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15 November, 00:24

What is the linking number in vivo for a plasmid that is 4200 bp in size, if there is normally 1 negative supercoil in every 20 base pairs of a circular DNA? Assume there are 10.5 bp per turn of the double helix.

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  1. 15 November, 00:32
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    190.

    Explanation:

    The linking number may be defined as the sum of the twist and writhes present in the DNA molecule.

    Linking number = twist + writhes.

    Writhe = Total size / base pairs in circular DNA

    Writhes = 4200 / 20 = 210.

    Writhes must be - 210 because the DNA is negatively supercoiled.

    Twist number = Total size / 10.4

    Twist number = 4200 / 10.5 = 400.

    Then, linking number = - 210 + 400

    Linking number = 190.

    Thus, the answer is 190.
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