explain the importance of mitosis in the production of genetically identical daughter cells during: growth of multicellular organisms, replacement of damaged or dead cells, repair of tissues by cell replacement, asexual reproduction

Replication and Division of Nuclei and Cells

Learning Objective

Explain why mitosis is essential for producing genetically identical daughter cells during:

  • Growth of multicellular organisms

  • Replacement of damaged or dead cells

  • Repair of tissues by cell replacement

  • Asexual reproduction

1. The Cell Cycle

1.1 Interphase – the preparatory period

PhaseMain ActivitiesKey Check‑points
G₁ (Gap 1)Cell growth; synthesis of proteins, RNA and organelles; cell “decides” whether to divide.G₁ checkpoint – assesses size, nutrients, DNA damage.
S (Synthesis)Complete replication of nuclear DNA → each chromosome becomes two sister chromatids; centrosomes duplicate.DNA‑damage checkpoint – ensures replication is accurate.
G₂ (Gap 2)Further growth; synthesis of mitotic proteins (e.g., cyclins); preparation of spindle apparatus.G₂/M checkpoint – verifies that DNA replication is finished and undamaged.

1.2 M Phase – mitosis and cytokinesis

The M phase is subdivided into the classic mitotic stages followed by cytokinesis.

2. Chromosome Structure (Key to Genetic Identity)

  • DNA – double‑helical polymer that carries the genetic code.

  • Histone proteins – form nucleosomes; DNA wraps around histone octamers, creating chromatin.

  • Chromatin → Chromosome – during prophase, chromatin condenses into visible chromosomes.

  • Sister chromatids – two identical copies of a replicated chromosome, joined at the centromere.

  • Centromere & kinetochore – site where spindle microtubules attach.

  • Telomeres – repetitive DNA at chromosome ends that protect coding DNA during replication; maintained by telomerase in stem and germ cells.

  • Ploidy – somatic cells are diploid (2n); each daughter cell receives one complete set of chromosomes, preserving the genetic blueprint.

3. The Mitotic Cycle (M Phase)

PhaseKey Events (Chromosomes, Nuclear Envelope, Spindle)
Prophase

  • Chromatin condenses into 2n distinct chromosomes (each with sister chromatids).

  • Centrosomes move to opposite poles; mitotic spindle begins to form.

  • Nucleolus disappears.

Prometaphase

  • Nuclear envelope fragments completely.

  • Spindle fibres attach to kinetochores on each centromere.

  • Chromosomes start moving via microtubule dynamics.

Metaphase

  • Chromosomes align at the metaphase plate (equatorial plane).

  • Each sister chromatid is connected to opposite spindle poles (bivalent attachment).

Anaphase

  • Sister chromatids separate at the centromere and are pulled toward opposite poles.

  • Kinetochore microtubules shorten; polar (astral) microtubules lengthen.

Telophase

  • Chromatids reach the poles and de‑condense into chromatin.

  • New nuclear envelopes form around each set of chromosomes.

  • Nucleoli reappear.

Cytokinesis

  • Animal cells: contractile actin‑myosin ring pinches the cell centre (cleavage furrow).

  • Plant cells: vesicles coalesce at the centre to form a cell plate, which becomes a new cell wall.

  • Result – two separate daughter cells, each with a full complement of chromosomes.

4. Regulation of Mitosis – Why Control Is Crucial

  • Check‑points (G₁, G₂, metaphase) monitor DNA integrity, replication completeness and spindle attachment.

  • Cyclins & CDKs drive the cycle forward; their timely degradation allows progression.

  • Loss of checkpoint control → uncontrolled division → tumour formation (e.g., p53 mutation permits division of DNA‑damaged cells).

5. Why Genetically Identical Cells Are Required

  • Uniform enzyme expression → consistent metabolic pathways throughout a tissue.

  • Correct proportions of structural proteins (collagen, keratin) maintain tissue strength and shape.

  • Identical receptors ensure reliable cell‑cell communication and coordinated responses to hormones/growth factors.

  • Preservation of developmental patterns during growth and repair.

6. Roles of Mitosis in Organisms

RoleDescriptionTypical Example
Growth of multicellular organismsIncreases cell number while keeping the original genetic blueprint, allowing organs and tissues to enlarge.Embryonic development; post‑natal growth of human limbs and long bones.
Replacement of damaged or dead cellsContinuous turnover of cells with limited lifespans ensures tissue function is maintained.Skin epidermis, intestinal epithelium, red blood cells.
Repair of tissuesRapid proliferation of cells at a wound site restores the original tissue architecture.Healing of a cut skin surface; regeneration of liver tissue after injury.
Asexual reproductionGeneration of a new organism from a single parent without genetic recombination; offspring are clones.Vegetative propagation in strawberry runners; budding in hydra; colony formation in *Chlamydomonas* algae.

7. Stem‑Cell Contribution to Cell Replacement

Specialised stem cells retain the capacity for unlimited mitotic divisions and can give rise to all cell types of a tissue. Examples include:

  • Haematopoietic stem cells in bone‑marrow → continuously produce red blood cells, white blood cells and platelets.

  • Epidermal stem cells in the basal layer of skin → replenish keratinocytes that are shed from the surface.

  • Intestinal stem cells in the crypts of Lieberkühn → generate new absorptive and secretory cells every few days.

8. Illustrative Examples

  • Growth: Chondrocytes in the epiphyseal growth plate undergo repeated mitosis, expanding the cartilage matrix; later ossification lengthens long bones.

  • Replacement: Mature erythrocytes survive ~120 days; bone‑marrow stem cells divide mitotically to replace the cells that are removed from circulation.

  • Repair: After a hepatic laceration, hepatocytes adjacent to the wound re‑enter the cell cycle, divide, and repopulate the lost tissue, restoring liver function.

  • Asexual reproduction: A single vegetative cell of the alga *Volvox* undergoes successive mitotic divisions, producing a multicellular, genetically identical colony.

Suggested diagram: The seven stages of mitosis (prophase → cytokinesis) showing chromosome condensation, spindle formation, alignment, separation, and formation of two daughter nuclei.

9. Summary

Mitosis is the fundamental mechanism that allows multicellular organisms to increase cell number, replace worn‑out cells, repair damaged tissue, and reproduce asexually while preserving the exact genetic information of the parent cell. Precise regulation through checkpoints, cyclins, CDKs and telomere maintenance ensures division occurs only when appropriate; failure of this control underlies many cancers. Mastery of mitosis therefore underpins understanding of development, tissue homeostasis, stem‑cell biology and disease.