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DNA Compaction
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DNA Compaction

DNA STRUCTURE
1. Primary structure: sequence of nucleotides
  • Pyrimidines: cytosine and thymine
  • Purines: guanine and adenine
  • Lends DNA polarity
2. Secondary structure: double helix stabilized by H-bonds
  • 10 base pairs per full (360 degree turn)
  • Adenine and thymine form TWO hydrogen bonds
  • Guanine and cytosine form THREE hydrogen bonds
  • 10 base pairs per full (360 degree) helical turn
3. Tertiary structure: relaxed or supercoiled
LEVELS OF COMPACTION
1. Nucleosome: comprises histone octamer and the DNA wrapped around it (1.75 supercoil) 2. Chromatin: DNA and associated proteins
  • heterochromatin: highly condensed, not transcribed
  • euchromatin: NOT highly condensed, regularly transcribed
3. Solenoid: nucleofilament
HISTONES
  • Small basic proteins rich in arginine and lysine
  • 5 classes of histones
  • Can be acetylated or methylated: regulates local DNA compaction
  • H1: binds spacer DNA (20-80 bp) and promotes tight packing of nucleosomes
CLINICAL CORRELATION
Anticancer drugs (chemotherapies)
  • Many bind to groove in DNA double helix to prevent DNA replication and transcription in cancerous cells

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