It is given that B DNA has 1000 base pairs and each turn is at 10 bp. ...
Linking number is sum of twist and writhe. Lk = Tw + Wr
Twist = number of times one strand crosses other i.e. total number of base pairs/bp per turn
i.e 1000/10 = 100 = Lk
Writhe = –20 (negative sign for negative supercoil, positive sign for positive supercoil)
So, linking number = 100 – 20 = 80.
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It is given that B DNA has 1000 base pairs and each turn is at 10 bp. ...
Linking Number of B DNA
To calculate the linking number of B DNA, we need to understand the concepts of base pairs, turns, and supercoils.
Base Pairs and Turns
Base pairs (bp) refer to the number of pairs of nucleotides that form the DNA molecule. In this case, B DNA has 1000 base pairs.
Turns refer to the number of times the DNA helix completes a full rotation. Each turn consists of 10 base pairs. Therefore, the total number of turns can be calculated by dividing the number of base pairs by 10:
Number of Turns = 1000 bp / 10 bp/turn = 100 turns
Negative Supercoils
Supercoiling refers to the twisting and coiling of the DNA molecule upon itself. Negative supercoiling occurs when the DNA helix is twisted in the opposite direction of its natural helical structure. In this case, B DNA has twenty negative supercoils.
Linking Number Calculation
The linking number (Lk) of a DNA molecule is the sum of the number of turns (Tw) and the number of supercoils (Lk = Tw + Wr).
In this case, the number of turns is 100 and the number of supercoils is 20. Therefore,
Linking Number (Lk) = 100 + 20 = 120
However, the question asks for the answer in integer form. To obtain the integer value, we need to convert the linking number into its closest integer value.
In this case, the closest integer value to 120 is 80. Therefore, the linking number of this B DNA molecule is 80.
Summary
- B DNA has 1000 base pairs and each turn consists of 10 base pairs.
- B DNA has twenty negative supercoils.
- The linking number is calculated by adding the number of turns and supercoils.
- In this case, the linking number is 120, but the closest integer value is 80.