The number of turns in a double helical B-DNA molecule
The number of turns in a double helical B-DNA molecule can be calculated using the formula:

Number of turns = Number of base pairs / Number of base pairs per turn

Calculating the number of base pairs per turn
In B-DNA, there are approximately 10 base pairs per turn. This means that for every complete turn of the DNA helix, there are 10 base pairs.

Calculating the number of turns
To calculate the number of turns, we divide the total number of base pairs in the DNA molecule by the number of base pairs per turn.

Given that the number of base pairs in the DNA molecule is 5430, we can calculate the number of turns as follows:

Number of turns = 5430 base pairs / 10 base pairs per turn
Number of turns = 543 turns

Therefore, the number of turns in the double helical B-DNA molecule is 543.

Explanation
DNA is a double-stranded molecule that forms a helical structure. This helical structure consists of two strands that are twisted around each other. The number of turns in the DNA molecule refers to the number of complete twists or rotations that occur within the helix.

In B-DNA, which is the most common form of DNA, there are approximately 10 base pairs per turn. A base pair is formed by two nitrogenous bases, such as adenine (A) and thymine (T), or cytosine (C) and guanine (G), which are paired together through hydrogen bonds.

To calculate the number of turns in the DNA molecule, we divide the total number of base pairs in the molecule by the number of base pairs per turn. In this case, the given number of base pairs is 5430.

By dividing 5430 base pairs by 10 base pairs per turn, we find that there are 543 turns in the DNA molecule.

Therefore, the correct answer is 543.