The stages of mitosis

Mitosis produces two cells strictly identical to the original. It is the division of growth and renewal.

The principle in one line

1 cell with 2n chromosomes  ->  2 cells with 2n chromosomes, identical

In humans, 2n = 46: one cell with 46 chromosomes yields two with 46 chromosomes, carrying exactly the same genetic information.

The four phases

PROPHASE
   the chromosomes CONDENSE and become visible
   the nuclear envelope breaks down, the spindle forms

      ~~~~~        ->        ||  ||  ||
   (DNA threads)        (visible chromosomes)

METAPHASE
   the chromosomes line up on the EQUATORIAL plane

          ---||---||---||---
              metaphase plate

ANAPHASE
   the two chromatids of each chromosome SEPARATE
   and migrate to opposite poles

        <-- |  |  |     |  |  | -->

TELOPHASE
   the chromosomes decondense, two nuclei re-form,
   then CYTOKINESIS separates the two cells

        (  ●  )   (  ●  )

What guarantees identity

Two mechanisms combine:

1. REPLICATION in phase S copies the DNA identically
2. ANAPHASE separates the two copies, one per daughter cell

Each daughter cell therefore receives one chromatid from each chromosome, that is, a complete and faithful copy of the genome. No information is lost, none is mixed.

The biological point of cloning

Every cell of an organism carries the same genome — a neuron and a skin cell have exactly the same DNA. What distinguishes them is not their information but the genes they express: each cell type uses only a fraction of the programme.

same DNA in every cell
        |
        +-- neuron     : expresses nervous-system genes
        +-- hepatocyte : expresses liver genes
        +-- myocyte    : expresses muscle genes

This is what makes cloning possible: in 1996, Dolly the sheep was obtained by transferring the nucleus of an adult mammary cell into an enucleated oocyte. The experiment proved that a specialised cell retains the whole genetic programme, merely switched off.

Asexual reproduction

In many organisms, mitosis alone suffices to reproduce:

taking a plant cutting   ->  genetically identical offspring
bacterial division       ->  clones
budding in hydra

Advantage: fast, no partner needed. Major drawback: no genetic diversity. A population of clones is vulnerable — a parasite able to kill one kills them all. This explains the disaster of the Irish famine of 1845: almost all cultivated potatoes descended by cutting from a handful of plants, and blight reached every one.

Summary

• Mitosis yields two identical cells with 2n chromosomes.
• Four phases: prophase, metaphase, anaphase, telophase, then cytokinesis.
• Identity comes from replication (phase S) followed by separation of the chromatids.
• All cells of an organism have the same DNA; they differ in the genes expressed.
• Asexual reproduction rests on mitosis: fast, but with no diversity at all.