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34 SECTION II BIOCHEmISTRY ``BIOCHEMISTRY—MOlECUlAR BIOCHEmISTRY ``BIOCHEMISTRY—MOlECUlAR
` `BIOCHEMISTRY—MOlECUlAR
Chromatin structure DNA exists in the condensed, chromatin form to
fit into the nucleus. DNA loops twice around a
histone octamer to form a nucleosome (“beads
DNA double-helix on a string”). H1 binds to the nucleosome
and to “linker DNA,” thereby stabilizing the
chromatin fiber.
H1 histone Phosphate groups give DNA a ⊝ charge. Lysine
(linker)
DNA and arginine give histones a ⊕ charge.
In mitosis, DNA condenses to form
chromosomes. DNA and histone synthesis
Nucleosome Euchromatin Supercoiled occurs during S phase.
(H2A, H2B, structure Mitochondria have their own DNA, which is
H3, H4) 2 Heterochromatin circular and does not utilize histones.
Metaphase
chromosome
Heterochromatin Condensed, appears darker on EM (labeled H HeteroChromatin = Highly Condensed.
in A ; Nu, nucleolus). Sterically inaccessible, Barr bodies (inactive X chromosomes) may be
A
E thus transcriptionally inactive. methylation, visible on the periphery of nucleus.
acetylation.
H
Nu
Euchromatin Less condensed, appears lighter on EM (labeled Eu = true, “truly transcribed.”
E in A ). Transcriptionally active, sterically Euchromatin is Expressed.
accessible.
DNA methylation Changes the expression of a DNA segment DNA is methylated in imprinting.
without changing the sequence. Involved with Methylation within gene promoter (CpG islands)
aging, carcinogenesis, genomic imprinting, typically represses (silences) gene transcription.
transposable element repression, and CpG Methylation Makes DNA Mute.
inactivation of the X chromosome.
Histone methylation Usually causes reversible transcriptional Histone Methylation Mostly Makes DNA Mute.
suppression, but can also cause activation
depending on location of methyl groups.
Histone acetylation Removal of histone’s ⊕ charge relaxed DNA Histone Acetylation makes DNA Active.
coiling transcription.
Histone deacetylation Removal of acetyl groups tightened DNA
coiling transcription.
FAS1_2019_01-Biochem.indd 34 11/7/19 3:16 PM
