Lesson at a Glance | He Tirohanga Whakamua
Timing: This is a 70-minute sequence. For shorter periods, pause after PMAT and begin the sourced harakeke application next lesson.
Ngā Whāinga Ako | Learning Intentions
Students will know
- How semi-conservative DNA replication occurs during S-phase: the parent strands are antiparallel, one running 5′→3′ and the other 3′→5′; helicase separates them and DNA polymerase adds complementary nucleotides while extending each new strand 5′→3′.
- The 4 stages of Mitosis (PMAT) normally creating 2 genetically near-identical diploid daughter cells for growth and repair.
- Factors influencing division rate: cell location (high in root meristems & skin stem cells; zero in mature neurons), organism age, nutrient availability, and enzyme rates.
Students will demonstrate
- By drawing a complete semi-conservative replication fork and 4-stage mitosis sequence.
- By completing formative Section 9 in their class exercise book or revision folder.
Curriculum alignment
- NZC (2007) · Science · Level 7 · Living World: “Explore the diverse ways in which animals and plants carry out the life processes.”
Do Now | Tīmatanga Whakaaro (10 min)
Semi-Conservative Copy Prompt:
"When a new strand of DNA is built, why does each new double helix retain one original parent strand and one newly synthesised strand?"
Unpack: This is called semi-conservative replication. Complementary base pairing and proofreading support high-fidelity copying, but they do not guarantee an exact copy. An uncorrected replication error can become a mutation inherited by a daughter cell.
DNA Replication (15 min)
1. Semi-conservative
Each new DNA molecule keeps one strand of the original and one newly built strand. That is what the name means — half of the original is conserved in each copy.
2. Unwinding
Helicase breaks the hydrogen bonds holding the base pairs together, and the double helix separates into two single strands. Each one is now a template.
3. Complementary pairing on antiparallel strands
The parent strands run in opposite directions: one 5′→3′ and the other 3′→5′. DNA polymerase pairs A with T and C with G, but can extend a new strand only in the 5′→3′ direction. This produces high-fidelity copies; proofreading repairs many, but not all, pairing errors.
4. Why it happens before mitosis
Each daughter cell needs a full set of instructions. Replication happens in interphase (S phase); mitosis only separates what replication already made. Get that order wrong and the rest of the lesson will not make sense.
Mahi | Do this: Given the template strand 3′-TACGGATCA-5′, write the complementary strand as 5′-ATGCCTAGT-3′. Label both directions, then sketch the other parent strand running 5′→3′ beside its antiparallel partner. Finally answer: if polymerase paired one base wrongly and it was not repaired, what could the daughter cell inherit?
4 Stages of Mitosis (PMAT) (15 min)
1. Prophase
Chromatin condenses into visible chromosomes (sister chromatids joined at centromere). Nuclear membrane dissolves.
2. Metaphase
Chromosomes line up along the cell equator (middle). Spindle fibres attach to centromeres.
3. Anaphase
Spindle fibres contract, pulling sister chromatids apart to opposite poles of the cell.
4. Telophase & Cytokinesis
New nuclear membranes reform around separated chromosomes. Cytoplasm divides (cytokinesis), normally forming 2 genetically near-identical cells; an inherited replication error can make them differ.
Apply it carefully: sourced harakeke botany and one possible consequence
What the sources establish. Manaaki Whenua describes harakeke leaves as arranged around the growing point, or rito, to form a fan; most fans have mature leaves on both sides of that central growth. Te Papa's teaching resource describes the two mātua protecting the rito and explains that whakapapa informs the practice of protecting the central three leaves. These are the sources for the botany and tikanga used here.
The bounded biology. New plant tissue requires cell division. It is therefore reasonable to investigate reduced future leaf production as one possible biological consequence of damage to actively growing tissue. That hypothesis is not a deeper reason for the tikanga, does not explain its origin or meaning, and carries no authority over it.
Mahi | Do this (in pairs): Draw a fan with the sourced growing point and mature outer leaves labelled. Mark where you predict mitotic activity would be higher and explain the evidence you would collect to test that prediction. Compare possible consequences of damage to actively growing tissue and removal of a mature outer leaf. Finish with one sentence naming what this cell-biology model cannot tell you about the tikanga.
Kaiako note: Treat student explanations as biological hypotheses tied to the named sources. Do not present mitosis as the origin, reason or authority for the harvesting practice.
📁 Formative Revision Exercise Book — Section 9: DNA Replication & Mitosis
Students complete Section 9 in their exercise book or class revision folder. This is formative classroom evidence, not an NZQA submission:
Section 9 Requirements:
1. Replication Fork Diagram: Draw helicase separating both parent strands. Label one parent 5′→3′ and the antiparallel parent 3′→5′; show complementary bases and label every new strand as extending 5′→3′.
2. PMAT Mitosis Sequence: Annotated 4-stage diagram illustrating Prophase, Metaphase, Anaphase, and Telophase with chromosome behaviour.
3. Division Rate Factors (Excellence): 1 paragraph explaining why plant root tip meristems have high mitotic index compared to mature leaf cells.
Kaiako — mātauranga handoff. The cell biology is ours to run. The tikanga of harvesting harakeke — including the rito, the awhi rito, and locally held decisions about returning or clearing offcuts — belongs to those who hold it, and carries whakapapa and practice a biology lesson cannot convey. Please involve kaiako Māori and, where a pā harakeke is held locally, the people who tend it, before teaching this section. Traditional records and current pest-management guidance differ about offcuts, so follow those local holders. Do not let the cellular explanation stand as the explanation of the practice.
Exit Verification | Ka Mutu Hoki (5 min)
Exit Check:
"My Section 9 diagram traces Helicase and DNA Polymerase in replication, and illustrates chromatid separation during Anaphase."
Teacher Planning & NCEA Alignment
NCEA Level 2 Biology Alignment (4 Credits External):
- DNA Replication & Mitosis: Demonstrate understanding of semi-conservative replication, cell cycle phases (Interphase, PMAT), and factors affecting mitotic rate.
- Cell Specialisation: Explain variation in division rate across different tissues and developmental stages.
Vocabulary: Semi-conservative replication, Helicase, DNA Polymerase, interphase, S-phase, Prophase, Metaphase, Anaphase, Telophase, cytokinesis, chromatids, meristem.
Paired replication and mitosis concept in Guided Media & Evidence →
Paired division-rate concept in Guided Media & Evidence →