Mitosis

10.2 M Phase — Mitosis

Mitosis is also called equational division because the daughter cells have the SAME chromosome number as the parent (no halving). It is the basis of growth, repair, and asexual reproduction in eukaryotes.

Mitosis occurs in somatic cells (body cells) — skin epidermis, intestinal lining, bone marrow, plant meristems. It is essentially the "copy-and-distribute" step: the cell has already copied its DNA in S phase; now it distributes the copies equally into two daughter cells.

Karyokinesis — Four Phases

The nuclear division (karyokinesis) happens in four sequential phases: Prophase → Metaphase → Anaphase → Telophase. After this, the cytoplasm divides (cytokinesis).

10.2.1 Prophase — Condensation

Early in mitosis, the cell prepares its chromosomes for movement:

• Chromosomes condense — chromatin coils and folds repeatedly into compact rod-shaped structures, now visible under light microscope.
• Each chromosome has two sister chromatids (produced in S phase) attached at the centromere.
• Centrioles (in animal cells), already duplicated in S phase, separate and move to opposite poles. From them, the spindle apparatus begins to form.
• Nucleolus disappears by end of prophase.
• Nuclear envelope begins to break down (completed in prometaphase, sometimes considered part of prophase in plants).
• The Golgi apparatus and endoplasmic reticulum fragment.

10.2.2 Metaphase — Alignment

The most photogenic phase:

• Nuclear envelope is fully disassembled — chromosomes have access to the spindle.
• Spindle fibres attach to kinetochores at each centromere — fibres from opposite poles pull on the two sister chromatids in opposite directions.
• Chromosomes align at the metaphase plate — the equator of the cell, halfway between the poles.
• The spindle assembly checkpoint verifies that every chromosome is properly attached to fibres from BOTH poles. If even one is mis-attached, anaphase is delayed.

10.2.3 Anaphase — Separation

The shortest mitotic phase but most dramatic:

• The centromere of each chromosome splits — sister chromatids are no longer joined.
• The separated sister chromatids (now called daughter chromosomes) are pulled toward opposite poles by shortening spindle fibres.
• Movement uses motor proteins (dynein, kinesin) and depolymerisation of microtubules. Speed: ~1 µm per minute.
• The result: at end of anaphase, each pole has a complete set of chromosomes (e.g., 46 in human cell).

10.2.4 Telophase — Reformation

The reverse of prophase:

• Daughter chromosomes arrive at their respective poles and begin to decondense (uncoil).
• The nuclear envelope reforms around each set of chromosomes from ER fragments.
• Nucleolus reappears.
• Spindle apparatus disassembles.
• The cell now has two daughter nuclei, but is still a single cell.

10.2.5 Cytokinesis — Cell Division

Cytokinesis is the actual physical splitting into two cells:

10.2.6 Significance of Mitosis

• Growth: A single fertilised egg becomes a multicellular body of trillions of cells, all through mitosis.
• Repair: Wound healing, replacement of dead cells (skin shedding, gut lining renewal every 2–3 days).
• Maintenance: Continuous replacement of short-lived cells (RBCs live 120 days; new ones from bone marrow mitosis).
• Asexual reproduction: Single-celled eukaryotes (yeast, Amoeba) and many plants reproduce by mitosis.
• Cellular constancy: Maintains identical chromosome number and genetic content across all body cells.

Worked Examples