DNA Structure

      • 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 on a string”).
        • H1 binds to the nucleosome and to “linker DNA,” thereby stabilizing the chromatin fiber.
        • Phosphate groups give DNA a ⊝ charge.
        • Lysine and arginine give histones a ⊕ charge.
        • In mitosis, DNA condenses to form chromosomes. DNA and histone synthesis occurs during S phase.
        • Mitochondria have their own DNA, which is circular and does not utilize histones
      • Heterochromatin
        • Condensed, appears darker on EM (labeled H in A ; Nu, nucleolus).
        • Transcriptionally inactive, sterically inaccessible.
        • in methylation

        • in acetylation

        • HeteroChromatin = Highly Condensed.
        • Barr bodies (inactive X chromosomes) may be visible on the periphery of nucleus.
      • Euchromatin
        • Less condensed, appears lighter on EM (labeled E in A ).
        • Transcriptionally active, sterically accessible.
        • Eu = true, “truly transcribed.”
        • Euchromatin is Expressed.
      • DNA methylation
        • Changes the expression of a DNA segment without changing the sequence.
        • Involved with genomic imprinting, X-chromosome inactivation, repression of transposable elements, aging, and carcinogenesis.
        • DNA is methylated in imprinting.
        • Methylation within gene promoter (CpG islands) typically represses gene transcription.
        • CpG Methylation Makes DNA Mute.
      • Histone methylation
        • Usually causes reversible transcriptional suppression, but can also cause activation depending on location of methyl groups.
        • Histone Methylation Mostly Makes DNA Mute.
      • Histone acetylation
        • Relaxes DNA coiling, allowing for transcription.
        • Histone Acetylation makes DNA Active.
      • Nucleotides
        • NucleoSide = base + (deoxy)ribose (Sugar)
        • NucleoTide = base + (deoxy)ribose + phosphaTe; linked by 3′-5′ phosphodiester bond.
          • 5′ end of incoming nucleotide bears the triphosphate (energy source for the bond).
          • Triphosphate bond is target of 3′ hydroxyl attack.
        • PURines (A,G)—2 rings.
          • PURe As Gold.
        • PYrimidines (C,U,T)—1 ring
          • CUT the PY (pie).
        • Deamination of cytosine forms uracil.
        • Deamination of adenine forms hypoxanthine.
        • Deamination of guanine forms xanthine.
        • Deamination of 5-methylcytosine forms thymine.
        • Uracil found in RNA; thymine in DNA.
          • Methylation of uracil makes thymine.
          • Thymine has a methyl.
        • G-C bond (3 H bonds) stronger than A-T bond (2 H bonds).  ↑G-C content→ ↑Melting temperature of DNA.
          • “C-G bonds are like Crazy Glue.”
        • Amino acids necessary for purine synthesis (Cats purr until they GAG):
          • Glycine
          • Aspartate
          • Glutamine

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