Final answer:
Heterochromatin is tightly packed and often contains inactive genes, whereas euchromatin has a looser structure with active genes. Histones play a key role in structuring both forms of chromatin, with tighter histone binding leading to heterochromatin and looser binding associated with euchromatin. Transitions between these forms are linked to gene regulation.
Step-by-step explanation:
Heterochromatin and euchromatin are two forms of chromatin that describe the condensation level of DNA and its associated proteins in the nucleus of eukaryotic cells. Heterochromatin is the tightly packed form of DNA, which makes it less accessible to enzymes for transcription and thus, is generally found in regions with genes not being expressed, such as centromeres and telomeres. On the other hand, euchromatin is more loosely packed and contains genes that are actively being transcribed.
Histones are core components of chromatin and play a crucial role in the condensation of DNA. DNA wraps around histones, forming nucleosome structures - the fundamental units of chromatin. In euchromatin, DNA is wrapped around nucleosomes but is not further compacted, allowing for easier access to transcription and replication enzymes. In contrast, additional non-histone proteins bind to the DNA in heterochromatin, leading to tighter packaging and less access to gene expression machinery. Transitions between euchromatin and heterochromatin occur during gene regulation. When genes in euchromatin are turned off, they may become packed into heterochromatin. Conversely, euchromatin formation may occur to activate gene expression in previously silent regions of DNA. These changes involve alterations in histone modifications and histone density.