Mitosis

From Hwiki

Mitosis divides genetic information during cell division.

Mitosis is the process by which a cell separates its duplicated genome into two identical halves. It is generally followed immediately by cytokinesis which divides the cytoplasm and cell membrane. This results in two identical daughter cells with a roughly equal distribution of organelles and other cellular components. Mitosis and cytokinesis together is defined as the mitotic (M) phase of the cell cycle, the division of the mother cell into two daughter cells, each the genetic equivalent of the parent cell. Mitosis occurs exclusively in eukaryotic cells. In multicellular organisms, the somatic cells undergo mitosis, while germ cells — cells destined to become sperm in males or ova in females — divide by a related process called meiosis. Prokaryotic cells, which lack a nucleus, divide by a process called binary fission.

The process of mitosis is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These stages are prophase, prometaphase, metaphase, telophase and anaphase. During the process of mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell. The cell then divides in cytokinesis, to produce two identical daughter cells.

Because cytokinesis usually occurs in conjunction with mitosis, "mitosis" is often used interchangeably with "mitotic phase". However, there are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei. This occurs most notably among the fungi and slime moulds, but is found in various different groups. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development.<ref name=Lilly>{{#if:Lilly M, Duronio R

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}}.</ref> Errors in mitosis can either kill a cell through apoptosis or cause mutations that may lead to cancer.

File:Mitosis-flourescent.jpg

A cell in late metaphase. All chromosomes (blue) but one have arrived at the metaphase plate.

Overview

File:Chromosomes during mitosis.svg

Chromosome during mitosis

The primary result of mitosis is the division of the parent cell's genome into two daughter cells. The genome is composed of a number of chromosomes, complexes of tightly-coiled DNA that contain genetic information vital for proper cell function. Because each resultant daughter cell should be genetically identical to the parent cell, the parent cell must make a copy of each chromosome before mitosis. This occurs during the middle of interphase, the period that precedes the mitotic phase in the cell cycle where preparation for mitosis occurs.<ref name=Blow>{{#if:Blow J, Tanaka T

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Each chromosome now contains two identical copies of itself, called sister chromatids, attached together in a specialized region of the chromosome known as the centromere. Each sister chromatid is not considered a chromosome in itself, and a chromosome does not always contain two sister chromatids.

In most eukaryotes, the nuclear envelope that separates the DNA from the cytoplasm degrades, and its fluid spills out into the cytoplasm. The chromosomes align themselves in a line spanning the cell. Microtubules, essentially miniature strings, splay out from opposite ends of the cell and shorten, pulling apart the sister chromatids of each chromosome.<ref>{{#if:Zhou J, Yao J, Joshi H

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}}.</ref> As a matter of convention, each sister chromatid is now considered a chromosome, so they are renamed to sister chromosomes. As the cell elongates, corresponding sister chromosomes are pulled toward opposite ends. A new nuclear envelope forms around the separated sister chromosomes.

As mitosis completes cytokinesis is well underway. In animal cells, the cell pinches inward where the imaginary line used to be,(the pinching of the cell membrane to form the two daughter cells is called cleavage furrow.) separating the two developing nuclei. In plant cells, the daughter cells will construct a new dividing cell wall between each other. Eventually, the mother cell will be split in half, giving rise to two daughter cells, each with an equivalent and complete copy of the original genome.

Prokaryotic cells undergo a process similar to mitosis called binary fission. However, prokaryotes cannot be properly said to undergo mitosis because they lack a nucleus and only have a single chromosome with no centromere.<ref>{{#if:Nanninga N

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Phases

Interphase

File:Cell cycle.png

The cell cycle

The mitotic phase is a relatively short action-packed period of the cell cycle. It alternates with the much longer interphase, where the cell prepares itself for division. Interphase is divided into three phases, G₁ (first gap), S (synthesis), and G₂ (second gap). During all three phases, the cell grows by producing proteins and cytoplasmic organelles. However, chromosomes are replicated only during the S phase. Thus, a cell grows (G₁), continues to grow as it duplicates its chromosomes (S), grows more and prepares for mitosis (G₂), and divides (M).<ref name=Blow/>

Mitosis is a continual and dynamic process. For purposes of description, however, mitosis is conventionally broken down into five subphases: prophase, prometaphase, metaphase, anaphase, and telophase, even though it is impossible to discern an exact "start" and "stop" of the phases.

Prophase

File:Prophase.jpg

Prophase: The two round objects above the nucleus are the centrosomes. Note the condensed chromatin.

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Mitosis

From Hwiki

Contents

Overview

Phases

Interphase

Prophase

Preprophase

Prometaphase

Metaphase

Anaphase

Telophase

Cytokinesis

Significance

Consequences of errors

Endomitosis

Timeline in pictures

References

Further reading

External links

Views

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