Open with a real phenomenon: in 2023, forensic labs used DNA recovered from a 1970s cigarette butt to identify the Long Island Serial Killer suspect after 40+ years. Ask: what molecular property of DNA lets it survive that long and still be read base by base? Take 2–3 student ideas aloud. Steer them toward the sugar-phosphate backbone (covalent, stable) and complementary base pairing (predictable, readable). Tell students that today they'll learn the exact chemistry that makes DNA both stable enough to survive decades and specific enough to identify one human out of 8 billion — and how RNA differs in ways that matter for the rest of Unit 6.

1. 6m

   The nucleotide: three parts, numbered carbons

   Content

   A nucleotide has three parts joined together: a phosphate group, a 5-carbon sugar, and a nitrogenous base. In DNA the sugar is deoxyribose; in RNA it is ribose. The carbons of the sugar are numbered 1' through 5' ("one-prime" through "five-prime") — this numbering is the key to everything else in the lesson. The base attaches at the 1' carbon. The phosphate attaches at the 5' carbon. The 3' carbon carries a free hydroxyl (-OH) group, and this is where the next nucleotide will be added. The single difference between deoxyribose and ribose is at the 2' carbon: deoxyribose has -H there, ribose has -OH. That extra -OH is why RNA is more chemically reactive and less stable than DNA.

   Delivery

   Walk students through the numbered carbons on the slide — 1' base, 5' phosphate, 3' hydroxyl. Emphasize that "5' end" and "3' end" are not arbitrary labels; they name which carbon of the terminal sugar is exposed. Ask a cold-call question: "Which carbon has the free -OH that grabs the next nucleotide?" (3'). Pre-empt the confusion that the prime symbol means something mysterious — it just distinguishes sugar carbons from base atoms, which are also numbered.

2. 5m

   Backbone chemistry: phosphodiester bonds vs. hydrogen bonds

   Content

   Two very different bonds hold DNA together, and confusing them is one of the most common AP mistakes. Along a single strand, nucleotides are joined by phosphodiester bonds — strong covalent bonds between the 3' -OH of one sugar and the 5' phosphate of the next. These form the sugar-phosphate backbone and are what makes DNA chemically durable. Across the two strands, the bases are held together by hydrogen bonds — weak, non-covalent attractions. A-T pairs share 2 hydrogen bonds; G-C pairs share 3. Individually each hydrogen bond is weak, but millions of them along a chromosome add up to a stable duplex — while still being weak enough that enzymes can pry the strands apart during replication and transcription.

   Delivery

   State the rule bluntly: covalent bonds run ALONG a strand; hydrogen bonds run ACROSS the helix. Ask students to predict which bonds break when DNA is heated in PCR (answer: hydrogen bonds — the backbone stays intact). Connect this to why DNA can survive on a 40-year-old cigarette butt: the backbone is covalent. Pre-empt the misconception that hydrogen bonds hold nucleotides together within a strand — this is the #1 wrong answer on FRQs.

3. 5m

   Antiparallel strands and base-pairing geometry

   Content

   The two DNA strands run in opposite directions — one 5'→3', its partner 3'→5'. This is called antiparallel. It is not a stylistic choice: the geometry of the sugars and the bases only allows a stable, uniform-width helix when the strands are oriented this way. Base pairing follows two rules that AP loves to test. First, purine (A or G — double ring) always pairs with pyrimidine (C, T, or U — single ring), so the rungs of the ladder are always the same width (about 2 nm). Second, hydrogen-bond donors and acceptors only line up correctly for A-T and G-C. That's why A does not pair with G, and C does not pair with T, even though textbooks sometimes hint that "any purine + pyrimidine" would fit — the H-bond geometry rules that out.

   Delivery

   Have students physically point their two index fingers in the same direction (parallel) and then flip one (antiparallel) to feel the difference. Emphasize that a strand written 5'-ATGC-3' pairs with 3'-TACG-5', which we usually rewrite as 5'-GCAT-3'. Watch for the flip error — students want to write the complement in the same left-to-right direction as the template. Pre-empt "A pairs with G because they look similar": pairing is purine-with-pyrimidine AND correct H-bond count/geometry.

4. 4m

   RNA vs. DNA — what actually changes

   Content

   RNA differs from DNA in three specific ways, and none of them is "length." First, the sugar: RNA has ribose (2'-OH), DNA has deoxyribose (2'-H). Second, the bases: RNA uses uracil (U) instead of thymine (T); U still pairs with A. Third, strandedness: RNA is typically single-stranded, though it can fold back on itself to form local double-stranded regions (as in tRNA cloverleaf structure and rRNA). RNA molecules are not necessarily short — a mammalian mRNA can be thousands of nucleotides long, and rRNA is longer still. The 2'-OH of ribose is what makes RNA chemically less stable — it can attack the backbone under basic conditions — which is fine for a short-lived messenger but bad for a genome.

