Page "Chromatin" Paragraph 2

The structure of chromatin depends on several factors.

The overall structure depends on the stage of the cell cycle: during interphase the chromatin is structurally loose to allow access to RNA and DNA polymerases that transcribe and replicate the DNA.

The local structure of chromatin during interphase depends on the genes present on the DNA: DNA coding genes that are actively transcribed (" turned on ") are more loosely packaged and are found associated with RNA polymerases ( referred to as euchromatin ) while DNA coding inactive genes (" turned off ") are found associated with structural proteins and are more tightly packaged ( heterochromatin ).

Epigenetic chemical modification of the structural proteins in chromatin also alter the local chromatin structure, in particular chemical modifications of histone proteins by methylation and acetylation.

As the cell prepares to divide, i. e. enters mitosis or meiosis, the chromatin packages more tightly to facilitate segregation of the chromosomes during anaphase.

During this stage of the cell cycle this makes the individual chromosomes in many cells visible by optical microscope.