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Understanding How DNA Is Packaged In A Cell

[For a printable pdf of this topic click here.]

We are all familiar with the images of DNA molecules as thread-like strands of double helix.  Nevertheless, our 46 chromosomes are not merely made up of naked strands of DNA.  The length of straightened-out DNA in a single cell is nearly two meters long.  And so, DNA must be packaged into cells in a highly compacted state so that it can fit inside the small space of the cell’s nucleus.  The DNA illustration at right is from  http://en.wikipedia.org/wiki/DNA

For a more highly magnified view, take a look at the following figure from 
http://genomics.energy.gov/gallery/  

To package DNA inside the nucleus, cells wrap their DNA strands around scaffolding proteins to form a coiled condensed structure called chromatin.  Chromatin is further folded into higher orders of structure that form the characteristic shape of chromosomes. 

Cells exert control over the compactness of the chromatin structure as a means to regulate gene expression. Genes in tightly condensed regions are not as accessible for gene expression.  The figure illustrates the winding of DNA to form a 30 nm fiber of chromatin (the scaffolding proteins are not shown).

Figure of chomatin from      http://en.wikipedia.org/wiki/Chromatin

Histone proteins act like molecular spools that coil the strands of DNA into bead-like units called nucleosomes. A nucleosome contains eight histones wrapped by DNA, and serves as the repeated primary unit for organizing the higher levels of chromatin structure.  There are two copies of four different types of histones in a nucleosome, each having specialized roles (H2A , H2B, H3, H4).

Illustration of the nucleosome from      http://en.wikipedia.org/wiki/Nucleosome

The figure below illustrates the organization of DNA within the chromatin structure. The lowest level of organization is the nucleosome, in which DNA strand is wound around the outside of a histone octamer. Nucleosomes are connected to one another by short stretches of linker DNA. At the next level of organization the string of nucleosomes is folded into a fibre about 30 nm in diameter, and these fibres are then further folded into higher-order structures. At levels of structure beyond the nucleosome the details of folding are still uncertain.

Figure. Reprinted by permission from Macmillan Publishers Ltd: From the following article:  Controlling the double helix Gary Felsenfeld and Mark Groudine Nature 421, 448-453 (23 January 2003) doi:10.1038/nature014

For an animation of how nucleosomes package DNA in a cell, visit this website:  http://www.johnkyrk.com/chromosomestructure.html



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