Copying the DNA Code

Overview

Students investigate how DNA is copied before cell division. Building on prior knowledge of DNA structure, nucleotides, complementary base pairing, chromosomes and genes, they model semi-conservative replication and explain the roles of key enzymes on leading and lagging strands.

Learning intentions

• WALT explain why DNA must replicate before a cell divides.
• WALT model semi-conservative DNA replication using complementary base pairing.
• WALT describe the roles of helicase, DNA polymerase and ligase.
• WALT distinguish between leading and lagging strand synthesis and explain proofreading.

Prior knowledge: Students should know that DNA is a double-stranded molecule made from nucleotides, that bases pair A–T and C–G, and that genes are sections of DNA found on chromosomes.

Success criteria

• I can describe semi-conservative replication as two DNA molecules, each containing one original strand and one new strand.
• I can use complementary base pairing accurately when building a new strand.
• I can explain the roles of helicase, DNA polymerase, ligase and proofreading.
• I can compare continuous leading-strand synthesis with discontinuous lagging-strand synthesis using the term Okazaki fragments.

Curriculum links

• Biological Science — Organism Diversity: inheritance, DNA, variation and environmental influences.
• Biological Science — Genetic material and inheritance: explaining connections between cells, chromosomes and DNA.
• Biological Science — Organism Diversity: describing relationships between DNA, genes and chromosomes using diagrams or physical representations.
• Science capabilities: understanding and communicating science, using evidence, and interpreting representations and models.

Lesson structure (60 minutes)

1. 0–7 min · Hook and retrieval. Open with the DNA copying hook and retrieval questions and display the question, “How can one cell make two genetically identical cells?” Students complete a quick think-pair-share, then answer three retrieval prompts: identify a nucleotide, state the base-pairing rules, and explain where genes are located. Teacher addresses misconceptions, especially that DNA replication is not the same as transcription.

2. 7–18 min · Teacher explanation. Use the DNA replication explanation slides to explain that replication occurs before cell division. Model the sequence: helicase unwinds and separates the two DNA strands by breaking hydrogen bonds; each original strand acts as a template; DNA polymerase adds complementary nucleotides in the 5′ to 3′ direction; and proofreading helps correct mismatched bases. Emphasise that replication is semi-conservative because each daughter DNA molecule contains one original strand and one newly built strand. Students annotate a simple diagram on the DNA replication model worksheet.

3. 18–25 min · Strand challenge. Demonstrate why the strands behave differently because DNA polymerase works in one direction. Explain that the leading strand is built continuously towards the replication fork, while the lagging strand is built discontinuously away from the fork in short sections called Okazaki fragments. Explain that ligase joins these fragments into one continuous strand. Students use the worksheet diagram to label helicase, DNA polymerase, leading strand, lagging strand, Okazaki fragments and ligase. Check understanding with “Which enzyme joins the fragments, and why are fragments needed?”

4. 25–45 min · Hands-on model building. In groups of four, students use coloured paper strips or cards to construct a short double-stranded DNA molecule, then simulate replication. One student acts as helicase and separates the strands, two students act as DNA polymerase and add complementary bases, and one student checks base pairing and proofreading. Groups show continuous synthesis on one side and separate Okazaki fragments on the other, then use a clip or strip of tape as ligase to join the fragments. Each group labels the original and new strands and records the final result on the DNA replication model worksheet. Teacher circulates, questioning: “Which strand is the template?”, “How do you know this base belongs here?” and “Where is the semi-conservative result?”

5. 45–53 min · Explain and assess. Groups compare models with a nearby group and use the slides’ discussion prompt to give a 60-second explanation of replication. Teacher uses mini-whiteboards for formative checks: students draw the outcome of replication, identify the enzyme that unwinds DNA, and explain the difference between the two strands. Provide immediate correction and ask students to improve one label or explanation on their worksheet.

6. 53–60 min · Exit ticket and plenary. Students complete the DNA replication exit ticket independently. They must define semi-conservative replication, match three enzymes to their roles, and explain why Okazaki fragments form. Finish with a whole-class response to: “How does accurate DNA replication support inheritance?” Collect tickets to identify next steps.

Resources

• the DNA replication teaching deck
• the DNA replication model worksheet
• Coloured paper DNA strips or nucleotide cards
• Scissors and glue sticks or removable tape
• Coloured pencils and black markers
• Mini-whiteboards and pens
• Group role cards or displayed role instructions
• Timer
• Optional printed dyslexia-friendly copies with enlarged diagrams

Assessment

• Listen to retrieval responses and question groups during model building, checking accurate use of complementary base pairing and enzyme terminology.
• Use mini-whiteboard responses to identify misconceptions about semi-conservative replication, strand direction and the role of ligase.
• Mark the exit ticket for: correct definition, accurate enzyme functions, and a clear explanation of leading and lagging strands. Use results to plan a short reteach or extension.

Differentiation

• Provide a colour-coded DNA template, a completed example of one base pair, labelled enzyme cards and sentence starters: “Helicase…”, “DNA polymerase…”, and “Replication is semi-conservative because…”.
• Give students who need support a pre-drawn replication fork and allow them to explain orally, record an audio response, or work with a supportive partner. Read instructions aloud and chunk the worksheet into one step at a time.
• Offer dyslexia-friendly reading options: use a clear sans-serif font, at least 14-point text, generous spacing, short lines, tinted or cream paper, diagrams alongside definitions, and no unnecessary italics or dense blocks of text. Provide text-to-speech or teacher-read instructions where available.
• For advanced learners, require students to add 5′ and 3′ labels, explain why DNA polymerase synthesises only in the 5′ to 3′ direction, and predict the consequences of a proofreading failure or a non-functioning ligase enzyme.