Space­sav­ing DNA

How It Works - - SCIENCE -

How is so much DNA packed into the space of a cell nu­cleus that’s only around two to ten mi­crons (0.002-0.01 mil­lime­tres) wide? Each dou­ble helix strand is wrapped around pro­teins called hi­s­tones to form struc­tures called nu­cle­o­somes, which un­der the mi­cro­scope have the ap­pear­ance of beads on a string. These nu­cle­o­somes coil up, fur­ther com­press­ing the DNA mol­e­cule into com­pact fi­bres. The fi­bres are then tightly folded to pro­duce the 250-nanome­tre-wide fi­bres that make up chro­mo­somes.

This ar­range­ment is ad­justable, so por­tions of DNA strands can ef­fec­tively be opened up when the mol­e­cule needs to be ‘read’ dur­ing tran­scrip­tion or repli­ca­tion. Since these struc­tural changes are re­versible, the DNA re­verts to its com­pact form when these pro­cesses are com­plete.

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