Interpretation of Photomicrographs, Diagrams and Microscope Slides of Cells in the Mitotic Cycle
Learning Objective (AO1 / AO2)
Interpret photomicrographs, diagrams and microscope slides of cells at different stages of the mitotic cell cycle and accurately identify the main stages of mitosis (prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis) in both animal and plant cells.
Key Facts (Exact Syllabus Wording – 5.1 & 5.2)
- Chromosomes are linear DNA‑protein complexes that become highly condensed during mitosis.
- Telomeres protect the ends of chromosomes; each mitotic division shortens them slightly. Telomerase can restore telomeres in germ cells, stem cells and many cancer cells.
- Stem cells in multicellular organisms undergo mitosis to replace specialised cells, maintaining tissue homeostasis.
- Uncontrolled mitosis → tumours; mutations in checkpoint proteins, telomere maintenance or growth‑factor signalling are common causes.
- In plants, mitosis produces spores or new individuals (asexual reproduction) and involves plant‑specific structures (pre‑prophase band, phragmoplast, cell plate).
Why Mitosis Matters (Syllabus 5 – Overview)
- Growth & repair: Generates two genetically identical daughter cells.
- Stem‑cell contribution: Replenishes differentiated cells in tissues such as skin, gut and blood.
- Telomere dynamics: Links cell division to ageing and cancer.
- Asexual reproduction in plants: Forms spores and new individuals without fertilisation.
- Uncontrolled division → tumours: Checkpoint failures lead to cancer.
Syllabus Coverage Check
| Syllabus Requirement | Current Coverage | Gap / Issue | Improvement Needed |
|---|
| 5.1 Replication & division of nuclei and cells – chromosome structure, telomeres, stem cells, tumour formation | All points present. | – | Add a concise “Key Facts” box (done above) mirroring exact syllabus wording. |
| 5.2 Chromosome behaviour in mitosis – outline mitotic cycle, describe chromosomes, nuclear envelope, spindle, identify each stage, include plant‑specific features | Comprehensive visual description, stage‑by‑stage table, plant vs. animal comparison. | Interphase, mitotic checkpoint, spindle‑assembly checkpoint and cyclin/CDK regulation are omitted. | Insert sections on Interphase, Cell‑cycle checkpoints and Cyclin/CDK control (below). |
Key Concepts (AO1)
- Sequential mitotic phases: prophase → prometaphase → metaphase → anaphase → telophase → cytokinesis.
- Chromosome structural changes: condensation, alignment, segregation, de‑condensation.
- Spindle apparatus: microtubules, centrosomes/centrioles (animals) or microtubule‑organising centres (plants), kinetochores.
- Plant‑specific structures: pre‑prophase band, phragmoplast, cell plate.
- Cell‑cycle checkpoints: G1/S, G2/M, spindle‑assembly checkpoint.
- Regulation by cyclins and cyclin‑dependent kinases (CDKs).
- Typical visual cues in photomicrographs that distinguish each stage.
Interphase and Cell‑Cycle Checkpoints (Context for 5.2)
- G1 phase: Cell grows, synthesises proteins, checks DNA integrity.
- S phase: DNA replication; sister chromatids are formed but remain attached at centromeres.
- G2 phase: Further growth, synthesis of microtubule proteins, checkpoint ensures complete replication and DNA repair before entering mitosis.
- Spindle‑assembly checkpoint (SAC): Monitors attachment of each kinetochore to spindle fibres; prevents anaphase onset until all chromosomes are correctly bioriented.
- Regulation by cyclins/CDKs:
- Cyclin B + CDK1 (MPF) drives entry into mitosis.
- Degradation of cyclin B by the APC/C complex triggers exit from mitosis.
