Genome organization in higher organisms

The complexity of higher organisms, which arises in the course of embryonic development from the much simpler fertilized egg, does not emerge by spontaneous generation nor by miracle. Such complexity must somehow be preexistent in the egg. Since the structure of organisms is genetically transmitted, it is DNA itself, the support of genetic information, that encodes this complexity. Here we propose a model of organization of the genome in DNA loops maintained by DNA crossings. In this model DNA sequences that do not encode proteins, which represent more than 98% of the human genome, are involved in the definition of DNA crossing points and can no longer be considered as junk, but instead play a fundamental part in the encoding of genetic information by modulating the transcriptional state of genome domains. The structural and physiological complexity of organisms has long been known to be related to the amount of DNA per cell, the "c-value". Not that this amount is proportional to the intuitive, visible complexity, since the c-value can vary greatly between organisms that are very similar. However, considering that the structural complexity of organisms is compressed in their genomes in the same way as computer files can be compressed using appropriate algorithms, there is a lower limit to the size of the message, and therefore of the genome, coding for a given complexity 1 . As a consequence, the minimal amount of DNA required to encode organisms of a class increases with the complexity of organisms in that class. For example, no mammal can be found with as little DNA per cell as the fruitfly Drosophila melanogaster 2 . The c-value of mammals does not vary by a large extent from one species to another, and does not differ by much more than a factor of 2 from the ~3,2x10 9 base pairs of the human genome. Therefore, at least about a gigabase of information seems required to encode genetically a mammal, using the mechanisms of embryonic development at work in mammals.