DNA Structure
High-Yield Summary
- A nucleoside = sugar + base (base attaches at sugar C1). A nucleotide = nucleoside + one or more phosphate groups (at sugar C5). Nucleotides are DNA's true building blocks.
- Ribose (RNA) has an –OH at C2; deoxyribose (DNA) has just H at C2. The missing –OH makes DNA more stable — built for long-term storage vs. RNA's temporary roles.
- Sugar-phosphate backbone links via phosphodiester bonds (3'-OH of one sugar to 5'-phosphate of next), giving DNA 5'→3' directionality. The backbone's negative charge matters for histone binding and gel electrophoresis.
- Purines (adenine, guanine) = two fused rings, found in both DNA and RNA. Pyrimidines (cytosine, thymine, uracil) = one ring; thymine is DNA-only, uracil is RNA-only, cytosine is shared.
- All nitrogenous bases are aromatic heterocycles (cyclic, planar, conjugated, obey Hückel's 4n+2 rule) — aromaticity enables base stacking, which stabilizes the helix.
- Naming pattern: base + ribose (no phosphate) = adenosine; + 1/2/3 phosphates = AMP/ADP/ATP. Same base + deoxyribose = deoxyadenosine → dAMP/dADP/dATP.
Key Terms
- Nucleoside
- Sugar + nitrogenous base (attached at C1), no phosphate.
- Nucleotide
- Nucleoside + one or more phosphate groups (attached at C5).
- Phosphodiester bond
- Bond linking the 3'-OH of one sugar to the 5'-phosphate of the next, forming the backbone.
- Purine
- Two-fused-ring nitrogenous base: adenine, guanine.
- Pyrimidine
- Single-ring nitrogenous base: cytosine, thymine, uracil.
- Aromatic heterocycle
- Ring with non-carbon atoms that is cyclic, planar, conjugated, and follows Hückel's rule (4n+2 pi electrons).
Ribose vs. Deoxyribose
| Feature | Ribose (RNA) vs. Deoxyribose (DNA) |
|---|---|
| Carbon-2 substituent | Hydroxyl (–OH) / Hydrogen atom only |
| Relative stability | Less stable, more reactive / More stable |
| Typical role | Temporary tasks (e.g. protein synthesis) / Long-term genetic storage |
Purines vs. Pyrimidines
| Feature | Purines vs. Pyrimidines |
|---|---|
| Ring structure | Two fused rings / One six-membered ring |
| Members | Adenine, guanine / Cytosine, thymine, uracil |
| In DNA | Both / Cytosine and thymine |
| In RNA | Both / Cytosine and uracil |
Common MCAT Trap
- "Deoxy" refers specifically to the missing C2 hydroxyl — check carbon 2 of a shown sugar structure to instantly tell DNA from RNA.
- Thymine and uracil are easy to swap: thymine = DNA only, uracil = RNA only. Cytosine is the pyrimidine shared by both.
- The phosphate backbone's negative charge isn't just trivia — it directly explains histone binding (positively charged histones) and why DNA migrates toward the positive electrode in gel electrophoresis.
Quick Recall
What's the structural difference between a nucleoside and a nucleotide?
What bond links adjacent nucleotides in the backbone, and between which carbons?
Which nitrogenous bases have two fused rings?
What is deoxyadenosine triphosphate more commonly abbreviated as?