5.1 Replication and Division

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.