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In biology, mitosis is the process of chromosome segregation and nuclear division that follows replication of the genetic material in eukaryotic cells. This process assures that each daughter nucleus receives a complete
copy of the organism's genome. In most eukaryotes mitosis is accompanied with cell division or cytokinesis, but there are many exceptions, for instance among the fungi. There is another process called meiosis, in
which the daughter nuclei receive half the chromosomes of the parent, which is involved in gamete formation and other similar processes.
Mitosis is divided into several stages, with the remainder of the cell's growth cycle considered interphase. Properly
speaking, a typical cell cycle involves a series of stages: G1, the first
growth phase; S, where the genetic material is duplicated; G2, the second growth phase; and M, where the nucleus divides through
mitosis. Mitosis is divided into prophase, prometaphase, metaphase, anaphase, and telophase.
The whole procedure is very similar among most eukaryotes, with only minor variations. As prokaryotes lack a nucleus and only have a single chromosome with no centromere, they cannot be properly said to
undergo mitosis.
The genetic material (DNA), which normally exists in the form of chromatin condenses into a highly ordered structure called a chromosome. Since the genetic material has been duplicated, there are two identical copies of each chromosome in
the cell. Identical chromosomes (called sister chromosomes) are attached to each other at a DNA element present on every
chromosome called the centromere. A protein called cohesin acts as a glue
joining two sister chromosomes (aka sister chromatids) along their lengths. When chromosomes are paired up and attached, each
individual chromosome in the pair is called a chromatid, while the whole unit
(confusingly) is called a chromosome. Just to be even more confusing, when the chromatids separate, they are no longer called
chromatids, but are called chromosomes again. The task of mitosis is to assure that one copy of each sister chromatid - and only
one copy - goes to each daughter cell after cell division.
The other important piece of hardware in mitosis is the centriole. Each
centrosome contains two centrioles. The two centrosomes of the cell serve as
organizing centers for microtubules. During prophase, the two centrosomes -
which replicate independently of mitosis - begin recruiting microtubules (which may be thought of as cellular ropes or poles) and
forming mitotic spindles. By increasing the length of the spindle
(growing the microtubules), the centrioles push apart to opposite ends of the cell nucleus. It should be noted that many
eukaryotes, for instance plants, lack centrioles although the basic process is still similar.
Prometaphase
Some biology texts do not include this phase, considering it a part of prophase. In this phase, the nuclear membrane dissolves
in some eukaryotes, reforming later once mitosis is complete. This is called open mitosis, found in most multicellular forms.
Many protists undergo closed mitosis, in which the nuclear membrane persists
throughout.
Now kinetochores begin to form
at the centromeres. This is a complex protein structure that may be thought of as an 'eyelet' for the microtubule 'rope' - it is
the attaching point by which chromosomes may be secured. The kinetochore is an enormously complex structure that is not yet fully
understood. The key feature of a kinetochore is that it contains a molecular motor that uses energy from ATP molecules to move a
chromatid along a microtubule, like a train on a train track. Two kinetochores form on each chromosome - one for each
chromatid.
When the spindle grows to sufficient length, the microtubules begin searching for kinetochores to attach to.
As microtubules find and attach to kinetochores, they begin to line up in the middle of the cell. Proper segregation requires
that every kinetochore be attached to a microtubule before separation begins. It is thought that unattached kinetochores control
this process by generating a signal - the mitotic spindle checkpoint - that tells the cell to wait before proceeding to anaphase. There are many
theories as to how this is accomplished, some of them involving the generation of tension when both microtubules are attached to
the kinetochore.
When chromosomes are bivalently attached - when both kinetochores are attached to microtubules emanating from each centriole -
they line up in the middle of the spindle, forming what is called the metaphase plate. This does not
occur in every organism - in some cases chromosomes move back and forth between the centrioles randomly, only roughly lining up
along the midline.
When every kinetochore is attached to a microtubule and the chromosomes have lined up along the middle of the spindle, the
cell proceeds to anaphase. This is divided into two phases. First, the proteins that bind the sister chromatids together are
cloven, allowing them to separate. They are pulled apart by the microtubules, towards the respective centrioles to which they are
attached. Next, the spindle axis elongates, driving the centrioles (and the set of chromosomes to which they are attached) apart
to opposite ends of the cell. These two stages are sometimes called 'early' and 'late' anaphase.
At the end of anaphase, the cell has succeeded in separating identical copies of the genetic material into two distinct
populations.
Now the nuclear membrane reforms around the genetic material and the chromosomes are unfolded back into chromatin. This is
often followed by cytokinesis or cleavage, where the
cellular membrane pinches off between the two newly separated nuclei, to form two new daughter cells.
- See also : genetics -- cell biology -- meiosis -- cell cycle -- daughter cell
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