Which statements about the modification of chromatin structure in eukaryotes are true?

1. DNA is not transcribed when chromatin is packaged tightly in a condensed form

2. Some forms of chromatin modification can be passed on to future generations of cells

3. Acetylation of histone tails in chromatin allows access to DNA for transcription

4. Acetylation of histone tails is a reversible process.

5. Methylation of histone tails in chromatin can promote condensation of the chromatin

6. Deacetylation of histone tails in chromatin loosens the association between nucleosomes and DNA

All of the statements are correct except 6. (Deacetylation of histone tails in chromatin loosens the association between nucleosomes and DNA)

Explanation:

Chromatin structure and its modifications can change the package of the DNA and consequently, alter the gene expression. The most common modifications of the chromatin are covalent modifications such as acetylation/deacetylation (by acetyltransferases and eacetylases), methylation (by methyltransferases), and phosphorylation (by kinases). This is the way of gene expression regulation.

The effects of modifications are different, for example methylation promotes condensation of the chromatin and as a consequence, prevents binding of transcription factors to the DNA (transcription is repressed).

Acetylation loosens the association between nucleosomes and DNA (because it neutralizes the positive charge of histones) and consequently promotes transcription. Deacetylation is a process opposite to acetylation.