   Delivery

   Correct the length misconception directly: "RNA is not defined by being short." Ask students to name where in the cell they'd expect each: DNA in the nucleus (stable, long-term storage), RNA moving between nucleus and cytoplasm (short-lived, working copy). Foreshadow that in 6.3–6.4 they will use these structural features to reason about transcription and translation.

1. 15m

   Build-a-Helix: Paper Nucleotide ModelingLab

   Targets SP2 (Visual Representations) and SP6 (Argumentation). Students work in pairs to physically build a short antiparallel DNA duplex from paper nucleotides, then swap one strand for RNA and describe the structural changes. Teacher setup: hand each pair an envelope with ~20 pre-cut nucleotides (5 each of A, T, G, C). Each paper piece shows the phosphate as a circle at one end (labeled 5'), the sugar as a pentagon in the middle with a small tab labeled 3'-OH at the other end, and the base drawn as either a double ring (purines A, G) or single ring (pyrimidines C, T). Students tape nucleotides end-to-end (5' phosphate to 3'-OH) to form strands. Walk around and check: (1) that within a strand they connect 3'-OH to next 5'-phosphate — this is the phosphodiester bond; (2) that when they pair the two strands, the 5' end of the top lines up with the 3' end of the bottom; (3) that they draw 2 short lines between A-T pairs and 3 short lines between G-C pairs to represent hydrogen bonds. After ~10 minutes of building, direct pairs to swap their top strand for the RNA version and answer the argumentation prompt on the handout. Student handout: Part 1 — Build the DNA duplex 1. Using your paper nucleotides, build a top strand with this sequence, reading left to right in the 5'→3' direction: 5'-A T G G C A T C-3' 2. Tape each nucleotide's 3'-OH tab to the next nucleotide's 5'-phosphate circle. The bonds you are making are called ______________ bonds and they are (covalent / hydrogen — circle one). 3. Now build the complementary bottom strand and place it under the top strand so the two strands are antiparallel. Write the bottom strand here in the direction you actually built it: 5'-______________-3' 4. Between each pair of bases, draw short pencil lines to represent hydrogen bonds. Use two lines for A–T and three lines for G–C. Part 2 — Label your model - Circle in red the 5' end of each strand. - Circle in blue the 3' end of each strand. - Count: how many total hydrogen bonds hold your 8-bp duplex together? ______ Part 3 — Swap in RNA 5. Remove your top DNA strand. Build a single-stranded RNA molecule that would be complementary to the bottom DNA strand. Write it 5'→3': 5'-______________-3' 6. List the three structural changes you had to make going from DNA to RNA: - Sugar: ______________ → ______________ - Base: ______________ replaces ______________ - Number of strands: ______________ Part 4 — Argue from evidence (SP6) 7. A classmate claims: "You can tell a nucleic acid is RNA just by looking at whether it is short." In 2–3 sentences, argue against this claim using specific structural features from your model. Reference at least two structural differences, not length.

   Materials

   • Pre-cut paper nucleotide pieces (A, T, G, C) — ~20 per pair, color-coded by base
   • Tape or paper clips
   • Rulers
   • Colored pencils (red and blue)

   Example outputs

   • Bottom strand built 5'-GATGCCAT-3' (i.e., complementary and antiparallel to 5'-ATGGCATC-3'). Total H-bonds: A-T pairs contribute 2 each and G-C pairs contribute 3 each; for this sequence there are 4 A-T pairs (8 H-bonds) + 4 G-C pairs (12 H-bonds) = 20 hydrogen bonds.
   • RNA complement to bottom strand 5'-GATGCCAT-3' written 5'→3' is 5'-AUGGCAUC-3'. Changes: deoxyribose → ribose; U replaces T; strand is single-stranded.
   • Argument (Part 4): "The claim is wrong because 'short' is not a defining feature. RNA is identified by its ribose sugar (2'-OH) and its use of uracil instead of thymine. A eukaryotic mRNA can be thousands of nucleotides long, and rRNA is longer still, so length alone cannot distinguish RNA from DNA."
   • Common error to correct on the fly: pairs writing the complement in the SAME left-to-right direction as the template (parallel), giving 5'-TACCGTAG-3' instead of the correct antiparallel complement.
2. 8m

