AP Biology 1.6 Nucleic Acids Study Notes - New Syllabus Effective 2025
AP Biology 1.6 Nucleic Acids Study Notes- New syllabus
AP Biology 1.6 Nucleic Acids Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the structure and function of DNA and RNA.
Key Concepts:
- Nucleic Acids
1.6 — Nucleic Acids (DNA & RNA)
🔹 What are Nucleic Acids?
Nucleic acids are macromolecules that store, transmit, and express genetic information in all living organisms.
There are two main types:
- DNA (Deoxyribonucleic acid)
- RNA (Ribonucleic acid)
🧱 Basic Building Blocks: Nucleotides
Each nucleotide (monomer unit) has 3 parts:
- 🧪 Phosphate group
- 🧬 5-carbon sugar
- DNA → Deoxyribose
- RNA → Ribose
- 🔡 Nitrogenous base
There are 5 bases total:
- Adenine (A)
- Guanine (G)
- Cytosine (C)
- Thymine (T) → only in DNA
- Uracil (U) → only in RNA
🧬 DNA Structure
- Double helix: Two strands twisted like a spiral staircase
- Base pairing (complementary): A 🧬 T, G 🧬 C
- Antiparallel: Strands run in opposite directions
- Held together by hydrogen bonds between nitrogenous bases
🧫 RNA Structure
- Single-stranded
- Uracil (U) replaces Thymine (T) → A pairs with U
- Has ribose sugar instead of deoxyribose
📚 Functions
| Nucleic Acid | Function |
|---|---|
| DNA | Stores genetic blueprint for building proteins |
| RNA | Helps carry out instructions from DNA to make proteins (involved in protein synthesis) |
📌 Quick Differences: DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Strands | Double-stranded | Single-stranded |
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Location | Mostly in nucleus | Nucleus & cytoplasm |
| Role | Long-term info storage | Protein synthesis helper |
1.6.A.1 — Nucleotide Structure in DNA & RNA
🔑 Key Points
🔹 Nucleotide = Basic Unit of Nucleic Acids
Each nucleotide has 3 parts:
- Sugar (5-carbon sugar)
- DNA → Deoxyribose
- RNA → Ribose
- Phosphate group (PO₄³⁻)
- Links the sugar of one nucleotide to the next
- Creates the sugar-phosphate backbone of DNA/RNA
- Nitrogenous base
- Carries genetic code
- 5 types:
- Adenine (A)
- Guanine (G)
- Cytosine (C)
- Thymine (T) → only in DNA
- Uracil (U) → only in RNA
🧬 DNA vs. RNA Nucleotides
| Component | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Structure | Double-stranded helix | Single-stranded |
🧠 Fun Fact (Memory Tip):
- “CUT the PY” → C, U, and T are pyrimidines
- “Pure As Gold” → A and G are purines
🧩 How it connects to function:
- The order of bases (A, T/U, G, C) = genetic code
- It’s the sequence that determines protein-building instructions
- Even a single base change can affect traits or cause mutations
1.6.A.2 – Nucleic Acid Directionality & Synthesis
📌 Key Concept:
Nucleic acids like DNA and RNA are long chains (polymers) made from nucleotides joined in a specific direction.
🧭 What does “direction” mean?
- Each nucleotide has:
- a 5′ (five-prime) phosphate group
- a 3′ (three-prime) hydroxyl group (–OH)
- This creates a directional backbone for the strand:
- The chain always runs from the 5′ end → 3′ end
🧪 Nucleic Acid Synthesis: How strands grow
- New nucleotides are always added to the 3′ end
- This forms covalent bonds (called phosphodiester bonds) between the sugar of one nucleotide and the phosphate of the next
🔄 The 5′ → 3′ directionality is essential for copying DNA, transcribing RNA, and making proteins accurately.
🧠 Why is this important?
- DNA replication, RNA transcription, and protein synthesis all depend on this directionality.
- Enzymes like DNA polymerase can only build in the 5′ → 3′ direction.
- The 3′ end is where new nucleotides are added.
🧬 Summary Table
| Term | Meaning |
|---|---|
| 5′ end | Has a phosphate group (PO₄³⁻) |
| 3′ end | Has a hydroxyl group (–OH) on the sugar |
| Strand grows at | 3′ end only |
| Bond formed | Covalent (phosphodiester) bond |
1.6.A.3 – Structure of DNA & RNA Base Pairing
🧠 Big Idea: DNA = Antiparallel Double Helix
- DNA is made of two strands of nucleotides that:
- Run in opposite directions (antiparallel)
- One strand runs 5′ → 3′, the other 3′ → 5′
- Twist into a double helix (like a spiral staircase)
🧬 Base Pairing Rules (Hydrogen Bonds):
| Molecule | Base Pairs | Bond Type |
|---|---|---|
| DNA | A ↔ T (2 hydrogen bonds) C ↔ G (3 hydrogen bonds) | Hydrogen bonding |
| RNA | A ↔ U (uracil replaces T) | Hydrogen bonding |
🔑 More bonds = more stable → G≡C base pairs are stronger than A=T
🧩 Why Antiparallel Structure Matters
- Allows complementary base pairing
- Enables DNA replication and transcription to occur properly
- Essential for forming the stable double-stranded helix of DNA
📌 Key Vocabulary
- 5′ end: Phosphate group end
- 3′ end: Hydroxyl (–OH) group end
- Antiparallel: Opposite direction of strands
- Hydrogen bond: Weak bond holding complementary bases together
1.6.A.4 – Structural Differences Between DNA & RNA
🧠 Core Concept: DNA vs. RNA – Same building blocks, different designs
Both DNA and RNA are made of nucleotides, but they differ in 3 important structural ways:
🧾 1. Sugar Difference
| DNA | RNA |
|---|---|
| Sugar = Deoxyribose | Sugar = Ribose |
| Lacks one oxygen on 2′ carbon → makes DNA more stable | Has extra –OH group → less stable, but more reactive |
🧪 That one oxygen makes RNA better for short-term use (like messages), and DNA better for long-term storage.
🧬 2. Nitrogen Base Difference
| DNA | RNA |
|---|---|
| Has Thymine (T) | Has Uracil (U) instead of Thymine |
| A ↔ T pairing | A ↔ U pairing |
✨ Uracil is structurally similar to thymine but is used only in RNA.
🧵 3. Strand Difference
| DNA | RNA |
|---|---|
| Double-stranded helix (antiparallel) | Single-stranded |
| Stable, tightly coiled | Flexible, can fold into many shapes (like tRNA, rRNA, etc.) |
🔍 Quick Comparison Table
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Base difference | Thymine (T) | Uracil (U) |
| Strands | Double-stranded (helix) | Single-stranded |
| Stability | More stable (long-term info) | Less stable (short-term use) |
| Location | Nucleus (mostly) | Nucleus & cytoplasm |
✅ Why It Matters:
- DNA stores genetic instructions permanently
- RNA carries out instructions temporarily (like a messenger or builder)