Which of the following is a characteristic of double-stranded DNA? Multiple Choice
A forms a single helix
B it contains amino acid subunits
C the backbone of one strand binds to the backbone of the adjacent strand
D hydrogen bonds hold together the two strands
E nitrogen bases project toward the outside of the helix
The Correct Answer and Explanation is :
The correct answer is D: Hydrogen bonds hold together the two strands.
Explanation:
Double-stranded DNA (deoxyribonucleic acid) is a molecule that carries genetic information in most organisms. The structure of DNA is often described as a double helix, resembling a twisted ladder. This structure is essential for the function of DNA in storing and transmitting genetic information.
- Hydrogen Bonds (Answer D):
The two strands of DNA are held together by hydrogen bonds between complementary nitrogenous bases. These bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Adenine pairs with thymine (A-T) through two hydrogen bonds, and cytosine pairs with guanine (C-G) through three hydrogen bonds. The hydrogen bonds are relatively weak, allowing the strands to separate during processes like DNA replication, but they are strong enough to hold the strands together under normal conditions. - Why the Other Choices Are Incorrect:
- A: Forms a single helix: Double-stranded DNA forms a double helix, not a single helix. The two strands twist around each other, forming the characteristic helical shape.
- B: Contains amino acid subunits: DNA does not contain amino acids; it consists of nucleotides. Each nucleotide is made up of a phosphate group, a sugar molecule (deoxyribose), and a nitrogenous base.
- C: The backbone of one strand binds to the backbone of the adjacent strand: This is incorrect because the backbones (composed of sugar and phosphate groups) of the two strands are not directly bound to each other. Instead, they run antiparallel to each other, meaning one strand runs in the 5′ to 3′ direction, and the other runs in the opposite direction (3′ to 5′).
- E: Nitrogen bases project toward the outside of the helix: The nitrogen bases actually project inward, where they form hydrogen bonds with complementary bases on the opposite strand. The sugar-phosphate backbone is on the outside of the helix.
Thus, the characteristic of double-stranded DNA is that hydrogen bonds hold the two complementary strands together, ensuring the structure and stability of the molecule.