The DNA-histone complex

• Where the association of histones with DNA is weak, the DNA-histone complex is less condensed (loosely packed). In this condition, the DNA is accessible by transcription factors which can initiate mRNA production. This in turn can switch the gene on.
• When the association between DNA and histones is stronger, the DNA-histone complex is more condensed (tightly packed). In this condition, the DNA is not accessible by transcription factors, meaning no mRNA can be produced. Hence, the gene is switched off.
• Condensation of the DNA-histone complex inhibits transcription, and can be brought about by decreased acetylation of histones OR increased methylation of DNA.