The shape of the lesson
By the end of the lesson, learners can
- A fork that copies both strands: Press Replicate and follow the fork from left to right.
- Why semi-conservative?: Three models were once possible: conservative (the parent molecule stays whole), semi-conservative (each new molecule has one old strand) and dispersive (old and new DNA mixed in both strands).
- Semi-conservative: each new molecule = one old strand + one new.
- DNA polymerase builds new strands 5′ to 3′ only.
- Leading: continuous. Lagging: fragments joined by DNA ligase.
- Both enzymes make phosphodiester bonds; hydrogen bonds form by pairing.
One picture every idea returns to
DNA is copied during the S phase of the cell cycle. The hydrogen bonds between bases break and the double helix unwinds, so each strand is exposed as a template. Free DNA nucleotides pair with the exposed bases by complementary base pairing (A with T, C with G). DNA polymerase joins the new nucleotides by forming phosphodiester bonds, but it can add nucleotides only to the 3′ end of a growing strand, so new DNA is always built in the 5′ to 3′ direction. On one template the new strand grows towards the replication fork continuously (the leading strand); on the other it must grow away from the fork, in short fragments (the lagging strand), which DNA ligase joins with phosphodiester bonds. Each daughter molecule has one original and one new strand: replication is semi-conservative.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, One old strand, one new.. Before a cell divides, every DNA molecule in its nucleus is copied, accurately, during the S phase of interphase. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: A fork that copies both strands | Press Replicate and follow the fork from left to right. Project tab 3 of the lab (Nucleic acids lab, semi-conservative replication) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Why semi-conservative? | Three models were once possible: conservative (the parent molecule stays whole), semi-conservative (each new molecule has one old strand) and dispersive (old and new DNA mixed in both strands). | Cooperative Concept Mapping |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 17 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why semi-conservative?, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Do not say DNA polymerase builds the lagging strand 3′ to 5′. | It never does: both new strands are made 5′ to 3′. The lagging strand is in fragments precisely because its 5′ to 3′ direction points away from the fork. |
| The intermediate band never disappears in later generations: the two original heavy strands are kept, so there are always exactly two hybrid molecules. | Only their proportion halves each generation. |
Semi-conservative replication of DNA: worksheet
6 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
State the roles of DNA polymerase and DNA ligase in DNA replication.
Describe the semi-conservative replication of a DNA molecule.
Explain why one new strand is made continuously but the other is made as a series of short fragments.
Bacteria grown for many generations in 15N are transferred to 14N. Predict the percentages of heavy, intermediate and light DNA after three generations, and state which model of replication your answer assumes.
Explain how the result after one generation in 14N rules out conservative replication.
Suggest why it is important that DNA is replicated with great accuracy before cell division.
Answer key and marking notes
- DNA polymerase joins free DNA nucleotides, paired with the template strand, into a new strand by forming phosphodiester bonds (adding to the 3′ end).
- DNA ligase joins the short fragments of the lagging strand by forming phosphodiester bonds between them.
- Hydrogen bonds between bases break and the double helix unwinds / the two strands separate.
- Both strands act as templates.
- Free DNA nucleotides pair with exposed bases by complementary base pairing: A with T, C with G.
- DNA polymerase joins adjacent nucleotides with phosphodiester bonds, building the new strand 5′ to 3′.
- Each new DNA molecule contains one original strand and one newly made strand.
- The template strands are antiparallel, and DNA polymerase can only add nucleotides in the 5′ to 3′ direction.
- On the leading strand template, 5′ to 3′ synthesis is towards the replication fork, so it can continue as the fork opens.
- On the lagging strand template, 5′ to 3′ synthesis is away from the fork, so new fragments must keep being started near the fork, and these are joined by DNA ligase.
- Heavy 0%; intermediate 25%; light 75%.
- After one generation 2 hybrid molecules; after two, 2 hybrid + 2 light; after three, 2 hybrid + 6 light (2 of 8 = 25%).
- Assumes semi-conservative replication.
- Conservative replication would keep the parent molecule intact, giving one heavy band and one light band.
- Only one band of intermediate density is seen, so every molecule contains one heavy and one light strand.
- Each daughter cell must receive an identical copy of the genetic information.
- Errors would change base sequences (mutations), which could change polypeptides and harm cell function, and would be passed on to all descendant cells.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement describes semi-conservative replication?
