynthesis with light harvesting pigments such as chlorophyll. The rRNA from chloroplasts matches rRNA from photosynthetic bacteria better than rRNA from the plant cell nucleus. Thus, the prevailing theory is that chloroplasts descended from photosynthetic bacteria trapped by the ancestors of modern-day plants. Some plants have lost the ability to photosynthesize but still contain defective chloroplasts. The term plastid refers to all organelles that are genetically equivalent to chloroplasts, whether functional or not. Since fungi do not contain chlorophyll, the green light-absorbing pigment of all other plants, it was once thought that fungi were degenerate plants that had lost their chlorophyll through evolution. However, fungi contain no trace of a plastid genome and rRNA analysis implies that the ancestral fungus was never photosynthetic, but split off from the ancestors or green plants before the capture of the chloroplast. If anything, rRNA sequencing implies that fungi are more closely related to animals than plants.

Chloroplasts and mitochondria possess small circular genomes.

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