Teaching DNA as a Chemical

Students usually meet DNA in biology as “the genetic code” — an abstract store of information. They rarely meet it as a chemical: a polymer with a charged backbone, held together by hydrogen bonds, that dissolves in water and precipitates in alcohol. Teaching DNA from the chemistry side makes the biology easier to understand: base pairing becomes a matter of hydrogen-bond geometry, and techniques like electrophoresis become applications of charge. This guide outlines three lessons for students aged roughly 15–18.

Learning goals

Students should be able to:

1. Identify the three components of a nucleotide and the bonds that join nucleotides.
2. Explain complementary base pairing in terms of hydrogen bonds and molecular shape.
3. Explain why DNA is negatively charged and use this to explain extraction and electrophoresis.
4. Use Chargaff’s rules in calculations.
5. Explain how the structure of DNA allows it to be copied accurately.

Prior knowledge: covalent and hydrogen bonding, polymers and condensation reactions, acids and ions.

Lesson 1: meet the molecule — extraction

Starter (5 minutes)

Hold up a strawberry. Ask: “If I could pull all the DNA out of this strawberry, what would it look like? A powder? A liquid? A double helix you could see?” Collect predictions.

Practical: DNA extraction (30 minutes)

Students extract DNA from strawberries using washing-up liquid, salt and ice-cold ethanol. The full method, equipment list and safety notes are in extracting DNA from strawberries.

As they work, ask them to predict the purpose of each ingredient before you explain:

• Mashing → breaks cell walls.
• Detergent → dissolves phospholipid membranes (link to how detergent removes grease; see phospholipids and membranes).
• Salt → Na⁺ ions shield DNA’s negative charges.
• Cold ethanol → DNA is insoluble in ethanol and precipitates.

Plenary (10 minutes)

Ask: “Is the white stringy stuff a double helix?” Guide students to scale: a double helix is 2 nm wide. What they see is millions of tangled strands. Compare with a rope made of thousands of threads.

Safety: ethanol is highly flammable — no flames; eye protection.

Lesson 2: building the molecule — models and bonds

Paper nucleotide model (25 minutes)

Give each student printed cut-outs of phosphate (circle), deoxyribose (pentagon, with carbons 1′–5′ labelled) and the four bases (A and G as larger two-ring shapes; T and C as smaller one-ring shapes). Each base cut-out shows the positions of hydrogen-bond donors (H) and acceptors (N or O lone pairs) along its pairing edge.

Tasks:

1. Build four nucleotides: base on carbon 1′, phosphate on carbon 5′.
2. Join them into a strand: link the 3′ carbon of one sugar to the phosphate of the next (phosphodiester bond, formed by condensation — a small “H₂O” card is removed each time).
3. Build a complementary strand. Students discover that the only pairs where donors line up with acceptors are A–T (2 hydrogen bonds) and G–C (3). They also find the second strand must run upside down — antiparallel.

This makes base pairing a chemical fact rather than a rule to memorise (see base pairing).

Discussion (10 minutes)

• Why do purines always pair with pyrimidines? (Constant width of the ladder.)
• Why is the backbone on the outside? (Polar, charged phosphates interact with water; bases, which are flatter and less polar, stack inside.)
• Why is each phosphate negatively charged at pH 7? (Its remaining –OH is strongly acidic.)

Chargaff’s rules (10 minutes)

Give students real base-composition data from several organisms and ask them to spot patterns: in every double-stranded sample, %A ≈ %T and %G ≈ %C, but the A+T : G+C ratio varies between species. Then practise calculations: “If 18 % of bases are cytosine, what percentage are adenine?” (32 %). These are the data Erwin Chargaff published in the late 1940s and early 1950s, which helped Watson and Crick.

Lesson 3: DNA in action — charge, copying and history

Demonstration or practical: gel electrophoresis (20 minutes)

If equipment is available, run pre-cut DNA samples or food dyes on an agarose gel. Otherwise, use an online simulation or a video.

Key chemistry:

• DNA is negatively charged, so it moves towards the positive electrode (anode).
• The gel acts as a molecular sieve: shorter fragments move faster.
• Every nucleotide carries one charge, so charge per unit length is roughly constant — size, not charge, decides speed.

Link to forensic and medical uses (see electrophoresis).

Copying: semi-conservative replication (15 minutes)

Using the paper models, “unzip” a double strand by breaking the hydrogen bonds (never the phosphodiester bonds), then build a new partner for each old strand. Ask: “Why is it important that the strands are held together by hydrogen bonds and not covalent bonds?” (Hydrogen bonds are weak enough to separate without breaking the backbone, yet many together keep the helix stable.)

Briefly present the Meselson–Stahl experiment using ¹⁵N and ¹⁴N — a nice link to isotopes and density. See DNA replication.

History snapshot (10 minutes)

Tell the story concisely: Friedrich Miescher isolated “nuclein” from white blood cells in 1869; Chargaff’s ratios; Rosalind Franklin’s X-ray diffraction image (Photo 51, 1952) showing a helical pattern; Watson and Crick’s model (1953). Discuss how the chemistry — hydrogen bonding and the correct tautomers of the bases — was essential to getting the model right.

Misconceptions to expect

Misconception	How to address it
“DNA is a living thing.”	It’s a molecule; it can be extracted, dried and stored in a freezer.
“Base pairs are held by covalent bonds.”	Hydrogen bonds between bases; covalent bonds within each strand.
“The helix is visible in the extraction.”	Discuss scale — 2 nm wide.
“DNA moves to the negative electrode because it’s ‘attracted to’ electricity.”	Opposite charges attract: negative DNA moves to the positive electrode.
“Only humans/animals have DNA.”	Extract from plants; all cellular life uses DNA.
“A–T and G–C is an arbitrary rule.”	Build the paper model; only these pairs line up hydrogen-bond donors and acceptors.

Questions to check understanding

1. Name the bond that joins nucleotides in a strand. (Phosphodiester.)
2. How many hydrogen bonds join G to C? (Three.)
3. A DNA sample has 28 % thymine. Calculate %G. (A = 28 %, so G + C = 44 %, G = 22 %.)
4. Explain why DNA moves towards the anode in electrophoresis. (Phosphate groups are negatively charged.)
5. Why does salt help in DNA extraction? (Na⁺ shields negative phosphates, allowing strands to aggregate and precipitate.)
6. Challenge: Explain why DNA rich in G–C pairs needs a higher temperature to separate its strands. (More hydrogen bonds per pair and stronger base stacking.)

Differentiation

• Support: coloured, pre-labelled model pieces; a partially completed flow chart for extraction.
• Stretch: research why RNA is less stable than DNA (the 2′-OH group can attack the backbone); calculate the mass of DNA in a human cell (see DNA and RNA chemistry practice).

Summary for the teacher

• Start with extraction to make DNA a tangible substance.
• Use paper models so students discover antiparallel strands and hydrogen-bonded pairing.
• Use charge to connect extraction, structure and electrophoresis.
• Keep asking “which bond?” — covalent in the backbone, hydrogen between strands.

For background reading, point students to DNA structure and nucleic acids.