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Chromosomal control of eukaryotic gene expression

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.

The DNA wraps around four pairs of histones.
The DNA wraps around four pairs of histones.

DNA can be modified by adding methyl groups ($$\ce{CH3}$$) to cytosine. This is called DNA methylation. Highly methylated genes cannot be expressed.

DNA methylation occurs in eukaryotes and prokaryotes. It is usually a permanent change to the DNA, but it is reversed when zygotes form.

DNA methylation prevents cells from reverting to stem cells after differentiation.

Changing the chromosomal structure can alter the gene expression of several genes at once.

A methyl group (a carbon with three hydrogens) is attached to cytosine to form methylcytosine.
A methyl group (a carbon with three hydrogens) is attached to cytosine to form methylcytosine.