DNA Structure
DNA and RNA are both nucleic acids built from repeating nucleotide units that carry genetic information.
DNA and RNA are both nucleic acids — long molecules built from repeating units that carry genetic information. Although they're built from similar types of building blocks, they play very different roles in the cell. This subtopic focuses on DNA: what its building blocks are, how they link together, and how they're named.
Key Takeaways
A nucleoside is a sugar plus a base; a nucleotide is a nucleoside plus one or more phosphate groups.
The sugar is what distinguishes DNA (deoxyribose, no carbon-2 hydroxyl) from RNA (ribose, carbon-2 hydroxyl present).
The sugar-phosphate backbone links via phosphodiester bonds and gives DNA its 5'-to-3' directionality; the negatively charged backbone matters for histone binding and gel electrophoresis.
Purines (adenine, guanine) have two fused rings; pyrimidines (cytosine, thymine, uracil) have one ring. Thymine is DNA-only; uracil is RNA-only.
All nitrogenous bases are aromatic heterocycles (cyclic, planar, conjugated, obeying Hückel's rule), and their aromaticity supports base stacking, which helps stabilize the double helix.
Nucleotide names follow base identity + phosphate count: adenosine → AMP → ADP → ATP (and the deoxy- equivalents for DNA).
Nucleic Acids: The Basics
DNA is a polymer, meaning it's made of many repeating units strung together. Each repeating unit — called a monodeoxyribonucleotide — contains three key parts:
A five-carbon (pentose) sugar
A nitrogenous base
One or more phosphate groups
Nucleosides and Nucleotides
These two terms are easy to mix up, but the distinction is simple:
A nucleoside is a sugar plus a base (the base attaches at carbon 1 of the sugar).
A nucleotide is a nucleoside with one or more phosphate groups attached (bonded to carbon 5 of the sugar).
Nucleotides are the true building blocks of DNA — strung together in a long chain, they form the DNA polymer. The phosphate groups aren't just structural: each one carries a negative charge, and those negative charges repel each other. That repulsion stores potential energy, which is exactly why nucleotides like ATP are used as energy carriers in the cell.
The Sugar That Tells DNA and RNA Apart
Both DNA and RNA contain a five-carbon sugar, but one small difference at carbon 2 has major consequences.
Feature | Ribose (RNA) | Deoxyribose (DNA) |
|---|---|---|
Carbon-2 substituent | Hydroxyl (–OH) group | Hydrogen atom only |
Relative stability | Less stable, more reactive | More stable |
Typical role | Temporary tasks (e.g., protein synthesis) | Long-term genetic storage |
The missing hydroxyl group is what "deoxy" means — lacking oxygen. Because RNA's extra –OH group makes it more reactive and more easily broken down, DNA is the molecule built for long-term storage of genetic information. On the MCAT, if you're shown a sugar structure, check carbon 2: a hydroxyl group there means RNA; just a hydrogen means DNA.
The Sugar-Phosphate Backbone
The DNA molecule is a long chain of nucleotides, but it isn't the bases that form the chain — it's the sugars and phosphate groups linking together to form the sugar-phosphate backbone.
This backbone gives DNA directionality: it's always read and built in a specific direction, from the 5' end to the 3' end. Those numbers refer to carbon positions on the sugar — the 5' carbon is where the phosphate group attaches, and the 3' carbon is where the next nucleotide connects. The bond linking them is called a phosphodiester bond, joining the 3'-hydroxyl group of one sugar to the 5'-phosphate group of the next nucleotide.
This linkage repeats again and again, forming a strong, stable backbone with the nitrogenous bases sticking out to the side, like rungs on a ladder. Because each phosphate group carries a negative charge, the entire DNA molecule carries an overall negative charge — a property that matters later for how DNA interacts with positively charged proteins like histones, and for techniques like gel electrophoresis.
DNA in cells is almost always double-stranded: two strands wrap around each other in a spiral to form the double helix, with one strand running 5' to 3' and the other running antiparallel (3' to 5'). The full mechanics of the double helix and base pairing are covered in the next subtopic.
Purines and Pyrimidines
Nitrogenous bases fall into two families:
Feature | Purines | Pyrimidines |
|---|---|---|
Ring structure | Two fused rings (one six-membered, one five-membered) | One six-membered ring |
Members | Adenine, guanine | Cytosine, thymine, uracil |
Found in DNA? | Yes (both) | Cytosine and thymine |
Found in RNA? | Yes (both) | Cytosine and uracil |
Thymine is found only in DNA, and uracil is found only in RNA — cytosine is the one pyrimidine shared by both.
Both purines and pyrimidines are aromatic heterocycles. A heterocycle is a ring structure that includes atoms other than carbon (here, nitrogen). To be aromatic, a molecule must meet four conditions:
Cyclic — it forms a closed ring.
