Biologia Celular

The Birth of Complex Cells

Humans, together with all other animals, plants and fungi, owe their existence to the momentous transformation of tiny, primitive bacteria into large, intricately organized cells

bout 3.7 billion years ago the first living organisms appeared on the earth. They were small,

A

single-celled microbes not very different from some present-day bacteria. Cells of this kind are classified as prokaryotes because they lack a nucleus (karyon in Greek), a distinct compartment for their genetic machinery. Prokaryotes turned out to be enormously successful. Thanks to their remarkable ability to evolve and adapt, they spawned a wide variety of species and invaded every habitat the world had to offer.

The living mantle of our planet would still be made exclusively of prokaryotes but for an extraordinary development that gave rise to a very different kind of cell, called a eukaryote because it pos- sesses a true nucleus. (The prefix eu is derived from the Greek word meaning “good.”) The consequences of this event were truly epoch-making. Today all mul- ticellular organisms consist of eukary- otic cells, which are vastly more complex than prokaryotes. Without the emer- gence of eukaryotic cells, the whole vari- egated pageantry of plant and animal life would not exist, and no human would be around to enjoy that diversity and to penetrate its secrets.

Eukaryotic cells most likely evolved from prokaryotic ancestors. But how? That question has been difficult to ad- dress because no intermediates of this momentous transition have survived or left fossils to provide direct clues. One can view only the final eukaryotic prod- uct, something strikingly different from

by Christian de Duve

any prokaryotic cell. Yet the problem is no longer insoluble. With the tools of modern biology, researchers have uncov- ered revealing kinships among a num- ber of eukaryotic and prokaryotic fea- tures, thus throwing light on the man- ner in which the former may have been derived from the latter.

Appreciation of this astonishing evo- lutionary journey requires a basic un- derstanding of how the two fundamen- tal cell types differ. Eukaryotic cells are much larger than prokaryotes (typically some 10,000 times in volume), and their repository of genetic information is far more organized. In prokaryotes the en- tire genetic archive consists of a single chromosome made of a circular string of DNA that is in direct contact with the rest of the cell. In eukaryotes, most DNA is contained in more highly structured chromosomes that are grouped within a well-defined central enclosure, the nu- cleus. The region surrounding the nu- cleus (the cytoplasm) is partitioned by membranes into an elaborate network of compartments that fulfill a host of functions. Skeletal elements within the cytoplasm provide eukaryotic cells with internal structural support. With the help of tiny molecular motors, these el- ements also enable the cells to shuffle

their contents and to propel themselves from place to place.

Most eukaryotic cells further distin- guish themselves from prokaryotes by having in their cytoplasm up to several thousand specialized structures, or or- ganelles, about the size of a prokaryotic cell. The most important of such organ- elles are peroxisomes (which serve as- sorted metabolic functions), mitochon-

...