October Update

Bought a standard poodle puppy.  Bringing him home October 5, so October will be full of housebreaking, and FUN.

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WONDER OF THE MOMENT

Entries in eukaryotic (2)

Thursday
Jul022009

Searching for Clues about Cell Origins

     I planned to write three or four articles about the origin of our kind of cells. The cells we are made of are called “eukaryotic” cells, meaning cells with a nucleus inside. But this topic turns out to have a life of its own. The more I think about it or research it, the more there is to say.
      Our species, Homo sapiens, has been on this planet for perhaps 100,000 years, at most 200,000. We know this because we have been able to retrieve and date human fossils: mummies, bones, or tools.
      We can also find and date fossils of other organisms, but this gets harder to do the longer ago the organism lived. All sorts of interference occurs. An organism may die in a location where its body immediately decays without a trace. It may die and be buried in a way that preserves some parts, hard ones like shell or bone or wood. But later geological activity, like mountain building, earthquakes, volcanoes, or erosion, may expose the fossil parts to decay or may destroy them.
      Finding and dating fossils gets much harder with organisms that didn’t have any hard parts. Yet, remarkably, some of these still leave traces, including bacteria, soft. one-celled, microscopic creatures. Some cyanobacteria (photosynthetic green bacteria from as early as 3.5 billion years ago) left stromatolites for us to find. These are stacks of calcium carbonate deposits that resulted from photosynthesis reactions. Photosynthesis removed carbon dioxide from the sea water in which the photosynthetic bacteria lived, and tiny particles of calcium carbonate powder resulted. This calcium carbonate deposited on top of the bacteria. As the bacterial cells reproduced by splitting in two, new cells remained on top of old cells. Cells by the thousands and millions and billions stacked up, and calcium carbonate deposited on each layer. And that’s what stromatolites look like. They look like rock built up out of layers and layers of material.
      Other such soft, one-celled, microscopic creatures left fossil traces. But a huge number did not. And even the traces we find may not tell us much about the inner workings of these single cells. So the puzzle is: what was going on inside the earliest cells to ever live on earth? We want to know, because we want to follow their evolution. There are clues. But these clues are a completely different kind of “fossil.” I’ll begin to talk about them in my next article.

Thursday
Apr232009

Cells: An Evolutionary Tale

         Since the 1960’s, we have discovered a lot about the evolution of cells.

         Fossil evidence indicated that bacteria had not only been the first living creatures, but they had had the earth to themselves for two billion years. Bacteria are single-celled organisms. Each one carries its genes, made of DNA, in a ring-shaped chromosome folded up in a special region of the cell. Smaller rings of genes, called plasmids, sometimes accompany this chromosome.

         Over two billion years, plenty of mutations took place in bacterial genes, resulting in vast numbers of different bacterial species. Also, being single-celled, bacteria were, and are, capable of picking up chromosome fragments from one another, introducing even more new species.

         About a billion and a half years ago, a new type of organism appeared in the fossil record. Like bacteria, they consisted of single cells. But unlike bacteria, these cells carried their chromosomes enclosed within a special membrane. These membrane-enclosed chromosomes formed a “nucleus” in the new cell type. To distinguish bacteria from the new cells, biologists call bacteria “prokaryotic,” meaning “before the nucleus;” and they called nucleated cells “eukaryotic,” meaning “true nucleus.” Besides the nucleus, the new eukaryotic cells contained a number of infinitesimal organs, called “organelles.” Some of these organelles were photosynthetic and made sugar from light energy. Some did the opposite, extracting energy from sugar to run cell processes.

         Over the next billion and a half years, mutations and gene trading resulted in vast numbers of new eukaryotic species. In some cases, eukaryotic cells joined into multicellular species, such as plants, animals and fungi.

         As François Jacob famously wrote, evolution acts like a tinkerer. Old devices and mechanisms get put to new uses. So it was unlikely that eukaryotic cells had sprung up on their own. It was much more likely that they had somehow evolved out of prokaryotic cells.

         In 1967, Lynn Margulis at Boston University suggested that the first eukaryotic cell could actually have been a group of prokaryotic cells that began living together. In fact, she found that the photosynthetic organelles, called “chloroplasts,” are quite similar to certain photosynthetic bacteria. She also found that the energy-harvesting organelles, called “mitochondria,” are quite similar to certain oxygen-using bacteria. And it turned out that chloroplasts and mitochondria have their own genes, exactly as we might expect, if they were actually bacteria that just happened to be living inside another cell. Margulis’ idea is called the “endosymbiont hypothesis” or the “endosymbiont theory.” It is the beginning of some interesting stories about cell evolution. Stay tuned!