Planar — all the atoms lie flat, in the same plane.
Conjugated — it has alternating single and double bonds, or lone pairs that allow electron delocalization.
It must have a specific number of pi electrons, following Hückel's rule: 4n + 2.
All the nitrogenous bases in DNA and RNA meet these criteria, and their aromaticity contributes to base stacking — the flat, aromatic bases stack on top of one another inside the helix, adding stability to the DNA molecule alongside hydrogen bonding.
Naming Nucleosides and Nucleotides
Nucleoside and nucleotide names follow a consistent pattern based on two things: which nitrogenous base is attached, and how many phosphate groups are added.
Phosphates | Ribose Version (RNA) | Deoxyribose Version (DNA) |
|---|---|---|
0 (nucleoside only) | Adenosine | Deoxyadenosine |
1 | Adenosine monophosphate (AMP) | Deoxyadenosine monophosphate (dAMP) |
2 | Adenosine diphosphate (ADP) | Deoxyadenosine diphosphate (dADP) |
3 | Adenosine triphosphate (ATP) | Deoxyadenosine triphosphate (dATP) |
When adenine attaches to ribose, it's called adenosine; attached to deoxyribose, it becomes deoxyadenosine. Adding phosphate groups builds up the nucleotide names shown above, and the same pattern extends to the other nitrogenous bases (guanine, cytosine, thymine, uracil).
Common MCAT Mistakes
Confusing nucleoside and nucleotide. A nucleoside is only sugar + base; it isn't a nucleotide until at least one phosphate group is attached at carbon 5.
Forgetting which carbon distinguishes DNA from RNA. The difference is at carbon 2 of the sugar — a hydroxyl group means ribose (RNA), a lone hydrogen means deoxyribose (DNA). Don't confuse this with the 3'/5' carbons that matter for the backbone linkage.
Thinking the bases form the backbone. The sugar-phosphate backbone is built from sugars and phosphates linked by phosphodiester bonds; the nitrogenous bases stick out to the side and don't participate in the backbone linkage itself.
Mixing up purines and pyrimidines by base count instead of ring structure. The distinction is structural: purines (adenine, guanine) have two fused rings, pyrimidines (cytosine, thymine, uracil) have one ring — not which nucleic acid they appear in.
MCAT-Style Concept Check
Question: A researcher is comparing two nucleotides. Nucleotide 1 has a hydroxyl group at the 2' carbon of its sugar; Nucleotide 2 has only a hydrogen atom at that position. Which of the following statements is correct?
A) Nucleotide 1 contains ribose and is more chemically stable than Nucleotide 2.
B) Nucleotide 2 contains deoxyribose and is found in DNA, while Nucleotide 1 contains ribose and is found in RNA.
C) Both nucleotides contain the same sugar, differing only in their nitrogenous base.
D) Nucleotide 1 contains deoxyribose because it has an extra oxygen-containing group.
Answer: B
Explanation: A hydroxyl group at the 2' carbon identifies ribose (found in RNA); a hydrogen atom alone at that position identifies deoxyribose (found in DNA) — the "deoxy" name reflects the missing oxygen. Option A is wrong because ribose (with the extra –OH) is actually less stable than deoxyribose, not more. Option C is wrong because the sugars are different, not the bases. Option D is wrong because it's Nucleotide 2, not Nucleotide 1, that has deoxyribose.
FAQ
What's the difference between a nucleoside and a nucleotide?
A nucleoside is a sugar bonded to a nitrogenous base at carbon 1. A nucleotide is a nucleoside with one or more phosphate groups attached at carbon 5 — it's the nucleotide, not the nucleoside, that serves as the actual building block of DNA.
How do you tell DNA's sugar apart from RNA's sugar?
Check carbon 2 of the sugar. Ribose (RNA) has a hydroxyl (–OH) group there; deoxyribose (DNA) has only a hydrogen atom. That missing oxygen is what "deoxy" refers to, and it's why DNA is more chemically stable than RNA.
What holds the DNA backbone together?
Phosphodiester bonds link the 3'-hydroxyl group of one sugar to the 5'-phosphate group of the next nucleotide, forming the sugar-phosphate backbone. This gives DNA a consistent 5'-to-3' directionality, and the backbone's negative charge (from the phosphate groups) is important for interactions with proteins like histones and for techniques like gel electrophoresis.
What's the difference between purines and pyrimidines?
Purines (adenine and guanine) have two fused rings; pyrimidines (cytosine, thymine, and uracil) have a single ring. Thymine appears only in DNA, and uracil appears only in RNA, while adenine, guanine, and cytosine are found in both.
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