mitosis

mitosis (/maɪˈtoʊsɪs/) is a part of the cell cycle when replicated chromosomes are separated into two
new nuclei. Cell division gives rise to genetically identical cells in which the number of chromosomes is
maintained.[1] In general, mitosis (division of the nucleus) is preceded by the S stage of interphase
(during which the DNA is replicated) and is often accompanied or followed by cytokinesis, which divides
the cytoplasm, organelles and cell membrane into two new cells containing roughly equal shares of
these cellular components.[2] Mitosis and cytokinesis together define the mitotic (M) phase of an
animal cell cycle—the division of the mother cell into two daughter cells genetically identical to each
other.

The process of mitosis is divided into stages corresponding to the completion of one set of activities and
the start of the next. These stages are prophase, prometaphase, metaphase, anaphase, and telophase.
During mitosis, the chromosomes, which have already duplicated, condense and attach to spindle fibers
that pull one copy of each chromosome to opposite sides of the cell.[3] The result is two genetically
identical daughter nuclei. The rest of the cell may then continue to divide by cytokinesis to produce two
daughter cells.[4] Producing three or more daughter cells instead of the normal two is a mitotic error
called tripolar mitosis or multipolar mitosis (direct cell triplication / multiplication).[5] Other errors
during mitosis can induce apoptosis (programmed cell death) or cause mutations. Certain types of
cancer can arise from such mutations.[6]

Mitosis occurs only in eukaryotic cells. Prokaryotic cells, which lack a nucleus, divide by a different
process called binary fission. Mitosis varies between organisms.[7] For example, animal cells undergo an
"open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, whereas
fungi undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus.[8] Most
animal cells undergo a shape change, known as mitotic cell rounding, to adopt a near spherical
morphology at the start of mitosis. Most human cells are produced by mitotic cell division. Important
exceptions include the gametes – sperm and egg cells – which are produced by meiosis.

Mitosis "function" or significance relies on the maintenance of the chromosomal set; each cell formed
receives chromosomes that are alike in composition and equal in number to the chromosomes of the
parent cell.

Mitosis occurs in the following circumstances:

Cell replacement: In some parts of the body, e.g. skin and digestive tract, cells are constantly sloughed
off and replaced by new ones. New cells are formed by mitosis and so are exact copies of the cells being
replaced. In like manner, red blood cells have a short lifespan (only about 4 months) and new RBCs are
formed by mitosis.

Regeneration: Some organisms can regenerate body parts. The production of new cells in such instances
is achieved by mitosis. For example, starfish regenerate lost arms through mitosis.

Asexual reproduction: Some organisms produce genetically similar offspring through asexual
reproduction. For example, the hydra reproduces asexually by budding. The cells at the surface of hydra
undergo mitosis and form a mass called a bud. Mitosis continues in the cells of the bud and this grows
into a new individual. The same division happens during asexual reproduction or vegetative propagation
in plants.

histopathology, the mitosis rate is an important parameter in various types of tissue samples, for
diagnosis as well as to further specify the aggressiveness of tumors. For example, there is routinely a
quantification of mitotic count in breast cancer classification.[62] The mitoses must be counted in an
area of the highest mitotic activity. Visually identifying these areas is difficult in tumors with very high
mitotic activity.[63] Also, the detection of atypical forms of mitosis can be used both as a diagnostic and
prognostic marker.[citation needed] For example, lag-type mitosis (non-attached condensed chromatin
in the area of the mitotic figure) indicates high risk human papillomavirus infection-related Cervical
cancer.[citation needed]

The primary mechanism by which organisms generate new cells is through cell division. During this
process, a single "parent" cell will divide and produce identical "daughter" cells. In this way, the parent
cell passes on its genetic material to each of its daughter cells. First, however, the cells must duplicate
their DNA. Mitosis is the process by which a cell segregates its duplicated DNA, ultimately dividing its
nucleus into two.

Cell division is a universal process among living organisms. In 1855, Rudolf Virchow, a German
researcher, made a fundamental observation about all living creatures: every cell originates from
another cell, or "omnis cellula e cellula," in the original Latin, as author Myron Shultz recounts in a 2008
article in the journal Emerging Infectious Diseases.
The mechanisms of cell division vary between prokaryotes and eukaryotes. Prokaryotes are single-celled
organisms, such as bacteria and archaea. They have a simple internal structure with free-floating DNA.
They use cell division as a method of asexual reproduction, in which the genetic makeup of the parent
and resulting offspring are the same. One common mechanism of asexual reproduction in prokaryotes is
binary fission. During this process, the parent cell duplicates its DNA and increases the volume of its cell
contents. Eventually, a fissure emerges in the center of the cell, leading to the formation of two identical
daughter cells.

