Which statement best describes the structure of a DNA molecule
A. DNA is single-stranded and straight molecule
B. DNA is a double-stranded molecule that looks like a spiral staircase
C. DNA is a double stranded molecule that is straight like a ladder
D. DNA is a single-stranded and curly molecule
The Correct Answer and Explanation is :
The correct answer is:
B. DNA is a double-stranded molecule that looks like a spiral staircase.
Explanation:
DNA, or deoxyribonucleic acid, is the hereditary material in nearly all living organisms. Its structure is famously described as a double helix, which resembles a spiral staircase or twisted ladder.
- Double-Stranded Molecule: DNA is composed of two long strands of nucleotides that run in opposite directions, making it antiparallel. These strands are held together by hydrogen bonds between complementary nitrogenous bases.
- Nitrogenous Bases: The four bases in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G). Adenine pairs with thymine (A-T) through two hydrogen bonds, while cytosine pairs with guanine (C-G) through three hydrogen bonds. These base-pairing rules are crucial for DNA replication and maintaining genetic fidelity.
- Helical Shape: The twisted structure results from the specific arrangement of sugar-phosphate backbones and the stacking of nitrogenous bases. The sugar-phosphate backbones form the sides of the “ladder,” while the base pairs form the rungs.
- Importance of the Structure: The double helix structure is not only elegant but also functional. It allows DNA to be compactly stored within the cell nucleus, protects genetic information from damage, and provides a mechanism for accurate replication during cell division.
- Misconceptions About DNA:
- It is not single-stranded under normal cellular conditions (option A and D are incorrect).
- It is not straight like a ladder; the helical twist is a defining feature (option C is incorrect).
The discovery of the double-helix structure by James Watson and Francis Crick in 1953, based on Rosalind Franklin’s X-ray diffraction data, was a monumental breakthrough in biology. It revealed how DNA carries genetic information and paved the way for modern molecular biology.