the Dawn of Life, a Mystery
Ann Gauger
At the dawn of life the first cells were something like the bacteria we see today. Their appearance by itself is a great wonder and mystery, yet another marvel had to take place -- the appearance of the first eukaryotic cells.
What does it take to go from a prokaryotic cell to a eukaryotic cell? Why does it matter? Every animal and plant, every fungus we know is made up of eukaryotic cells. The differences between the two kinds of cells are immense. I discussed this briefly in a post a few days ago, but rather than simply trying to convince you with words, this time I have included two transmission electron micrographs of the two kinds of cells. These are very high-resolution images, allowing us to see the two cells' structures down to the level of DNA strands.
The first, Bacillus subtilis, is pictured above. It is representative of most bacteria. The micrograph I have shown in cross section is about 600 nanometers across (a nanometer is one millionth of a millimeter). A rod-shaped organism, B. subtilislives in the soil, and the cow and human gut. It has been studied extensively as a model genetic organism.
In prokaryotes, which B. subtilis is, the DNA is not separated from the cytoplasm by any membranes. It can be seen as those faint squiggles barely visible in the center of the cell's cytoplasm. B. subtilis normally reproduces by cell division, but forms spores when resources become scarce. This is a common strategy among single celled organisms, and even some multicellular ones.
The latter is a eukaryotic single-celled green alga about ten to thirty microns in size (a micron is one thousandth of a millimeter), with a nucleus, a large chloroplast that takes up most of the cell's interior, mitochondria, a storage compartment called a pyrenoid, and two flagella used for swimming. The chloroplast is where photosynthesis takes place, the process whereby light and carbon dioxide are used to manufacture glucose. The mitochondria are the powerhouses of the cell, using glucose and oxygen to create ATP, the cell's energy currency. Because it has both a chloroplast and mitochondria, C. reinhardtii is capable of living by photosynthesis when there is light, and by carbon metabolism when it is dark. It normally uses an asexual mode of reproduction, dividing four to eight times before separating into individual organisms. When things get tough, though, the little green cells switch to sexual reproduction and then go dormant, like B. subtilis does.
In the upper left of the image above we can see the heart-shaped pyrenoid, where the products of photosynthesis are stored. The thin black lines around the periphery of the cell are the chloroplast's thylakoids, the membranes where photosynthesis takes place. The central irregular disc with the black spot is the membrane-bound nucleus. Mitochondria are scattered about, but are harder to see. The white roundish circles around and to the right of the nucleus are various kinds of vacuoles (membrane bound compartments for processing waste and digesting things).
I am not telling you this to give you a short course in cell biology, but to illustrate how different the cell types are. The details I have described represent some of the white spaces in the evolutionary story that must be accounted for if evolution by undirected processes is true. Stories exist for how mitochondria and chloroplasts came to be present in eukaryotic cells -- they mainly involve the incorporation of ancient bacteria into the incipient eukaryotic cell. The proposed process has been given the name endosymbiosis. There is no single proposed mechanism for the evolution of the nucleus or the other structures I have named.
I deliberately call such evolutionary accounts "stories." To become a eukaryote like C. reinhardtii involves enormous changes in cell organization that affect every aspect of cellular life. Most of these structures are common to eukaryotic cells, and most are membrane-bound. Membranes mean there must be transport mechanisms in or out of each compartment. DNA replication and division becomes more complicated because the nuclear membrane must break down and reform at each division. Nuclear genes have somehow come to specify proteins necessary for mitochondrial function; they must be transcribed, the RNA exported to the cytoplasm, made into protein, and then the proteins must be transported into the mitochondrion. Specific problems associated with the replication of chromosomes versus circular DNA as in bacteria have to solved. There are more differences to be dealt with than I can cover -- exons and introns, and the separation of mRNA production in the nucleus from protein synthesis in the cytoplasm, just to name two. All of these problems must be solved somehow if the story of undirected evolution is true.
How many new steps were needed to accomplish all this? Even if it happened one organelle at a time, that it happened at all is a wonder. And all this had to happen before the appearance of multicellular animals, since animals and plants have these structures in common.
These are some of the things Darwin didn't know, but now we do. I wonder what Darwin would have said if he had known these details.
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