Stages of Mitosis

An extremely cool video about metaphase and chromosome structure!
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Mitosis Metaphase
Compare this simplified diagram with the photomicrograph at left.

Mitosis metaphase
Chromosomes (blue), kinetochores (pink), microtubules (green). Other parts of the cell are unstained and therefore invisible.

Related Topics:

Meiosis >>

Chromatids >>

Interphase >>

Eukaryotes >>

Interkinesis >>

Kinetochores >>

Chromosomes >>

Centromeres >>

Telomeres >>

DNA >>

Histones >>

Nucleus >>

Base pairs >>

Cell >>

Cytokinesis >>

DNA replication >>

Microtubules >>

By the end of prophase (see the discussion of prometaphase on the previous page) the nuclear envelope has entirely vanished and the chromosomes have condensed (see note about chromosome condensation below), which means that they have become tightly coiled and are now clearly visible even under an ordinary light microscope. In addition, the microtubules of the spindle apparatus have attached to the centromeres at their kinetochores. The centrosomes are now at opposite ends ("poles") of the cells.

Now, during metaphase — the second stage of mitosis in the eukaryotic cell cycle — the chromosomes, pulled by the spindle fibers, line up along the middle of the cell, halfway between the centrosomes in the middle of the dividing cell. The chromosomes are now maximally condensed.

In mitosis, individual replicated chromosomes, each composed of two sister chromatids, move to the equatorial plate during this step (whereas during the first division of meiosis, pairs of replicated chromosomes (tetrads) line up at this stage). This lack of pairing between homologs during mitosis is a fundamental distinction between mitosis and meiosis. Mitosis, unlike meiosis, produces identical daughter cells, because each homolog divides into two identical chromatids during anaphase.

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Etymology: The name of this stage of mitosis is derived from two Greek words, meta, meaning "after," "later" or "more advanced," and phasis, meaning "stage."
Chromosome condensation is the reorganisation of the long thin chromatin strands, of which chromosomes are composed during interphase, into compact short masses during prophase of mitosis (as well as in prophase I of meiosis). Chromosome condensation is carried out primarily by the condensin complex and occurs as the strands of DNA are tightly wrapped around histone molecules that act much like the reel of a fishing pole (see picture). If condensation did not occur as meiosis and mitosis began, the long spaghetti-like strands of the uncondensed interphase chromosomes would become hopelessly tangled like a fisherman's knot during chromosome segregation. As it is, they are neatly packed and ready for delivery to the poles!
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