Journey from Gene to Brain in 25,000 Genes or Less, The

Phi Kappa Phi Forum, Winter 2005 by Marcus, Gary

The "THEN" is the better-known part of a gene: the template for building a particular protein. The insulin gene specifies how to build the insulin protein, the keratin gene specifies how to build keratin, and so forth. The lesser-known "IF" (or, more formally, the regulatory region) specifies where and when that protein should be built. The recipe for hemoglobin, for example, is followed only in red-blood precursors, the recipe for human-growth hormone only in the pituitary gland. Some genes are expressed only in the brain, others only in the kidneys or the liver, or in a particular kind of cell, or a particular place within a cell. And many genes are just as choosy about when they are expressed. Some genes (like those that build proteins that help convert sugar to energy) are on almost all of the time in almost every cell, but most genes are on (or most active) only at select times, during particular situations (for example, during cell division or gastrointestinal inflammation), or during particular moments in embryological development (such as during the leg-growing, tail-shedding process of tadpole-to-frog metamorphosis).

IF some situation holds true, THEN build a particular protein. The net result is a kind of mass empowerment: every gene is a free agent, authorized to act on its own. As soon as the IF part of a gene's IP-THEN rule is satisfied, the process of translating the template part of a gene into its corresponding protein commences. By switching on only at specific times and places, these hordes of IF-THENs modulate the growth of proteins in different ways in different cells.

Those IF-THENs guide every step of development, every bit of the flurry of cell division, cell migration, and cell differentiation that scientists call embryogenesis. The brain, for example, begins as a simple sheet of cells that gradually curls up into a tube that sprouts bulges, which over time differentiate into ever more complex shapes, including the convoluted sheet that we recognize as the human cerebral cortex. And in one way or another, each step of this embryological origami is driven by the instructions carried in the genome's twenty-five thousand IF-THENs.

GENOME AS COMPRESSION SCHEME

But we are only a step closer. How can twenty-five thousand such genes come together to build all the initial complexity of the human brain? By anticipating a trick from computer science: compression. By ferreting out redundancy, computer scientists can store and transmit information in highly efficient ways, and it turns out that nature does much the same thing.

For instance, you may have seen the GIF format (pronounced jiff), a way of storing picture files compactly. One of the oldest compression schemes, GIF relies on patterns of repeated pixels (the colored dots of which digital images are made). If a whole series of pixels are of exactly the same color, the software that creates GIF files assigns a code to represent the color of those pixels, followed by a number to indicate how many pixels in a row are of the same color. Instead of listing every blue pixel individually, a GIF-compressed picture saves space by storing only the code for blue and the number of repeated blue pixels. In this way, two numbers can stand for fifty or a hundred pixels. (Today there are dozens, perhaps hundreds, of different compression schemes, ranging from JPEGs for photographs to MP3s for music and MPEG-4s for movies, each one specialized in a different way.)