The necessity to compact DNA presents a problem for the cell since DNA has to be accessible for replication (all of the DNA), transcription (some of the DNA), and repair (only damaged regions). Define heterochromatin and euchromatin in terms of accessibility and the level of compaction. How does the cell make DNA in highly compacted states available for these processes to occur?

Heterochromatin in terms of accessible of the cell's machinery is termed as the region whereby the nucleosomes are tightly packed toghether such that the cell's transcriptional machinery is unable to access this regions. Examples of regions where this is found is at the centromeres, the telomeres and some other few regions in the chromosomes.

Euchromatin in terms of accessibility is the region wherein the nucleosomes are loosely packed such that the cellular processes machines is able to get access to the DNA.

The cell makes DNA in highly compatible state accessible by understood some epigenetic mechanisms such as modification of the histone tails. An example is acetylation; this allows addition of an acetyl group to specific regions on the tail of particular histones that allows for the unwrapping of the history from the DNA making them accessible. This is carried out by some enzymes known as histone acetyltransferases (HATs). Another example is phosphorylation: this does the same as acetylation.