Thursday, December 18, 2008

Encyclopedia A. dubia of Human Knowledge

All the information needed to make a human being is contained in the human genome. These instructions are written in a kind of computer code in the sequence of three billion base pairs of DNA which make up the 46 chromosomes that appear in just about every cell of the human body. This represents about 750 megabytes of information (slightly more than the information on a typical compact disk or "CD"), divided into about 20,000 genes. Each gene produces one or more of the proteins needed for the design, growth, and repair of the human body, and for the digestive, metabolic, reproductive, and other processes that keep the body going.

Actually only about 1.5% of the genome codes for proteins; a small part of the rest is RNA genes, regulatory sequences, and introns (spacer regions within a gene that do not provide actual code); but about 95% is so-called "junk DNA" that has no known use.

Where did all this junk come from? There are occasional mutations and errors in the gene duplication process--although these are very few and far between. Nevertheless, over hundreds and thousands of duplications, the small fraction of genetic material that is broken and useless gradually grows. The cell's chemical processes that carefully and faithfully reproduce the genetic material each time a cell divides have no way of telling the useful DNA from the broken DNA that is no longer useful, so ALL of the genome's DNA pairs are replicated--in perfect sequence--regardless of whether or not they carry useful information. If the DNA that has been damaged was essential for biological functions, the cell dies and the individual dies or has no offspring. Otherwise the individual simply carries along this extra baggage of junk DNA and passes it on to future generations.

Presumably, then, the junk DNA is left over from past genetic coding material that, through mutations and evolutionary changes, has lost its place in the puzzle of life. Some of it--about 15%--codes for proteins of no known value; the rest of the 95% doesn't code for anything at all. In other words, within each human cell there is over 700 megabytes of junk computer code that doesn't appear to have any function but is faithfully carried along in tact from generation to generation.

Some creatures have more genetic material than humans, some have less. The winner on the low side is the pufferfish, Takifuga rubripes, which has a genome about one-tenth the size of the human genome (although it has a comparable number of genes). On the high side, the tiny Amoeba dubia has some 670 billion base pairs of DNA in its genome, more than 200 times the number in the human genome. In fact, this little amoeba, barely visible to the naked eye, is carrying around a collection of nearly 1,000 CDs of information space that it is not using, but that it is faithfully duplicating, generation after generation, century after century, millennium after millennium.

Suppose those CDs were put to a good use. Suppose that several encyclopedias, unabridged dictionaries (in several languages), atlases of maps, and even recordings of music and literary works were carefully encoded into that DNA. The little amoeba would go on its way, exploring, eating, and mating, but carrying an extensive record of human knowledge and accomplishments with it. Our geneticists could even give it a slight competitive genetic advantage over other amoebae--make it a little stronger, a little faster, or a little more durable--so that, over a few decades, the amoebae carrying the "Encyclopedia Amoeba dubia of Human Knowledge" would gradually spread all over the world. There would be trillions of copies.

Perhaps millions of years from now when the human race is long extinct, some creature will evolve the scientific and technical knowledge to decode the genomes of numerous species, and will find the record of what we humans have accomplished. On the other hand, before we go mucking around with the junk DNA of any species, perhaps we should look carefully and see if some past civilization has left its record there for us to find.