   G-C Content and Duplex Stability — Data Analysis

   Targets SP4 (Representing and Describing Data) and SP1 (Concept Explanation). Students analyze real melting-temperature data across organisms and connect the pattern back to hydrogen-bond count. Run this as a silent-solo 5 minutes, then 3 minutes of pair-share and full-class debrief. Student handout — G-C content and DNA melting temperature (Tₘ) The melting temperature Tₘ is the temperature at which half of a DNA sample's strands have separated. Higher Tₘ = more stable duplex. Data table - Organism A (deep-sea psychrophile): G-C content = 32%, Tₘ = 74 °C - Organism B (Escherichia coli, gut bacterium): G-C content = 50%, Tₘ = 87 °C - Organism C (Thermus aquaticus, hot-spring bacterium): G-C content = 68%, Tₘ = 95 °C - Organism D (unknown thermophile from a 90 °C vent): G-C content = ______, Tₘ = ______ Questions 1. Describe the trend between G-C content and Tₘ in one sentence. ______________ 2. Explain the trend at the molecular level. Your answer must reference the number of hydrogen bonds per base pair. ______________ 3. Predict a reasonable G-C content range for Organism D and justify. ______________ 4. A student writes: "Higher G-C content makes DNA more stable because G-C pairs have stronger covalent bonds." Identify the error and correct it. ______________

   Materials

   • Handout (below) — one per student

   Example outputs

   • Q1: As G-C content increases, Tₘ increases — the duplex is more heat-stable when it contains more G-C pairs.
   • Q2: Each G-C pair is held by 3 hydrogen bonds while each A-T pair is held by only 2. More G-C means more hydrogen bonds per unit length, so more thermal energy is required to separate the strands.
   • Q3: Roughly 60–70% G-C, because Organism D lives at ~90 °C, similar to T. aquaticus (68% G-C, Tₘ = 95 °C).
   • Q4: The error is calling the bonds covalent. G-C pairs are held by 3 HYDROGEN bonds (not covalent bonds). Covalent phosphodiester bonds run along the backbone, not between paired bases.

1. A single DNA strand is written 5'-TACGGATC-3'. Write the complementary strand in the correct antiparallel orientation, labeling the 5' and 3' ends. Then explain in one sentence why the strands must be antiparallel rather than parallel. (Targets SP1 Concept Explanation and SP2 Visual Representations.)

   short answerComplement: 3'-ATGCCTAG-5' (equivalently written 5'-GATCCGTA-3'). Explanation: the sugars in each strand are oriented so that the 3'-OH and 5'-phosphate only line up correctly for stable phosphodiester geometry and consistent base-pair width when the two strands run in opposite directions; a parallel arrangement cannot form the proper hydrogen-bond geometry or a uniform ~2 nm helix diameter.

2. A short nucleic acid segment has the sequence 5'-AUGCCGAU-3' and is single-stranded. Argue whether this molecule is DNA or RNA, citing at least two specific structural features. (Targets SP6 Argumentation.)

   short answerIt is RNA. Evidence: (1) the sequence contains uracil (U), which is used in RNA instead of thymine; (2) the molecule is described as single-stranded, which is typical of RNA. A stronger answer also notes that if we could inspect the sugar, RNA would show ribose with a 2'-OH rather than deoxyribose. Students should NOT argue from length.

3. Which statement correctly describes the bonds in a DNA double helix? A) Hydrogen bonds link adjacent nucleotides within one strand; covalent bonds pair the two strands. B) Phosphodiester bonds link adjacent nucleotides within one strand; hydrogen bonds pair the two strands. C) Phosphodiester bonds link both the backbone and the paired bases. D) Hydrogen bonds link both the backbone and the paired bases. (Targets SP1 Concept Explanation.)

   multiple choiceB. Phosphodiester (covalent) bonds link nucleotides along the sugar-phosphate backbone of a single strand; hydrogen bonds hold the two strands together across the helix by pairing A–T (2 H-bonds) and G–C (3 H-bonds).

4. A 100-bp DNA fragment from organism X has 40 adenines. Using Chargaff's rules (a consequence of complementary base pairing), determine the number of thymines, guanines, and cytosines, and calculate the total number of hydrogen bonds holding this duplex together. (Targets SP5 Statistical Tests and Data Analysis.)

   calculation100 bp = 200 bases total. A = 40, so T = 40 (A pairs with T). Remaining bases = 200 − 80 = 120, split evenly between G and C: G = 60, C = 60. H-bond count: each A-T pair contributes 2 H-bonds and each G-C pair contributes 3. A-T pairs = 40 → 40 × 2 = 80 H-bonds. G-C pairs = 60 → 60 × 3 = 180 H-bonds. Total = 80 + 180 = 260 hydrogen bonds.

• ☐Pre-cut paper nucleotide sets (5 each of A, T, G, C per pair) with 5'-phosphate circles and 3'-OH tabs — enough for pairs
• ☐Tape or paper clips
• ☐Rulers
• ☐Red and blue colored pencils
• ☐Printed Build-a-Helix handout (one per pair)
• ☐Printed G-C Content handout (one per student)
• ☐Printed exit-ticket / formative assessment (one per student)