DNA modification by histones
The chromosomal structure depends on the proteins that package (fold) the DNA. This structure affects gene expression.
Histone proteins bind to eukaryotic DNA, causing it to fold and giving rise to the chromosomal structure.
Enzymes bind to histones to alter how tightly they wrap the DNA. Extra histones and structural proteins can also be incorporated.
The level of folding alters the ability of RNA polymerase to bind to the DNA and therefore determines whether the gene is transcribed.
No transcription takes place in the metaphase chromosome (the highly condensed chromosome found during mitosis) because the DNA is too tightly packed.
Chromosomal regions may be permanently compressed to prevent transcription.