DNA supercoiling
1. a supercoil is when the double-helix twists around itself2. supercoils can be positive or negative but natural DNAs exist in the negative
supercoiled form
3. DNA can be supercoiled if it is circular or if is linear and has fixed ends
4. supercoiled DNA is more compact than relaxed DNA
5. Negatively supercoiled DNA molecules are easier to unwind than relaxed molecules- DNA unwinding is required for replication and transcription
Topoisomerases are enzymes that catalyze changes in DNA supercoiling
1. Type I topoisomerases function by breaking a phosphodiester bond of one strand, passing the other strand through the break and resealing the break—they can only remove supercoils
2. Type II topoisomerases function by breaking both strands and passing a double
strand region through the break before resealing the break—require ATP
3. Topoisomerases are targets of numerous chemotherapeutic drugs: adriamycin, VP16
(tenoposide), VM26 (etoposide), camptothecin
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