- Aeach new DNA molecule contains one strand from the parent molecule and one new strand
- Bthe parent DNA molecule stays intact and a completely new molecule is made
- Ceach strand of the new molecules contains short pieces of old and new DNA
- Donly half of the parent DNA molecule is copied in each cell cycle
The diagram shows a replication fork. Which statement about new strand Y is correct?
- Ait is made 5′ to 3′ away from the fork, as fragments joined by DNA ligase
- Bit is made 3′ to 5′ towards the fork, as fragments joined by DNA ligase
- Cit is made continuously 5′ to 3′ towards the fork by DNA polymerase
- Dit is made 5′ to 3′ away from the fork, and needs no DNA ligase
Which enzyme forms the phosphodiester bonds between neighbouring nucleotides along the leading strand?
- ADNA polymerase
- BDNA ligase
- CRNA polymerase
- DATP synthase
A cell in G1 contains 6.8 pg of DNA. How much DNA does it contain at the end of the S phase?
- A13.6 pg
- B6.8 pg
- C3.4 pg
- D27.2 pg
During replication, what determines which free nucleotide is added opposite each base of the template strand?
- Acomplementary base pairing: A with T and C with G
- BDNA polymerase choosing nucleotides in a fixed order
- Cthe base sequence of the mRNA for that gene
- Dcomplementary base pairing: A with U and C with G
Bacteria with only 15N in their DNA are moved to a 14N medium and divide four times. What percentage of the DNA molecules are of intermediate density?
- A12.5%
- B25%
- C6.25%
- D50%
Bacteria grown for many generations in 14N are moved to a 15N medium. If replication is semi-conservative, what is seen after two generations?
- A50% intermediate DNA and 50% heavy DNA
- B50% intermediate DNA and 50% light DNA
- C25% light DNA and 75% heavy DNA
- D100% DNA of intermediate density
A cell is treated with a drug that inactivates DNA ligase but not DNA polymerase. Which result is expected after one round of replication?
- Athe lagging strand stays as separate short fragments
- Bthe leading strand is made as short fragments instead
- Cno new nucleotides are added to either template strand
- Dboth new strands are made 3′ to 5′ instead
Quiz key
In semi-conservative replication each daughter molecule keeps one complete parental strand and gains one new strand. Keeping the parent intact is the conservative model, and mixed old and new pieces in each strand is the dispersive model; both were ruled out by density experiments. The whole molecule is copied, not half of it.
Y is in fragments, so it is the lagging strand: DNA polymerase builds each fragment 5′ to 3′, which on this template points away from the fork, and DNA ligase joins the fragments. DNA polymerase never builds 3′ to 5′. Continuous synthesis towards the fork describes X, the leading strand. Without ligase the fragments would remain separate pieces.
DNA polymerase joins each new nucleotide to the 3′ end of the growing strand with a phosphodiester bond. DNA ligase only seals gaps between separate fragments, which the leading strand does not have along its length. RNA polymerase makes RNA in transcription. ATP synthase makes ATP from ADP and phosphate; it does not join nucleotides into a chain.
Every DNA molecule is replicated once during the S phase, so the mass doubles to 13.6 pg. 6.8 pg would mean no replication. 3.4 pg is the halving that happens later, when the cell divides. 27.2 pg would need two rounds of replication.
Free nucleotides pair with the exposed template bases by complementary base pairing (A-T, C-G); DNA polymerase then joins the one that has paired. The enzyme does not choose a fixed order: the template does. mRNA is not involved in replication. Uracil occurs in RNA, not DNA.
One parent molecule gives 2, 4, 8, then 16 molecules. Only the two original heavy strands survive, each in a hybrid molecule, so 2 out of 16 = 12.5%. 25% is the answer after three generations. 6.25% (1 in 16) counts only one heavy strand. 50% is the result after two generations.
After one generation every molecule is light/heavy (intermediate). In the second, each hybrid gives one hybrid and one heavy/heavy molecule: 50% intermediate, 50% heavy. Half intermediate and half light is the result of the usual heavy-to-light experiment, not this one. No light/light molecules can form once all new strands are heavy. 100% intermediate is the first-generation result.
DNA polymerase still builds both new strands, but the lagging-strand fragments cannot be joined, so they remain separate. The leading strand is built continuously by DNA polymerase and does not depend on ligase along its length. Nucleotides are still added because DNA polymerase works. No enzyme can switch DNA synthesis to the 3′ to 5′ direction.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.