Plant vs. Animal Mitosis (Syllabus 5.2)
| Feature | Animal Cells | Plant Cells |
|---|
| Centrioles / Centrosomes | Present; organise spindle poles, visible asters. | Absent; microtubule‑organising centres (MTOCs) form at opposite poles. |
| Pre‑prophase band | Not present. | Transient ring of microtubules & actin that marks future division plane (disappears before metaphase). |
| Spindle formation | Spindle emanates from centrosomes; asters visible. | Spindle forms from MTOCs; no distinct asters. |
| Chromosome movement | Along spindle fibres attached to kinetochores. | Same mechanism; fibres often appear thicker in photomicrographs. |
| Cytokinesis | Cleavage furrow invaginates from the plasma membrane. | Cell plate forms from vesicles coalescing at the centre; later becomes a new cell wall. |
Stages of Mitosis – Visual Summary
| Stage | Chromosome Appearance | Spindle / Cellular Features | Key Visual Cues (Animal) | Key Visual Cues (Plant) |
|---|
| Prophase | Chromatin condenses into short, thick chromosomes; each chromosome = 2 sister chromatids joined at centromere. | Centrioles migrate to opposite poles (animal); MTOCs nucleate microtubules (plant). Nucleolus fades. | Fuzzy, elongated chromosomes; nuclear envelope intact; nucleolus still visible. | Same, plus a faint pre‑prophase band around the periphery. |
| Prometaphase | Chromosomes further condense; kinetochores become distinct. | Nuclear envelope fragments; spindle fibres attach to kinetochores. | Scattered chromosomes; visible spindle fibres radiating from poles; nuclear envelope broken. | Identical, with disappearance of the pre‑prophase band. |
| Metaphase | Chromosomes line up at the metaphase plate; sister chromatids side‑by‑side. | Spindle fibres fully attached; tension evident. | Sharp, straight line of chromosomes across the centre; no movement. | Metaphase plate present; thin phragmoplast‑like microtubule array may be visible at the equator. |
| Anaphase | Sister chromatids separate and move toward opposite poles. | Spindle fibres shorten (anaphase A) and poles move apart (anaphase B). | Two distinct groups of chromosomes moving away from centre; V‑shaped spindle. | Same V‑shape; early phragmoplast formation at the centre. |
| Telophase | Chromatids reach opposite poles and begin de‑condensing into chromatin. | New nuclear envelopes form; spindle disassembles. | Two nuclei visible; chromosomes appear less distinct, more diffuse. | Two nuclei plus a developing cell plate (disc of vesicles) at the centre. |
| Cytokinesis | Chromosomes fully de‑condensed; cell division completes. | Animal: cleavage furrow deepens.
Plant: cell plate expands outward to become a new cell wall. | Two separate daughter cells with distinct membranes; no mitotic spindle. | Two daughter cells separated by a visible cell plate. |
Interpreting Photomicrographs (AO2)
- Check the nuclear envelope: intact = prophase; fragmented = prometaphase; re‑forming = telophase.
- Assess chromosome morphology: fuzzy & diffuse = early (pro‑/prometaphase); crisp X‑shaped pairs = metaphase; clearly separated chromatids = anaphase; de‑condensed cloud‑like chromatin = telophase.
- Locate spindle fibres: thin radiating lines (animal) or thicker bundles (plant). V‑shape indicates anaphase; radial, less tense arrangement indicates telophase.
- Identify the division plane: cleavage furrow (animal) or cell plate / pre‑prophase band (plant).
- Look for scale bars and staining method:
- Scale bar → estimate cell size and stage‑specific dimensions.
- Common stains: Feulgen (DNA), DAPI (fluorescent DNA), Haematoxylin‑eosin (general morphology), or fluorescent tubulin antibodies for spindles.
- Consider checkpoint status: If kinetochores are attached but chromosomes are not aligned, the spindle‑assembly checkpoint is still active (usually seen in prometaphase).
Common Mistakes and How to Avoid Them
- Prophase vs. Prometaphase: Remember the nuclear envelope is intact in prophase.
- Missing the metaphase plate: Rotate the image mentally; a true plate appears as a single, sharp line, not a scattered cluster.
- Calling early anaphase “prometaphase”: Sister chromatids must be clearly separated; partial separation is still prometaphase.
- Overlooking plant‑specific cytokinesis: Look for a faint disc of vesicles (cell plate) rather than a furrow.
- Ignoring scale bars: Without a size reference it is easy to mis‑identify a late telophase cell as interphase.
- Neglecting checkpoint context: If chromosomes are mis‑aligned but spindle fibres are present, the SAC is active – this is typical of prometaphase, not metaphase.
Practice Worksheet – Photomicrograph Identification (AO3)
For each image, answer the three questions. Use the checklist in the next section to guide your response.
- Image A (Animal cell – Feulgen stain)
- What mitotic stage is shown? State the key visual cue(s) you used.
- Describe the status of the nuclear envelope and spindle fibres.
