The genetic code specifies all the proteins that a cell makes. The second code, superimposed on the first, sets the placement of the nucleosomes, miniature protein spools around which the DNA is looped. The spools both protect and control access to the DNA itself.
The discovery, if confirmed, could open new insights into the higher order control of the genes, like the critical but still mysterious process by which each type of human cell is allowed to activate the genes it needs but cannot access the genes used by other types of cell.
I blogged about another discovery made a few months back involving nucleosomes/histones (DNA segments) and their function in determining growth in children and adults:
Imagine this: fundamentally, our DNA is spun around little protein spools called histones. These histones provide a structure for DNA and assist in gene regulation, or the process of turning genes on and off. Genes, obviously, cause development.
There are two histone sites where the methyl-marking (methylation) occurs; the K4 site and the K27 site. At the K27 site, the DNA is bound very tightly to the histone complex, not allowing any strands to become available for transcription (expression of DNA into protein). These sites are marked in adults.
At the K4 site, the strands are loosely bound, more accessible. K4 sites are marked in young, undifferentiated cells, like embryonic stem cells.
What this says, in a nutshell, is that they have found the areas where gene expression is a go (marked K4 region, in young cells) and where it is shut down (K27, in adults).
Certain genes do not have to be "turned on" when we are fully grown - especially genes that direct limb or organ growth - and the marked K27 site prevents that area of DNA to be accessed.
However, the K4 site would need to marked in the blastocyst and the DNA made available in order to start the differentiation process, so the cells could be directed into whatever is needed: liver cells, muscle tissue, bone, neurons, etc.
We're getting closer by the month, it seems.