DNA and Chromosome Organisation

Overview

In lesson 19 of the unit Cells: The Basis of Life, students distinguish DNA, genes, chromosomes and sister chromatids. They examine how DNA is organised in the nucleus and connect DNA replication with chromosome behaviour during the cell cycle.

Learning intentions

Students will:

• define and distinguish DNA, genes, chromosomes and sister chromatids
• explain how DNA is organised within the nucleus
• describe why DNA replication occurs before cell division
• use accurate biological language to interpret chromosome diagrams

Success criteria

• I can explain the relationship between DNA, genes and chromosomes.
• I can identify a chromosome and a pair of sister chromatids in a diagram.
• I can describe when DNA replication occurs in the cell cycle.
• I can explain how replication ensures new cells receive genetic information.

Curriculum links

• Cells as the basis of life — cell structures and functions
• Cells as the basis of life — cell division and the processes required for life
• Cells to systems — how specialised cells depend on genetic information
• The basis of life — identifying components of living things and their functions

Lesson structure (60 minutes)

1. 0–5 min · Hook and retrieval. Open with the hook and retrieval slides showing a tightly packed chromosome beside an uncoiled DNA molecule, and ask, “Are these different types of genetic material?” Students complete a quick retrieval task naming the nucleus, cell membrane and cytoplasm, then share an initial explanation.

2. 5–17 min · Direct teaching: the genetic information hierarchy. Use the DNA-to-chromosome teaching slides to model the sequence: DNA is a molecule; a gene is a section of DNA containing information; DNA coils around proteins to form chromatin; condensed chromatin forms chromosomes in the nucleus. Students construct a four-step annotated diagram on the DNA and chromosome organiser and add one relationship sentence for each term.

3. 17–27 min · Chromosome structure. Display the chromosome structure slides and model the difference between an unreplicated chromosome and a replicated chromosome made of two identical sister chromatids joined at a centromere. In pairs, students label diagrams on the worksheet and answer: “Does a replicated chromosome contain one DNA molecule or two?” The teacher checks responses and addresses the common misconception that two sister chromatids are two chromosomes.

4. 27–40 min · Cell-cycle application. Present the cell-cycle sequence slides to locate DNA replication in interphase, before mitosis. In groups of four, students use the worksheet sequence to explain what happens during growth, DNA replication and cell division. Each group prepares a 30-second explanation using the sentence frame: “DNA replication occurs before division so that…”. Select two groups to share; peers identify whether the explanation refers to copied genetic information and distribution to daughter cells.

5. 40–53 min · Guided practice and feedback. Students independently complete the worksheet’s comparison table and short-answer questions, including a diagram showing chromosomes before and after replication. Circulate to question students: “Where is the DNA?”, “What makes these chromatids sisters?” and “When in the cell cycle does replication occur?” Use the practice and worked-example slides to reveal answers progressively, allowing students to self-correct in a different colour.

6. 53–60 min · Plenary and exit check. Return to the plenary slides and revisit the hook question. Students complete the worksheet exit ticket: define a gene, identify sister chromatids, and explain in two sentences why DNA replication is necessary before cell division. Collect responses to identify students requiring reteaching in lesson 20.

Resources

• DNA, chromosomes and the cell cycle slide deck
• DNA and chromosome organiser
• Whiteboard and markers
• Projector or interactive display
• Coloured pens or pencils
• Student exercise books
• Timer

Assessment

• Listen to pair and group explanations for accurate distinctions between DNA, genes, chromosomes and sister chromatids.
• Check annotated diagrams and circulate during independent work, using questioning to identify misconceptions about chromosome number and DNA replication.
• Use the final exit ticket to assess whether students can locate replication in the cell cycle and explain its purpose before cell division.

Differentiation

• Provide a partially completed hierarchy diagram, a word bank and sentence starters for students needing literacy or conceptual support: “A gene is…”, “A chromosome consists of…” and “Replication is needed because…”.
• Use colour coding consistently: DNA in blue, genes highlighted on DNA, chromosomes in red and sister chromatids in purple. Provide enlarged diagrams or digital zoom for students with visual-access needs.
• Pair students strategically and allow oral responses before written responses. Explicitly teach and display the terms genetic information, chromatin, centromere, replication and cell cycle.
• Challenge confident students to explain why a chromosome is counted as one chromosome both before replication and while its two sister chromatids remain joined, and to relate this to chromosome number after cell division.