BIOL308 Lecture Notes - Lecture 6: Dna Supercoil, Pyrimidine, Purine
16 Jul 2014
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Document Summary
Primary structure of dna: sugar-phosphate "chain" with purine and pyrimidine bases as side chains. Secondary structure of dna double helical structure [hydrogen bonding between bases; stacking interactions; phosphate backbone "outside"] Tertiary or higher order structure double stranded dna [both circular and linear] makes complexes with porteins - supercoil. Important: reduce stress on dna by twisting the double helix. Topological isomers - dna differing only in their states of supercoiling. A circular dna without any superhelical turn is known as relaxed molecule. Important for packing of dna - circular or linear. Dna helix becomes topographically linearized [locally uncoiled] during replication and transcription. Dna functions in replication and transcription depending on its topological state. Strain is released mainly by writhing into superhelical turns. It is easier [energetically "cheaper"] to bend long dna then to untwist. Supercoiling puts strain on a molecule and dna must rotate to take the strain off. Situation in circular dna molecules and locally in long linear dna molecules.
