5. The Mitotic Cell Cycle

A chromosome is a condensed structure made from a long DNA molecule associated with histone proteins. Packaging DNA as chromatin and then condensing it for division makes the genetic material manageable and helps it be distributed in an organised way.

• DNA and chromatin: DNA carries genetic information. Histone proteins help package the DNA; DNA plus associated proteins forms chromatin.
• Chromosome: Further condensation of chromatin produces a chromosome that can be moved and separated during nuclear division.
• Sister chromatids: After DNA has been copied, one replicated chromosome consists of two genetically matching sister chromatids. They carry corresponding genetic information but are still joined as one replicated chromosome.
• Centromere: The centromere joins the sister chromatids and provides the chromosome region used for their coordinated separation during division.
• Telomeres: Telomeres are repeated DNA sequences at the ends of linear chromosomes. Here they are identified as chromosome-end regions; their replication-buffer role belongs to the dedicated telomere card (4563).

Packaging reduces the tangle of long DNA molecules and condensation makes each replicated chromosome a discrete structure. When sister chromatids are separated at the centromere, one matching DNA copy can be allocated to each daughter nucleus; the structure therefore supports orderly distribution without changing the genetic information carried by the copies.

A chromosome, chromatid and sister chromatid are related but not interchangeable terms. DNA replication creates matching sister chromatids within one replicated chromosome; it does not immediately double chromosome number. Telomeres are DNA end regions, not histone proteins or a substitute for the full telomere mechanism.

Mitosis is nuclear division that normally produces two genetically matching daughter nuclei with the same chromosome number as the parent nucleus. Cytokinesis then separates the cell around those nuclei.

• Genetic matching: DNA is copied before mitosis, and the sister chromatids are separated so each daughter nucleus receives one matching copy of the genetic information.
• Chromosome number: The chromosome number is maintained because each daughter receives one complete set corresponding to the parent nucleus; mitosis is not a reduction division.
• Growth: Repeated mitotic divisions increase cell number, allowing a multicellular organism or tissue to grow.
• Replacement: New cells replace damaged or dead cells while preserving the tissue’s usual genetic information.
• Repair: Cell division supplies replacement cells for damaged tissue; it does not reverse damage inside the original cell.
• Asexual reproduction: In organisms that reproduce asexually, mitotic descendants can remain genetically matching the parent, so one parent can produce new individuals without fusion of gametes.

Once DNA has been copied, accurate separation of the matching chromosome copies lets one daughter nucleus inherit each complete set. Cytokinesis turns the two nuclei into separate cells, so the same mechanism can add cells for growth, replace cells during turnover, repair tissue or produce a new asexual individual.

Mitosis replaces a damaged cell; it does not repair that cell by undoing its injury. “Genetically matching” describes the intended nuclear DNA outcome, while mutation or environmental effects can still create differences. The detailed G1–S–G2–M sequence belongs to card 4562.

The cell cycle is the regulated sequence from one cell division to the next. Interphase prepares the cell and copies its DNA, mitosis divides the nucleus, and cytokinesis divides the cytoplasm to form separate daughter cells.

1. G1 — growth and normal activity: The cell grows and carries out its usual synthesis and functions, building the material needed for a later division.
2. S phase — DNA replication: The cell copies its DNA. Each chromosome is thereby prepared as a replicated chromosome with matching sister chromatids; the cell has not yet divided.
3. G2 — further preparation: The cell continues to grow and prepares for nuclear division, including checking that the copied genetic material and division machinery are ready.
4. Interphase result: G1, S and G2 together prepare one cell with duplicated genetic material for the accurate separation of matching copies.
5. Mitosis — nuclear division: The replicated chromosome copies are separated into two genetically matching nuclei with the same chromosome number as the parent nucleus.
6. Cytokinesis — cytoplasmic division: The cytoplasm and cell boundary partition around the nuclei, producing two separate daughter cells. Animal cells constrict; plant cells build a new partition.

