5. Linking number: Linking number has understood units of “turns”, i.e., if Lk =

Question

5. Linking number: Linking number has understood units of “turns”, i.e., if Lk = 20, that means 20 turns in that double helix. (See Lehninger Principles, 5th ed., pp. 956-957 for more understanding.)

A. Relaxed B-form DNA has 10.5 bp per turn. Suppose you have a 4620 bp molecule of closed circular double-stranded plasmid DNA in its relaxed form. What is the linking number of that DNA?

B. Suppose you have a 4620 bp molecule of closed circular DNA with a linking number (Lk) of 420.
1) How many bp per turn does that molecule have?
(please show your work)

Explanation / Answer

Linking number corresponds to the number of turns in a DNA double helix.

If LK is 20 then the number of turns are 20.

A). Relaxed B-form DNA has 10.5 bp per turn

Total base pairs are 4620

Therefore number of turns are 4620 / 10.5 = 440turns

It is known that 1LK = 1 turn.

So, linking number of DNA is 440.

B). circular DNA have 4620bp.

linking number is 420

so, the number of turns are 420.

420 turns have 4620bp

Each turn cotains 4620 / 420 = 11bp.

So, number of bp per turn that molecule have is 11bp.

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