AMED3001 Cancer
Conventional Therapy & Therapy-Induced DNA Damage
Week 10 opens Module 3 with conventional cancer treatment. It covers surgery, radiotherapy and chemotherapy and their DNA-level mechanisms, the cell-cycle checkpoints they exploit, why cancer stem cells escape and drive relapse, and how DNA damage is measured with the comet assay from the Module 3 practical. The comet-assay technique and interpretation feed the Module 3 practical report (6%), and therapy mechanisms are core short-answer material in the final (50%, confirm on Canvas).
What this chapter covers
- 01Three conventional modalities: surgery (localised, early-stage), radiotherapy, chemotherapy
- 02Radiotherapy: ionizing radiation → DNA double-strand breaks → cell death; fast-dividing cells most sensitive
- 03Chemotherapy classes: alkylating agents (DNA damage), antimetabolites (block DNA synthesis), taxanes (mitotic-spindle disruption)
- 04Cell-cycle checkpoints (G1/S, G2/M, M) and the DNA-damage response
- 05Drug resistance: efflux pumps (P-glycoprotein), DNA-repair upregulation, tumour heterogeneity
- 06Cancer stem cells escape chemo by G0 quiescence → therapy-induced damage can promote survival → relapse
- 07Comet assay (single-cell gel electrophoresis): alkaline (pH ~13) vs neutral (best for double-strand breaks); doxorubicin, DAPI stain
Interpreting a comet assay: control vs doxorubicin-treated cells
- +1The comet assay measures DNA damage in single cells. Cells are embedded in agarose, lysed to remove membranes and proteins leaving the DNA, then electrophoresed and stained (e.g. DAPI) for fluorescence microscopy.
- +1Damaged, fragmented DNA is smaller and more mobile, so it migrates further from the nucleus toward the anode, forming a ‘comet tail’; intact DNA stays in the head.
- +1A longer, brighter tail means MORE DNA damage. Doxorubicin-treated cells therefore show longer tails (greater tail moment) than untreated controls.
- +1Variant: the ALKALINE comet assay (pH ~13) is most common and detects single- and double-strand breaks; the NEUTRAL comet assay is best for detecting double-strand breaks specifically.
Key terms
- Radiotherapy
- Treatment using ionizing radiation to induce DNA double-strand breaks and kill cancer cells; precise and locally controlled but limited to the treated field and damaging to nearby normal tissue.
- Alkylating agent
- A chemotherapy class that damages DNA directly by adding alkyl groups, triggering apoptosis in fast-dividing cells; contrast with antimetabolites (block DNA synthesis) and taxanes (disrupt the mitotic spindle).
- DNA-damage response
- The network of checkpoints and repair pathways that detects DNA damage and either arrests the cycle for repair or triggers cell death; conventional therapy works by overwhelming it in cancer cells.
- Cell-cycle checkpoint
- A control point (G1/S, G2/M, M) where the cell checks for DNA damage or errors and either pauses for repair or induces death; loss of checkpoints permits uncontrolled division.
- Drug resistance (P-glycoprotein)
- The failure of therapy to control a tumour; mechanisms include drug-efflux pumps such as P-glycoprotein, upregulated DNA repair and tumour heterogeneity.
- Comet assay
- Single-cell gel electrophoresis that quantifies DNA damage: fragmented DNA migrates into a comet tail whose length/intensity reflects damage; alkaline (pH ~13) detects all breaks, neutral is best for double-strand breaks.
Conventional Therapy & Therapy-Induced DNA Damage FAQ
How do radiotherapy and chemotherapy actually kill cancer cells?
Radiotherapy uses ionizing radiation to cause DNA double-strand breaks; chemotherapy classes act differently — alkylating agents damage DNA directly, antimetabolites block DNA synthesis, and taxanes disrupt the mitotic spindle. All target fast-dividing cells, which is why normal fast-dividing tissues also suffer side effects.
Why does chemotherapy fail to eliminate cancer stem cells?
Cancer stem cells are often quiescent in G0, outside the cell cycle, so fast-dividing-cell chemotherapy cannot target them. Therapy-induced DNA damage can even promote their survival, so a small resistant population regrows the tumour and drives relapse.
Which comet-assay variant detects double-strand breaks?
The neutral comet assay is best for double-strand breaks; the alkaline version (pH ~13) is the most commonly used and detects both single- and double-strand breaks. Knowing this distinction is expected in the Module 3 practical report.
Can AI help me write up the comet-assay practical?
Sia can explain the comet-assay steps, the role of the lysis solution, and how to interpret tail length, and can check your reasoning — but it does not write your graded Module 3 report for you, and USyd academic-integrity rules apply. Confirm the report brief on Canvas.
Exam move
Separate the three modalities and, for chemotherapy, the three drug classes with their distinct DNA-level mechanisms — that precise mapping is where the marks are. Understand why cancer stem cells (G0 quiescence) survive and drive relapse, and rehearse the comet-assay method and interpretation for the Module 3 practical report (6%): what the tail means, and the alkaline-versus-neutral distinction. Ask Sia to quiz mechanisms and comet-assay steps; keep it warm for the final and confirm the practical brief on Canvas.
Working through Conventional Therapy & Therapy-Induced DNA Damage in AMED3001? Sia is AskSia’s AI Biology tutor — ask any AMED3001 Conventional Therapy & Therapy-Induced DNA Damage question and get a clear, step-by-step explanation grounded in how AMED3001 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
