1. In E. coli , plasmids are compacted through negative supercoiling just like t

Question

1. In E. coli, plasmids are compacted through negative supercoiling just like the chromosome is compacted. Assuming there are 10.2 base pairs per helical turn, what is the linking number of a plasmid consisting of 6,987 base pairs when the plasmid is in the relaxed state?

2. To be compacted in the E. colicell, the plasmid above is underwound by 23 complete turns. What is the linking number of this compacted plasmid?

3. How many negative supercoils are introduced into the plasmid by the compaction process described above?

Explanation / Answer

1. linking number of relaxed plasmid = total base pair/ base pair per turn

= 6987/ 10.2

= 685

2. linking number of compacted plasmid = change in linking number / linking number in relaxed state

= 708/ 685

= 1.033

3. 708 supercoils were introduced

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