Friday, October 14, 2011

Development of many living beings starts with a clock

The discovery of a new type of biological clock has given us an astonishing insight into the development of many organisms, including ourselves. A genetic clock that 'unwinds' in a time-dependent fashion gives rise to the basic components of our body, that later become fully developed and distinguished parts. The so-called Hox genes are responsible for creating the basic layout of many animals, but it was not known how they are regulated and create the right body parts at the right time. A team of scientists have found that the Hox genes, that lay next to each other on the genome, 'unwind' with the precision of a clock, so that the right Hox gene has its effect at the right time. This mechanism reveals one of the most basic concepts of life, a mechanism that ultimately decides the development of an organism.

It was already known that the Hox genes have an important role in the development of life. The bodies of most organisms are subdivided into several distinct parts, like segments. The creation of these segments start early on, so each part goes on to develop on its own, while, of course, still being connected to the other parts of the body. This, eventually gives rise to different forms of tissue: your head, hands, feet, spine, etc. It was noted that the Hox genes responsible for the initial subdivision lay next to each other on the genome, in the same way that the body layout is planned. This basically means that the Hox gene for the head segment is on 'top' in the DNA, while the Hox gene for your feet is at the very bottom. Of course, Hox genes do not actually make feet, but they do create the basic body plan, in the form of segments. Scientists were wondering why the layout on the DNA exactly matches the layout of the body, but it seems now that the answer has been found.
Hox expression in fruit flies, where the genes were originally discovered. Colour matches body segment.
Initially, right after development of the organism starts, the part of the DNA that holds the Hox genes is tightly packed, in a so-called chromatin structure. This means the genetic code is winded up on a spool, which is called a histone. When the histones are tightly packed, so is the DNA, and the code can not be 'read' in order to make proteins. This effectively silences the genes, and this process is frequently used in the body to turn parts of the DNA on and off, so that different cells assert different behaviour because their DNA is regulated differently.
Scientists have now discovered that the Hox 'spool' slowly unpacks during development. It takes about 48 hours to create all the 30 segments that the human body has (which corresponds to the number of vertebrae we have), and it is also in this time that the Hox genes, one by one, unwind from the tightly packed chromatin structure. Every 90 minutes, a new body segment is created during early development, and it also takes 90 minutes for a Hox gene to be 'freed' from the chromatin structure during this time. The researchers found that  the unpacking of a Hox gene is closely associated with the creation of the body segment it is responsible for.

The time by which the chromatin unpacks seems strictly regulated, as the time by which a new gene is uncovered is precisely determined. Now that this mechanism is discovered, the question remains is, what triggers the chromatin clock?

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