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Prometaphase

Prometaphase

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Prometaphase is the phase of mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

 following prophase
Prophase
Prophase, from the ancient Greek πρό and φάσις , is a stage of mitosis in which the chromatin condenses into a highly ordered structure called a chromosome in which the chromatin becomes visible. This process, called chromatin condensation, is mediated by the condensin complex...

 and preceding metaphase
Metaphase
Metaphase, from the ancient Greek μετά and φάσις , is a stage of mitosis in the eukaryotic cell cycle in which condensed & highly coiled chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells...

, in eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

 somatic
Somatic
The term somatic means 'of the body',, relating to the body. In medicine, somatic illness is bodily, not mental, illness. The term is often used in biology to refer to the cells of the body in contrast to the germ line cells which usually give rise to the gametes...

 cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

s. In Prometaphase, The nuclear envelope
Nuclear envelope
A nuclear envelope is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus from the cytosol...

 breaks into fragments and disappears. The tiny nucleolus
Nucleolus
The nucleolus is a non-membrane bound structure composed of proteins and nucleic acids found within the nucleus. Ribosomal RNA is transcribed and assembled within the nucleolus...

 inside the nuclear envolope, also dissolves. Microtubules emerging from the centrosomes at the poles (ends) of the spindle reach the chromosomes, now highly condensed. At the centromere region, each sister chromatid has a protein structure called a kinetochore. Some of the spindle microtubules attach to the kinetochores, throwing the chromosomes into agitated motion. Other spindle microtubules make contact with microtubules coming from opposite pole. Forces exerted by protein "motors" associated with spindle microtubules move the chromosomes toward the center of the cell.

Open and closed mitosis


In "open mitosis", microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...

s which have radiated from the two centrosome
Centrosome
In cell biology, the centrosome is an organelle that serves as the main microtubule organizing center of the animal cell as well as a regulator of cell-cycle progression. It was discovered by Edouard Van Beneden in 1883...

s located at the opposite poles of the cells invade the nuclear space
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

as the nuclear envelope
Nuclear envelope
A nuclear envelope is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus from the cytosol...

 disassembles. This allows the microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...

s to reach for the chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s to align them at the metaphase plate to ensure proper segregation of the genetic material.

Most fungi and many protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...

s, such as algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, undergo a variation called "closed mitosis" where the microtubules are able to penetrate an intact nuclear envelope through a spindle pole body
Spindle pole body
The spindle pole body is the microtubule organizing center in yeast cells, functionally equivalent to the centrosome. Unlike the centrosome the SPB does not contain centrioles. The SPB organises the microtubule cytoskeleton which plays many roles in the cell...

.

Two types of microtubules


The microtubules are composed of two types, kinetochore microtubules and non-kinetochore microtubules.
  • Kinetochore microtubules begin searching for kinetochores to attach to.
  • A number of non-kinetochore microtubules find and interact with corresponding nonkinetochore microtubules from the opposite centrosome to form the mitotic spindle
    Mitotic spindle
    In cell biology, the spindle fibers are the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells...

    .

Transition from prometaphase to metaphase


The role of prometaphase is completed when all of the kinetochore microtubules have attached to their kinetochores, upon which metaphase
Metaphase
Metaphase, from the ancient Greek μετά and φάσις , is a stage of mitosis in the eukaryotic cell cycle in which condensed & highly coiled chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells...

 begins. An unattached kinetochore, and thus a non-aligned chromosome, even when most of the other chromosomes have lined up, will trigger the spindle checkpoint
Spindle checkpoint
In order to preserve one cell's identity and its proper functioning, it is necessary to maintain constant the appropriate number of chromosomes after each cell division...

 signal. This prevents premature progression into anaphase
Anaphase
Anaphase, from the ancient Greek ἀνά and φάσις , is the stage of mitosis or meiosis when chromosomes move to opposite poles of the cell....

 by inhibiting the anaphase-promoting complex
Anaphase-promoting complex
Anaphase-Promoting Complex, also called cyclosome , is an E3 ubiquitin ligase that marks target cell cycle proteins for degradation by the 26S proteasome. The APC/C is a large complex of 11–13 subunit proteins, including a cullin and RING subunit much like SCF...

until all kinetochores is attached and all the chromosomes aligned.

Early events of metaphase can coincide with the later events of prometaphase, as chromosomes with connected kinetochores will start the events of metaphase individually before other chromosomes with unconnected kinetochores that are still lingering in the events of prometaphase.