BIOC 302 Lecture Notes - Lecture 3: Linking Number, Dna Supercoil, Ionic Strength
Document Summary
Dna stability is sensitive to a number of factors. These include ph, ionic strength, temperature and presence of organic compounds that disrupt hydrogen bonds and the hydrophobic effect. Human cells manage to pack the 2 meter very thin (20 diameter) dna threads into the nucleus. This requires very efficient packing but to allow for cellular processes access has to be permitted. In this regard the concept of supercoiling (remember the phone cord?) as a process to pack dna was discussed. Linking number as it pertains to cccdna was defined. In nature, bacterial dna is supercoiled due to an underwound state. Underwound refers to a state (topology) where there are fewer helical turn than expected from the 10. 5 bp/turn (same as saying more bp/turn). Purified dna from bacteria has characteristic degrees of supercoiling (underwound). The example shown was that of closed circular covalent dna (cccdna) like maybe seen in a bacterial plasmid. Topoisomers can be generated using naturally occurring enzymes: topoisomerases.
