Within the cell cycle, mitosis
is the name given to nuclear division, and it consists of a number of phases:
(division of the cytoplasm) is normally next, resulting in two cells.
Each of the cells produced after mitosis is genetically identical
|Events within cell
Chromosomes become shorter and thicker (condensation) and absorb stain strongly..
Under the light microscope, they can be seen to be double (composed of 2 chromatids).
Mitotic spindle - a structure composed of protein fibres - forms between centriole pairs, which gradually move apart.
Nuclear membrane disintegrates.
Spindle occupies central section of cell - from pole to pole.
Centromeres (near middles of chromosomes) radiate kinetochore filaments which attach to the spindle.
Chromosomes become aligned with centromeres across equator of cell, attached to the spindle.
Viewed from the side, they form a fairly distinct line; from other angles they can be seen as a metaphase plate.
Two sets of chromosomes move apart in opposite directions.
Distinctive V shapes are formed, pointing towards poles.
Nuclear membranes re-form around the 2 separate sets of chromosomes.
Chromosomes uncoil and decondense.
Cleavage furrow forms across cell equator.
Cytoplasm divides and two daughter cells separate.
Practical work observing stages of mitosis
This usually involves
- the preparation of stained squashes of cells from
plant root tips
- set-up and use of an optical microscope to identify
the stages of mitosis in these stained squashes
- calculation of a mitotic index
- measurement of the apparent size of cells in the root tip
- calculation of their actual size using the formula:
Actual size = size of image / magnification
This is generally seen as 'required' practical work - see web reference below
The microtubule-organizing center (MTOC)
is a structure found in eukaryotic cells from which microtubules emerge.
Its main function is the organization of the spindle apparatus, which separates the chromosomes during cell division after mitotis (and meiosis).
Don't get the centromere confused with the centrioles or the centrosome
[Another type of MTOC: the basal bodies associated with cilia and flagella].
In animal cells, the MTOC associated with spindle formation is the centrosome
, which consists of 2 centrioles
Proteins involved in the spindlle
Image of the mitotic spindle in a human cell showing microtubules in green, chromosomes (DNA) in blue, and kinetochores in red.
From Wikimedia Commons, the free media repository
Microtubules are composed of tubulin
, which is built up from α and β sub-units to form hollow tubules.
Within the cell, astral filaments
radiate from the MTOC towards the cell membrane at each pole of the cell. Other filaments move into the centre of the cell. K-fibres
radiate from the kinetochores which spread our sideways from the centromere regions of the chromosomes.
Each tubule has a polarity ('plus' and 'minus' ends), and subunits are added to one end of the microtubule.
Other motor proteins powered by ATP move along the microtubules causing movement in one direction or another.
move in a walking action, usually towards the positive end of a microtubule.
are motor proteins that move along microtubules, towards the minus-end.
Cytokinesis involves actin and myosin
[Actin and myosin are proteins which move relative to one another in muscle fibres.]
A contractile ring of actomyosin forms in the centre of the equatorial region of the cell, on the inner edge of the cell membrane. This causes the cleavage furrow which pinches the cytoplasm into two and brings the cell membrane in as well.