Developing a Body: Molecular Machines
The year 2013 has so far been a spectacular year for the science of how a living body assembles itself. For example, how do the Hox genes program a limb bud to form the correct order and placement of limbs? The Hox genes (found in humans, mice, even insects) have long been known to exist in a long series that approximates the order of the body parts specified, such as head, thorax, abdomen, tail. On a finer scale, the HoxD cluster can be regulated to develop the order of bones within a limb.
The order of gene expression involves removal of “epigenetic” markers–that is, molecules attached to the DNA or DNA-binding proteins called histones. The removal of epigenetic markers is regulated by “enhancer” sequences of DNA. These enhancers are found surprisingly far from the gene itself, in a region called a “gene desert” (formerly known as “junk DNA.”) These “gene deserts” are regions of DNA that do not themselves encode protein but do have DNA sequence that binds specific proteins that interact with RNA polymerase. The RNA polymerase then transcribes the Hox genes in order, directing development of limb segments in turn.
This candy-factory model of limb development, one part at a time, is just one of many revealing pictures emerging from the science of development. Others include the darwinian selection of neurons during brain development, and the roles of environment and microbes in development.