The epigenetic address: A model for embryological development

I present a model for embryological development according to which a cell arrives at its mature state after a series of binary choices (A or B) between developmental alternatives. The state of a cell at any time in its developmental trajectory is given as its epigenetic address, a sequence of code letters, AABBAA..., corresponding to the instructions the cell received at each "epigenetic crisis" at which its status was altered. Each A or B instruction has the effect of moving a marker one position along a sequence of regulatory units preceding genes that code for the molecules specific for the different cell types ("luxury molecules"). The regulatory units are of type A or B. Different genes are preceded by different sequences of A, B ... units. In this way, the epigenetic address of any cell type corresponds to the set of "luxury" proteins appropriate to that cell type. Indeed, the type of a cell merely reflects the set of gene products present in the cell as a result of the address-coded switching on of genes. Possible molecular bases for such a model, one in which the nucleosomes carry the epigenetic message and another in which cell specific "address RNA" controls the post-transcriptional processing of messenger RNA, are presented.