DNA replication must occur before chromosome copies are distributed: without two matching copies, each daughter nucleus could not receive a complete corresponding set. Mitosis handles nuclear distribution; cytokinesis completes the physical separation of the daughter cells.

Interphase is active preparation, not a single resting stage, and S phase is DNA replication rather than mitosis. Mitosis divides the nucleus; cytokinesis divides the cytoplasm. The broader biological importance of the resulting matching cells belongs to card 4561, while telomere buffering belongs to card 4563.

Telomeres are repeated, non-coding DNA sequences at the ends of chromatids/chromosomes. They act as a buffer so that the end-copying limit of linear DNA does not immediately remove important coding DNA during repeated cell division.

• Location and composition: Telomeres occupy chromosome ends and contain repeated DNA sequences that do not code for proteins; they are DNA regions, not protein caps.
• The copying problem: The normal DNA-copying machinery cannot copy the very end of a linear DNA molecule completely, so a small terminal section may be left out during replication.
• Buffer function: Because the telomere is non-coding repeated DNA, progressive shortening can remove buffer sequence before important genes near the chromosome end are affected.
• Why shortening matters: Each repeated division can reduce the remaining buffer. Once the protective reserve is sufficiently depleted, continued accurate replication/division becomes limited rather than allowing coding DNA to be lost without consequence.
• Cautious cell-outcome link: Telomere shortening is associated with a replicative limit and can contribute to cellular senescence, but this card does not claim that it alone explains all ageing or disease.

Telomeres convert an unavoidable end-copying limitation into loss of expendable repeated sequence first. This protects coding regions near chromosome ends during repeated replication, while the gradual loss of the buffer explains why a cell’s capacity for continued division can eventually become restricted.

Telomeres are not genes and are not the same as the whole chromosome-end structure discussed in the chromosome card 4560. Do not add telomerase mechanisms, named diseases or a universal ageing claim: the supported conclusion is a protective buffer with progressive shortening and a cautious link to replicative limitation.

A stem cell is an undifferentiated cell that can divide by mitosis and produce a daughter cell that remains a stem cell or a daughter cell that differentiates into a specialised cell. Potency describes the range of specialised cell types it can produce.

• Self-renewal: A stem-cell division can maintain the stem-cell pool, so suitable stem cells remain available for future replacement and repair.
• Differentiation: A daughter cell can become specialised, changing gene expression and cell function to fit a tissue role rather than simply becoming larger.
• Potency: Multipotent adult stem cells can produce a limited range of related cell types. For example, bone-marrow stem cells can produce several blood-cell types.
• Adult source and use: Small populations of adult stem cells remain in tissues such as bone marrow, skin or gut and can contribute to normal cell replacement and tissue repair.
• Therapy boundary: Introducing suitable adult stem cells into damaged tissue is a research-based treatment opportunity for some diseases or injuries, but outcome depends on correct tissue matching, differentiation and control of the cells.
• Evidence boundary: The mapped SME/syllabus evidence here supports multipotent adult stem cells. It does not establish an embryonic-stem-cell source/use comparison, so no embryonic claim or ethical conclusion is added.

Self-renewal preserves a reservoir; differentiation converts some descendants into the specialised cells needed by a tissue. Together these properties explain how stem cells can support growth, routine replacement and repair, while limited potency and the need to control cell fate set boundaries on therapeutic use.

Stem cells are not all equally potent, and “can divide repeatedly” does not mean that every stem cell can make every cell type or that a therapy is automatically safe or effective. A differentiated cell is not automatically a stem cell again; tumour formation and uncontrolled division are handled separately in card 4565.

A tumour is an abnormal mass of cells formed when changes in a cell’s control of division or survival allow repeated, unregulated cell proliferation. The altered cell passes the change to descendants as the clone expands.