The cells of eukaryotes, on the other hand, have an organized central compartment, called the nucleus,
and other structures, such as mitochondria and chloroplasts. Most eukaryotic cells divide and produce
identical copies of themselves by increasing their cell volume and duplicating their DNA through a series
of defined phases known as the cell cycle. Since their DNA is contained within the nucleus, they undergo
nuclear division as well. "Mitosis is defined as the division of a eukaryotic nucleus," said M. Andrew
Hoyt, a professor of biology at Johns Hopkins University, "[though] many people use it to reflect the
whole cell cycle that is used for cell duplication."

Like prokaryotes, single-celled eukaryotes, such as amoeba and yeast, also use cell division as a method
of asexual reproduction. For complex multicellular eukaryotes like plants and animals, cell division is
necessary for growth and the repair of damaged tissues. Eukaryotic cells can also undergo a specialized
form of cell division called meiosis, which is necessary to produce reproductive cells like sperm cells, egg
cells and spores.

Stages of the eukaryotic cell cycle

The eukaryotic cell cycle is a series of well-defined and carefully timed events that allow a cell to grow
and divide. According to Geoffery Cooper, author of "The Cell: A Molecular Approach, 2nd Ed." (Sinauer
Associates, 2000) most eukaryotic cell cycles have four stages:

G1 phase (first gap phase): During this phase cells that are intended for mitosis, grow and carry out
various metabolic activities.

Chromosomes, become highly compacted during mitosis, and can be clearly seen as dense structures
under the microscope.
The resulting daughter cells can re-enter G1 phase only if they are destined to divide. Not all cells need
to divide continuously. For example, human nerve cells stop dividing in adults. The cells of internal
organs like the liver and kidney divide only when needed: to replace dead or injured cells. Such types of
cells enter the G0 phase (quiescent phase). They remain metabolically active and only move into the G1
phase of the cell cycle when they receive the necessary molecular signals, according to Cooper.

Mitosis, a process of cell duplication, or reproduction, during which one cell gives rise to two genetically
identical daughter cells. Strictly applied, the term mitosis is used to describe the duplication and
distribution of chromosomes, the structures that carry the genetic information.

Mitosis

BIOLOGY

WRITTEN BY: The Editors of Encyclopaedia Britannica

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Alternative Title: karyokinesis

Divide into four phases the reproduction process of chromosomes in plant and animal cells

Divide into four phases the reproduction process of chromosomes in plant and animal cells

Mitosis has four stages: prophase, metaphase, anaphase, and telophase.

Encyclopædia Britannica, Inc.

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Mitosis, a process of cell duplication, or reproduction, during which one cell gives rise to two genetically
identical daughter cells. Strictly applied, the term mitosis is used to describe the duplication and
distribution of chromosomes, the structures that carry the genetic information.

Mitosis

QUICK FACTS

KEY PEOPLE

Walther Flemming
Daniel Mazia

RELATED TOPICS

Meiosis

Cleavage

Cell division

Endomitosis

A brief treatment of mitosis follows. For a full treatment, see growth: In cells; cell: Mitosis and
cytokinesis.

TOP QUESTIONS

What is mitosis?

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Prior to the onset of mitosis, the chromosomes have replicated and the proteins that will form the
mitotic spindle have been synthesized. Mitosis begins at prophase with the thickening and coiling of the
chromosomes. The nucleolus, a rounded structure, shrinks and disappears. The end of prophase is
marked by the beginning of the organization of a group of fibres to form a spindle and the disintegration
of the nuclear membrane.

The chromosomes, each of which is a double structure consisting of duplicate chromatids, line up along
the midline of the cell at metaphase. In anaphase each chromatid pair separates into two identical
chromosomes that are pulled to opposite ends of the cell by the spindle fibres. During telophase, the
chromosomes begin to decondense, the spindle breaks down, and the nuclear membranes and nucleoli
re-form. The cytoplasm of the mother cell divides to form two daughter cells, each containing the same
number and kind of chromosomes as the mother cell. The stage, or phase, after the completion of
mitosis is called interphase.