The First Eukaryotes

Recall that eukaryotic cells differ from prokaryotic cells in several ways. Eukaryotic cells are larger, their DNA is organized into chromosomes in a cell nucleus, and they contain membrane-bound organelles. How did eukaryotic cells evolve? Researcher Lynn Margulis (1938—) proposed that early prokaryotic cells may have developed a mutually beneficial relationship. A large body of evidence suggests that between about 2.0 billion and 1.5 billion years ago, a type of small aerobic prokaryote was engulfed by and began to live and reproduce inside of a larger, anaerobic prokaryote. This theory is called endosymbiosis (EN-doh-siM-bie-OH-sis) and is modeled in Figure 14-11. The eukaryotes provided a beneficial environment, and the prokaryotes provided a method of energy synthesis.

Scientists infer that endosymbiotic aerobic prokaryotes evolved into modern mitochondria, which perform aerobic respiration in eukaryotic cells. In a later case of endosymbiosis, photosynthetic cyanobacteria may have evolved into chloroplasts, which perform photosynthesis in modern eukaryotic plant and algae cells. There is compelling evidence to support these hypotheses for the origin of these organelles. Both chloroplasts and mitochondria replicate independently from the replication cycle of the cell that contains them. Moreover, chloroplasts and mitochondria contain some of their own genetic material, which differs from that of the rest of the cell. Finally, the DNA of these organelles is found in a circular arrangement that is characteristic of prokaryotic cells.

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