BIOC 302 Lecture Notes - Lecture 7: Gc-Content, Linking Number, Topoisomerase
Document Summary
Linking number (lk) = the number of imes a single strand crosses over the other in dsdna. Lk is a topological property it cannot change, unless the covalent linkage is broken (i. e. by a topoisomerase) To determine lk: example: in a circular dna of 2100 base pairs. 10. 5 is the base pair per turn for type b dna: lk has to be an integer (cannot have 0. 5 of a turn) Changes in lk: overwound base pairs per turn is decreased lk is increased, underwound base pairs per turn is increased lk is decreased. If the strands are cut, but not yet joined together again, the lk is undeined. Argarose gel electrophoresis: uilizes a polysaccharide matrix with large pores: the negaive charge (from the phosphates) is proporional to the length of dna, giving a constant charge/mass raio. Separaion is on the basis of size. Supercoiling reduces the efecive size of the molecule, allowing it to migrate faster.
