Chromatin

Chromatin is the complex of DNA, histone proteins, and other molecules that packages eukaryotic genomes. Its changing organization helps control access to genes and other DNA sequences.

Nucleosome: The basic chromatin unit, formed by DNA wrapped around a histone protein core. Nucleosomes package DNA into chromatin and help determine which DNA regions are accessible.
Histone: A family of basic proteins around which eukaryotic DNA is wrapped. Histones form the protein core that organizes DNA into nucleosomes.
Transcription: The synthesis of RNA from a DNA template. Transcription depends on whether chromatin permits RNA polymerase and regulatory proteins to reach DNA.
Barr body: The condensed, inactive X chromosome visible in many female mammalian cells. A Barr body is a visible example of chromosome-wide chromatin condensation.
Chromatin remodeling: The repositioning, restructuring, or removal of nucleosomes by specialized protein complexes. Remodeling complexes change DNA access without altering its sequence.
DNA: A molecule that stores hereditary information in sequences of nucleotides. Chromatin packages the DNA molecule while preserving its encoded information.
Epigenetics: The study of heritable changes in gene activity that do not alter DNA sequence. Chromatin marks and states are central mechanisms of epigenetic regulation.
Position-effect variegation: A mosaic pattern of gene expression caused by a gene’s proximity to heterochromatin. It demonstrates that chromatin can silence genes according to their genomic neighborhood.
Histone modification: Chemical changes to histone proteins that influence chromatin structure and gene activity. Histone modifications help recruit proteins that activate or repress nearby genes.
Linker DNA: The stretches of DNA connecting neighboring nucleosomes. Linker DNA connects nucleosome cores and contributes to higher-order packing.

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