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