**Core Concept**
DNA is a double-stranded helix composed of nucleotides, each consisting of a sugar molecule (deoxyribose), a phosphate group, and a nitrogenous base. The sugar and phosphate molecules form the backbone of the DNA molecule, while the nitrogenous bases project inward from the backbone and pair with their complementary bases to form the rungs of the ladder.

**Why the Correct Answer is Right**
The correct answer highlights the fundamental structure of DNA. The deoxyribose-phosphate backbone is the linear framework of the DNA molecule, with the sugar and phosphate molecules alternating in a repeating pattern. The nitrogenous bases, which are adenine (A), guanine (G), cytosine (C), and thymine (T), project inward from the backbone and pair with their complementary bases through hydrogen bonding, forming the stacked rungs of the DNA helix. This specific arrangement of the sugar-phosphate backbone and the paired bases is critical for the stability and function of the DNA molecule.

**Why Each Wrong Option is Incorrect**
**Option A:** This option incorrectly identifies uracil as a purine. In reality, uracil is a pyrimidine, while adenine and guanine are purines. Additionally, cytosine is a pyrimidine, not a purine.
**Option B:** James Watson and Francis Crick discovered the structure of DNA in 1953, not 1973. Their famous model of the DNA double helix was published in the journal Nature in 1953.
**Option D:** DNA is primarily composed of a right-handed helix, not a left-handed helix. The right-handed helix is the more stable and common form of DNA, while the left-handed helix is less common and is known as Z-DNA.

**Clinical Pearl / High-Yield Fact**
The sugar-phosphate backbone of DNA is susceptible to degradation by enzymes, particularly DNases, which can break the phosphodiester bonds between the sugar and phosphate molecules. This can lead to DNA fragmentation and degradation, which is an important consideration in molecular biology and forensic science.

**✓ Correct Answer:** C. Deoxyribose - phosphate backbone with bases stacked inside