Te gel electrophoresis pattern in Figure 4 23 was determined by soaking the gel in a solution of ethidium bromide (EtBr) This is a uorescent molecule with a planar structure Te at molecule intercalates, or ts directly between two adjacent base pairs in a double helix In doing so, it unwinds the dou...

a Ethidium bromide induces positive super helical writhe because T becomes less po ...

Te gel electrophoresis pattern in Figure 4.23 was determined by soaking the gel in a solution of

Question:

Te gel electrophoresis pattern in Figure 4.23 was determined by soaking the gel in a solution of ethidium bromide (EtBr). This is a ﬂuorescent molecule with a planar structure:

Te ﬂat molecule intercalates, or ﬁts directly between two adjacent base pairs in a double helix. In doing so, it unwinds the double helix by 26o for each ethidium molecule bound.
a. If EtBr was added to relaxed, closed circular DNA, would you expect positive or negative super coiling to occur? Explain.
b. If the circular DNA were nicked (had a single-stranded break) on one strand, what would be the aﬀect on super coiling?
c. If negatively super coiled DNA is titrated with EtBr, the electrophoretic mobility decreases at ﬁrst, but then increases at higher EtBr concentrations. Explain.