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?
Sign in to unlock this chapter
Biology chapters 1–3 are free. Sign up to unlock every chapter.