dna topology: nature's headphone cord problem
TRANSCRIPT
DNA is really long, and the nucleus is really
tiny
A single cell contains
six feet of DNA.
Ten cell nuclei = thickness of 1 human hair.
=
You’d be right.“The longer a string is, the more easily it gets
tangled.” –Captain Obvious
D.M. Raymer and D.E.Smith, “Spontaneous knotting of an agitated string”, PNAS 104:42, 2007
Yet somehow, DNA stays organized
Different chromosomes are confined to discrete territories.
Here, chromosomes 2 and 9 are stained green and red.
DNA is stained blue.
Mayer et al., BMC Cell Biology 2005, 6:44.
Spooling DNA onto histones
Histones are positively charged proteins.
DNA winds onto them – step one, of many layers of winding.
Diagram by Thomas Splettstoesser
Local structure…DNA wraps around
histones to form nucleosomes.
Nucleosomes line up to form 30 nanometer fibers.
…becomes larger-scale structure
DNA is further wound into chromatin fibers.
Chromosomes only look tidy when the cell is preparing to divide.
Most of the time they’re unpacked and much messier.
Chromosome domainsScaffold/Matrix
Attachment Regions: Specific regions of DNA attach to the inner surface of the nucleus.
This is still somewhat mysterious.
Each chromosome stays in its own zone.
Bolzer et al., (2005) PLOS Biol 3(5)
Things get tangled anyway
The entire chromosome has to unwind every time it gets copied.
Regions have to unwind to transcribe genes.