• Control failure: A mutation alters a gene or control system that normally limits cell division, promotes appropriate cell death or checks abnormal cells.
• Abnormal proliferation: The affected cell continues dividing when normal controls would stop division or remove the cell. Its descendants inherit the relevant change and form an expanding clone.
• Tumour mass: Repeated abnormal mitosis produces an accumulation of cells rather than the controlled replacement or growth produced by normal tissue division.
• Benign boundary: A benign tumour remains local and does not invade nearby tissue in the supported comparison.
• Malignant boundary: A malignant tumour invades nearby tissue and can spread through blood or lymph to establish secondary growths; this spread is metastasis.
• Normal/stem-cell contrast: Normal mitosis is regulated and supplies matching cells for growth, replacement or repair. Stem-cell self-renewal maintains a controlled reservoir with differentiation potential; tumour proliferation is a loss of that control, not normal self-renewal.

The key transition is regulatory: a cell that should pause, differentiate, die or remain within tissue instead keeps contributing descendants. Whether the resulting growth remains local or becomes invasive depends on the tumour behaviour, so “uncontrolled division” is the starting mechanism rather than a claim that every tumour spreads.

A carcinogen or mutation can increase risk but does not guarantee a tumour, and not every mutation affects division control. Do not invent named genes, cancer types or a specific carcinogenic pathway here; distinguish regulated mitosis and stem-cell self-renewal from pathological loss of control.

After DNA replication, each replicated chromosome contains two matching sister chromatids joined at a centromere. Mitosis uses the spindle to align and then separate those copies so each daughter nucleus receives one corresponding set of genetic material.

1. Condense and prepare: The replicated chromosomes condense so each pair of sister chromatids can be moved as a discrete structure; the spindle forms across the cell.
2. Connect: Spindle fibres attach to the chromosome region at the centromere, with the two sister chromatids connected to opposite poles. This gives the copies opposing pulling directions.
3. Align: The replicated chromosomes arrange at the cell equator. Alignment checks that each sister chromatid is positioned to move toward a different pole.
4. Separate: The centromeres divide and the sister chromatids are pulled toward opposite poles. Once separated, each chromatid is counted as an individual chromosome.
5. Reach and enclose: The chromosome groups arrive at opposite poles, decondense and become enclosed in two new nuclei. Each nucleus therefore receives one matching copy of each chromosome.

Opposite-pole spindle attachment is the key distribution logic: alignment precedes centromere separation, and separation converts one replicated chromosome into two matching chromosome copies moving apart. This is how mitosis preserves the chromosome set in each daughter nucleus rather than sending both copies to one side.

An aligned chromosome with joined sister chromatids is before anaphase; separated groups moving to opposite poles are after centromere separation. The named PMAT stage sequence belongs to 4568, while deciding a stage from a micrograph belongs to 4569; this card explains the chromosome mechanism rather than image-specific clues.

Mitosis coordinates three structural changes: the nuclear envelope opens access to condensed chromosomes, the spindle positions and moves the chromosome copies, and the cell boundary later partitions the cytoplasm around the two new nuclei.

• Nuclear envelope: It breaks down during the early part of mitosis so spindle fibres can interact with the condensed chromosomes, then reforms around each chromosome group at the poles as new nuclei form.
• Spindle: Spindle fibres extend across the cell and attach to chromosome regions at the centromeres. Opposite-pole connections align the copies; shortening of the attached fibres contributes to poleward movement after separation.
• Chromosome link: These structural changes make accurate chromosome distribution possible: access, attachment and movement are coordinated rather than independent events.
• Cell-surface membrane: The membrane remains the cell boundary during nuclear division, then changes shape during cytokinesis. Animal cells constrict to form a cleavage furrow; plant cells build a new partition that separates the daughter cells.
• Result: Nuclear-envelope reformation creates two nuclei, and boundary/cytoplasmic partition completes the transition from one divided nucleus to two separate daughter cells.

The nuclear envelope must no longer block spindle–chromosome interaction, the spindle must provide directional forces, and the cell boundary must wait until the nuclei are organised before partitioning the cytoplasm. Together these behaviours connect chromosome movement to physical separation without treating cytokinesis as another nuclear stage.

