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 DNA (7)

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!

Thursday
Jul192007

What we didn't expect from the Human Genome Project

The history of DNA research is a tale of patient researchers laboring day after day on myriad tiny problems.  It is a tale of myriad answers leading at last to profound insights.

    This is what happened with the Human Genome Project.  The question the HGP set out to answer was:

• What are all the genes in a human being?

    Before the Project began, geneticists had learned a lot.  They knew that genes work by manufacturing proteins.  They knew that genes do this indirectly:  Enzymes in the cell nucleus unroll and unzip the DNA double helix and copy a target gene into messenger RNA.  The messenger RNA carries the gene’s code to cell parts outside the nucleus to direct protein manufacture.  Finally, geneticists knew that humans have around 100,000 proteins in their bodies.  So researchers expected the HGP to take years and to turn up about 100,000 genes.

    But the geneticists working on the Project, devised new, speedier techniques for decoding DNA.  A lot sooner than anticipated, the whole human genome was known.  And there weren’t 100,000 genes—there were only about 30,000!  Or maybe only 25,000!  A humbling conundrum.

• How do the 100,000 proteins come from only 25,000 genes?

    Before the Human Genome Project, something else had come to light:  When a messenger RNA gets copied from one of our genes, it gets “edited.”  Molecules called spliceosomes cut the RNA message into fragments, remove some of the fragments, and splice the rest back together again.  The spliced message is what actually gets translated into a protein.  But the spliced message isn’t always the same.  The set of fragments that get spliced together can differ.  So that alternative proteins result from the same messenger RNA and therefore from the same gene!

    Is this how 25,000 genes make 100,000 proteins?  How did this incredible system evolve?   Some biologists think the first active catalytic molecules of life were RNA, while others think they were protein.  Intriguingly, spliceosomes have some of both.  Could alternative splicing be connected to the earliest molecules of life?  When we investigate this editing of RNA, are we seeing far back into life’s beginnings, just as we see far back into the beginnings of the universe when we investigate the oldest light we can find with the Hubble Telescope?

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