Читать книгу Cell Biology - Stephen R. Bolsover - Страница 17

Prokaryotic cells are simpler in their organization than eukaryotic cells and are assumed to be more primitive. According to the fossil record, prokaryotic organisms precede, by at least 1.5 billion years, the first eukaryotes that appeared some 2 billion years ago. It seems highly likely that eukaryotes evolved from prokaryotes, and the most likely explanation of this process is the endosymbiotic theory. The basis of this theory is that some eukaryotic organelles originated as free‐living bacteria that were engulfed by larger cells in which they established a mutually beneficial relationship. For example, mitochondria would have originated as free‐living aerobic bacteria and chloroplasts as photosynthetic cyanobacteria. The endosymbiotic theory provides an attractive explanation for the fact that mitochondria and chloroplasts contain their own DNA and ribosomes both of which are more closely related to those of bacteria than to all the other DNA and ribosomes in the same cell. The case for the origin of other eukaryotic organelles is less persuasive. Nevertheless, while it is clearly not perfect, most biologists are now prepared to accept that the endosymbiotic theory provides at least a partial explanation for the evolution of the eukaryotic cell from prokaryotic ancestors.

TABLE 1.1. Differences between prokaryotic and eukaryotic cells.

	Prokaryotes	Eukaryotes
Size	Usually 1–2 μm	Usually 5–100 μm
Nucleus	Absent	Present
DNA	Usually a single circular molecule (= chromosome)	Multiple linear molecules (chromosomes)a
Cell division	Simple fission	Mitosis or meiosis
Internal membranes	Rare	Complex
Ribosomes	70Sb	80S (70S in mitochondria and chloroplasts)
Cytoskeleton	Rudimentary	Microtubules, microfilaments, intermediate filaments
Motility	Rotary motor (drives bacterial flagellum)	Dynein (drives cilia and flagella); kinesin, myosin
First appeared	3.5 × 109 years ago	2 × 109 years ago

^a The tiny chromosomes of mitochondria and chloroplasts are exceptions; like prokaryotic chromosomes they are often circular.

^b The S value, or Svedberg unit, is a sedimentation rate. It is a measure of how fast a molecule moves in a gravitational field, and therefore in an ultracentrifuge.