Questions & Answers

If a length of DNA has 45,000 base pairs, how many complete turns will the DNA molecule take?
A. 4,500
B. 45,000
C. 45
D. 450

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Hint: DNA stands for deoxyribonucleic acid. DNA is the hereditary genetic material of humans and almost all living beings. The structure of DNA is a double helix structure proposed by Watson and Crick. Nucleotides are monomeric units of nucleic acids. There are four nitrogenous bases i.e. A, U, G, C in the case of RNA and A, T, G, C in the case of DNA.

Complete answer: Structure of DNA:
1. Two long polynucleotide strands are coiled around the central axis.
2. Strands are wrapped forming the right-handed helix structure.
3. Strands of DNA are antiparallel i.e. one strand is in direction of 5’ to 3’ and another is 3’ to 5’ end.
4. Both the strands are attached by hydrogen bonds between complementary base pairs.
5. Guanine (G) pairs with Cytosine (C) by forming three hydrogen bonds in between whereas, Adenine (A) pairs with Thymine (T) by forming two hydrogen bonds.
6. The angle of interaction between base pairs forms major and minor grooves.
7. The diameter of each helix in DNA is 20 Angstrom.
8. The diameter of each helix in DNA is 3.32 Angstrom.
9. Approximately 10.4 base pairs are present per helical turn.
For calculation purposes, base pair per helical turn is considered as 10 base pairs.
If a length of DNA has 45,000 base pairs then,
\[{\text{Number of turns in and DNA molecule will be = total base pairs / 10}}{\text{.}}\]
The number of turns in DNA\[{\text{ = 45000/10 = 4500 turns}}\].
So, the correct answer to the given question is option A, i.e., DNA will take 4500 turns if it consists of 45000 base pairs.

Note: There are three forms of DNA or three types of conformations of DNA namely, A-form, B-form, and Z-form of DNA. The base pair per turn in B-form is 10.4. Base pair per turn in A-form is 10.7 and Base pair per turn in Z-form is 12. If the conformation of DNA is not mentioned in the question we have to consider it as, B-form of DNA.