MicroRNAs are parts of nuclear and mitochondrial genomes which pertain to the not coding genome. They have a partly unspecific role of inhibition, preventing the weakest part of the genetic regulatory networks to be expressed and preventing the appearance of a too large number of attractors in these networks, i.e., forcing the network to have only one or two possible dynamic behaviours for fulfilling a precise function. They have also a great influence on the chromatin clock, which ensures the real mode of updating of genetic regulatory networks. We analyze this influence as well as their impact on important functions like p53 network control.