Transcription, like replication, displaces nucleosomes from DNA, and reassembly appears to occur in the wake of the RNA polymerase. Most transcribed genes thus retain a nucleosome structure, although the pattern of nucleosome phasing characteristic of non transcribed genes is lost, resulting in a smear of DNA fragments following digestion with micrococcal nuclease and a restriction enzyme, rather than a discrete band.
Experiments which examine the progress of the polymerase complex through the nucleosome have shown that pausing occurs about half-way through the core DNA, which may reflect the build-up of torsional strain enzyme attempts to negotiate the first coil released from the nucleosome. The strain is release the enzyme moves past this point, indicating that the octamer is expelled. The octamer then reassociates with DNA behind the enzyme, perhaps because it remains attached to the nontran scribed strand, or perhaps because it is transiently associated with the enzyme itself.
In heavily transcribed genes sucha s the rRNA genes of Zampbrush chromosomes (q.v.), the extended conformation of chromatin indicates that it is nucleosome-free. This probably reflects failure ofthe displaced histones to reassemble on post transcribed DNA because of a following transcriptionalelongation complex. In very active genes, there would be a convoy of RNA polymerases which would maintain an definite nucleosome-free region of chromatin.manipulating nanomachines, now available commercially. In general, natural bionanomachines are remarkably robust.
THE UNFAMILIAR WORLD OF BIONANOMACHINES:
Experiments which examine the progress of the polymerase complex through the nucleosome have shown that pausing occurs about half-way through the core DNA, which may reflect the build-up of torsional strain enzyme attempts to negotiate the first coil released from the nucleosome. The strain is release the enzyme moves past this point, indicating that the octamer is expelled. The octamer then reassociates with DNA behind the enzyme, perhaps because it remains attached to the nontran scribed strand, or perhaps because it is transiently associated with the enzyme itself.
In heavily transcribed genes sucha s the rRNA genes of Zampbrush chromosomes (q.v.), the extended conformation of chromatin indicates that it is nucleosome-free. This probably reflects failure ofthe displaced histones to reassemble on post transcribed DNA because of a following transcriptionalelongation complex. In very active genes, there would be a convoy of RNA polymerases which would maintain an definite nucleosome-free region of chromatin.manipulating nanomachines, now available commercially. In general, natural bionanomachines are remarkably robust.
THE UNFAMILIAR WORLD OF BIONANOMACHINES:
Biological machinery is different from anything we build with our familiar,human-sized technology. Natural biomolecules have organic, visceral, and often unbelievable shapes, unlike the tidy designs of toasters and tractors.They perform their jobs in a foreign environment, where jittery thermal motion is constantly pushing and pulling on their component parts. They are held together by a complex collection of bonding and non bonding forces. At their small scale, bionanomachines are almost immune to the laws of gravity and inertia that dominate our machines. The world of bionanotechnology is an unfamiliar, shifting world that plays by different rules.
