| Popis: |
Dna is the central repository of information to keep cells and organisms alive. In human cells, the 46 chromosomes amount to a length of about 2 m, with a diameter of 2 nm, and are packed in a way that allows for access by transcription, replication and repair machinery, fitting within a globular nucleus with a radius of 5000 to 10000 nm. Efficiency of packing is obtained by several levels of packing mechanisms (Figure 1), both general (due to general principles, irrespective of Dna sequence) and speci fic, i.e. mediated by proteins that recognize specifi c motives (sequences) and bring in close proximity parts of Dna that may be very distant in the genomic sequence. In both cases, general packing and specifi c aggregation, the underlying mechanisms are not entirely described or understood. The fi rst level, mediated by histon octamers, produces a ber of 11 nm, which in turn is organized into a 30 nm-wide structure. Further packing is at work in cells, and the research community engaged in the study of chromatin conformation is producing increasing knowledge that will finally allow for a clear vision of the nuclear machinery that regulates Dna metabolism. |