- Identify any potential artefacts (e.g., out‑of‑focus, overlapping cells).
- Image B (Plant root tip – DAPI fluorescence)
- Stage identification and visual cue(s).
- Note any plant‑specific structures visible (pre‑prophase band, phragmoplast, cell plate).
- Comment on image quality (scale bar, staining intensity).
- Image C (Animal cell – cytokinesis)
- Is this cleavage furrow or cell plate? Explain.
- What stage preceded this image?
- Are the daughter nuclei clearly visible?
- Image D (Plant cell – late telophase)
- Identify the stage and describe the appearance of chromosomes.
- What evidence shows the new nuclear envelope is forming?
- Is the cell plate complete? How can you tell?
Marking Rubric (0‑6 pts per image)
| Criteria | 0 pts | 1 pt | 2 pts |
|---|
| Correct stage identified | No answer or wrong stage | Stage named but not justified | Correct stage with appropriate visual cue(s) cited |
| Explanation of nuclear envelope / spindle status | Absent or incorrect | Partial description | Clear, accurate description linked to the image |
| Recognition of plant‑/animal‑specific features & image quality | None mentioned | One feature mentioned | Both relevant features and quality issues identified |
Suggested Diagrams and Photomicrographs (AO1 / AO2)
- Diagram 1: Fully labelled spindle apparatus showing centrioles, microtubules, kinetochores, metaphase plate, and for plants the pre‑prophase band and phragmoplast.
- Photomicrograph A (Animal – Metaphase) – Feulgen‑stained, clear metaphase plate, scale bar 10 µm.
- Photomicrograph B (Plant – Anaphase) – DAPI‑stained root tip, separated sister chromatids, early phragmoplast visible.
- Photomicrograph C (Animal – Cytokinesis) – Cleavage furrow midway, nuclei in each daughter cell.
- Photomicrograph D (Plant – Telophase/Cell‑plate) – Developing cell plate with two distinct nuclei.
Quick Reference Checklist for Students (AO1)
- Is the nuclear envelope intact, fragmented or re‑forming?
- How condensed are the chromosomes? (fuzzy → early, crisp X‑shaped → metaphase, separated → anaphase, diffuse → telophase)
- Is there a clear metaphase plate?
- Are sister chromatids clearly separated?
- Do spindle fibres show a V‑shape (anaphase) or a radial, less tense arrangement (telophase)?
- Animal cell: is a cleavage furrow present?
Plant cell: is a cell plate or pre‑prophase band visible? - Is a scale bar present and is the staining appropriate?
- Does the image suggest the spindle‑assembly checkpoint is active (unattached kinetochores) or satisfied (aligned chromosomes)?
Glossary (Key Terminology – AO1)
- Centrioles – Paired cylindrical organelles that organise spindle poles in animal cells.
- Kinetochores – Protein complexes on the centromere where spindle fibres attach.
- Metaphase plate – Imaginary plane at the cell centre where chromosomes align during metaphase.
- Phragmoplast – Plant‑specific microtubule‑actin structure that guides cell‑plate formation.
- Pre‑prophase band – Ring of microtubules in plant cells that predicts the future division site.
- Telomeres – Repetitive DNA sequences at chromosome ends that protect genetic information.
- Cleavage furrow – Ingression of the plasma membrane that divides animal cells.
- Cell plate – New wall material that forms in the centre of a plant cell during cytokinesis.
- Spindle‑assembly checkpoint (SAC) – Surveillance mechanism ensuring all kinetochores are attached before anaphase onset.
- Cyclin‑dependent kinase (CDK) – Enzyme that, when bound to a cyclin, drives progression through specific cell‑cycle phases.
Summary
Understanding mitosis requires knowledge of (i) the structural changes chromosomes undergo, (ii) the organisation and regulation of the spindle apparatus, (iii) the distinctive features of plant and animal cells, and (iv) the checkpoints that safeguard accurate division. By systematically observing the nuclear envelope, chromosome morphology, spindle configuration and cytokinetic structures, learners can reliably identify each mitotic stage in photomicrographs and diagrams. The added sections on interphase, checkpoint control and cyclin/CDK regulation fill the remaining syllabus gaps, while the checklist, practice worksheet and rubric provide targeted, exam‑style training aligned with Cambridge International AS & A Level Biology (9700).