The nuclear envelope is not simply absent for the whole cell cycle, the spindle is not a passive scaffold, and the cell-surface membrane does not disappear during mitosis. Card 4566 owns the detailed sister-chromatid movement chain; card 4568 owns the named PMAT stage labels.

Mitosis is the ordered division of one nucleus into two genetically matching nuclei. The four named stages are prophase, metaphase, anaphase and telophase (PMAT); interphase prepares the cell but is not one of these four stages.

1. Prophase — condense and prepare: Chromosomes condense and become distinct; each still has two sister chromatids joined at a centromere. The nuclear envelope breaks down and the spindle forms. The nucleus is being prepared for chromosome movement; the cytoplasm has not yet divided.
2. Metaphase — align and connect: Replicated chromosomes arrange at the cell equator. Spindle fibres attach at the centromere regions and connect the sister chromatids to opposite poles. This alignment provides the starting point for equal separation.
3. Anaphase — separate and move: Centromeres divide, so sister chromatids become individual chromosomes. Spindle fibres shorten and pull the separated chromosomes to opposite poles, forming two genetic-material groups.
4. Telophase — re-form nuclei: Chromosomes reach opposite poles and begin to decondense. A nuclear envelope reforms around each chromosome group and the spindle is dismantled. Cytokinesis then divides the cytoplasm, completing separation into two cells.

The PMAT sequence links chromosome state to movement: condensation makes chromosomes movable, spindle attachment and equatorial alignment establish opposite-pole routes, centromere division permits sister chromatids to separate, and arrival at the poles allows two nuclei to reform. Cytokinesis follows nuclear division to partition the cytoplasm, rather than being an additional PMAT stage.

Do not include interphase in PMAT: DNA replication occurs before mitosis. Card 4566 explains the chromosome-separation logic in more detail, and card 4567 covers coordinated nuclear-envelope, spindle and cell-boundary behaviour; this card is the ordered stage map. A micrograph or slide is interpreted separately in card 4569.

Stage identification in a mitosis micrograph or root-tip section is an evidence-matching task: inspect chromosome position and state first, then use nuclear-envelope and cell-separation clues when they are visible, and assign the PMAT stage only when the combined evidence fits.

1. Check the specimen and evidence: Confirm that the field is a suitable stained root-tip or other mitotic preparation. Note the cell orientation, image quality and whether the nucleus, chromosome group or cell boundary is actually visible; do not infer a hidden feature from an unclear image.
2. Inspect chromosome condensation: Look for distinct condensed chromosomes, chromosomes arranged in a line at the equator, sister chromatids separated into two groups, or chromosomes gathered at poles and beginning to decondense.
3. Inspect nuclear and spindle clues: If resolvable, check whether the nuclear envelope is breaking down or absent, whether spindle fibres are guiding an equatorial arrangement or poleward movement, and whether nuclear envelopes are reforming. These clues support the chromosome evidence rather than replacing it.
4. Match the stage: Prophase is condensation and preparation for movement; metaphase is equatorial alignment; anaphase is separated chromatids moving towards opposite poles; telophase is pole arrival with decondensation and reforming nuclei. Cytokinesis or a new cell partition is supporting evidence for completion of division, not a separate PMAT stage.
5. Cross-check before deciding: Use at least two compatible observations where possible—for example, equatorial alignment plus spindle attachment for metaphase, or two separated chromosome groups plus poleward movement for anaphase. Compare neighbouring cells only as context: a root-tip field contains cells at different stages, not one shared stage.

The decisive evidence changes through PMAT: chromosomes first become compact and movable, then align, then separate, and finally reach opposite poles while nuclei reform. This sequence explains why a cell with joined chromatids at the equator is not yet in anaphase, whereas two chromosome groups moving apart are evidence for anaphase.

Do not identify a stage from one vague shape, cell size or stain intensity alone. Section angle, overlap, image resolution and an obscured nuclear envelope can hide useful clues, so report only what the image supports and use the strongest visible combination. The ordered PMAT map belongs to card 4568; this card teaches how to apply it to an image or slide.