When explaining genetic coding to a group of students, the instructor discusses gene activation and ...

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Although solving the structural problem of how to fit a huge amount of DNA into the nucleus, the chromatin fiber, when complexed with histones and packaged into various levels of compaction, makes the DNA inaccessible during the processes of replication and gene expression. Several chemical interactions are now known to affect this process. One of these involves the acetylation of a histone amino acid group that is linked to opening of the chromatin fiber and gene activation. Another important chemical modification involves the methylation of histone amino acids, which is correlated with gene inactivation. Several repair mechanisms exist, and each depends on specific enzymes called endonucleases that recognize distortions of the DNA helix, cleave the abnormal chain, and remove the distorted region. Replication of mtDNA depends on enzymes encoded by nuclear DNA. Thus, the protein-synthesizing apparatus and molecular components for oxidative metabolism are jointly derived from nuclear and mitochondrial genes.

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