Name: chromosome movement during mitosis
Uploaded: 2021-04-07T12:12:01.000Z
Duration: 2 min 11 s
Channel: JoVE

Während der Mitose oszillieren die biorientierten Chromosomen, um ihre Spindelansätze zu stabilisieren, und lagern sich anschließend entlang der Metaphasenplatte am Äquator der mitotischen Spindel an.
Mehrere Kräfte wirken auf die Chromosomen, nachdem sie an der mitotischen Spindel befestigt sind.
Eine große polwärts gerichtete Kraft, die entlang des Kinetochor-Mikrotubuli ausgeübt wird, als Ergebnis der Plus-End-Depolymerisation, zieht das Kinetochor und sein zugehöriges Chromosom in Richtung des Spindelpols. Am Kinetochor verbinden Ndc80-Proteinkomplexe das Kinetochor mit dem Mikrotubuli durch mehrere niedrig affine Attachments entlang der Mikrotubuli-Seiten.
Während der Depolymerisation von Mikrotubuli plus Ende brechen die Ndc80-Attachments und bilden sich an neuen Stellen neu, um die Kinetochor-Mikrotubuli-Verbindung aufrechtzuerhalten. Der Mechanismus zieht das Chromosom allmählich in Richtung des Spindelpols, während sich der Mikrotubuli in der Länge verkürzt.
Eine zweite polwärts gerichtete Kraft ergibt sich aus dem Mikrotubuli-Fluss. Die Mikrotubuli-Minus-Ende-Depolymerisation erzeugt einen Minus-Ende-gerichteten Fluss, der eine Bewegung der Mikrotubuli in Richtung des Spindelpols verursacht. Die Koordination der Plus-End-Polymerisation kompensiert die Minus-End-Depolymerisation, so dass die Mikrotubuli ihre Länge beibehalten können.
Eine dritte Kraft, die polare Auswurfkraft oder der polare Wind, die von den Motorproteinen Kinesin-4 und 10 erzeugt wird, drückt die Chromosomen von den Spindelpolen weg. Kinesin-4 und 10 verbinden chromosomale Arme mit interpolaren Mikrotubuli. Diese nach Plus-Enden gerichteten Motorproteine bewegen das Chromosom in Richtung Spindeläquator.
Ein ausgewogenes Zusammenspiel dieser vielfältigen gegensätzlichen Kräfte ermöglicht es, dass sich die biorientierten Chromosomen genau entlang der Metaphasenplatte ausrichten, um die Chromosomentrennung vorzubereiten.

<ol>
	<li>Brouhard, G. J. und A. J. Hunt. "Mikrotubuli-Bewegungen an den Armen mitotischer Chromosomen: In vitro quantifizierte polare Auswurfkräfte." Verfahren der Nationalen Akademie der Wissenschaften 102, Nr. 39 (2005): 13903–8. <a href="https://www.pnas.org/content/102/39/13903">[Quelle]</a></li>
	<li>Dumont, Sophie und Timothy J. Mitchison. "Kraft und Länge in der mitotischen Spindel." Current Biology 19, Nr. 17 (2009). <a href="https://www.sciencedirect.com/science/article/pii/S096098220901450X">[Quelle]</a></li>
	<li>Khodjakov, Alexey und Tarun Kapoor. "Mikrotubuli-Fluss: Wofür ist er gut?" Current Biology 15, Nr. 23 (2005). <a href="https://www.sciencedirect.com/science/article/pii/S0960982205013928">[Quelle]</a></li>
</ol>

kräfte, die auf chromosomen wirken

Forces Acting on Chromosomes

Während der Mitose treten Chromosomenbewegungen durch das Zusammenspiel von Kräften auf Piconewton-Niveau auf. In der Prometaphase helfen diese Kräfte beim Aufbau der Chromosomen oder der Kongression in der Äquatorialebene, was letztendlich zu ihrer Ausrichtung auf der Metaphaseplatte führt. Die auf die Chromosomen wirkenden Kräfte sind raum- und zeitabhängig; daher variieren sie mit der Position der Chromosomen, während die Zelle die Mitose durchläuft.

Mikrotubuli und Motorproteine üben zwei Arten von Kräften auf Chromosomen aus – polwärts und antipolwärts, auch bekannt als polare Ausstoßkräfte. Die Depolymerisation von Kinetochor-Mikrotubuli erzeugt die polwärts gerichtete Kraft und zieht das Chromosom zum Spindelpol. Im Gegensatz dazu führt die Polymerisation des Kinetochor-Mikrotubulus zu polaren Ausstoßkräften, die das Chromosom zum Äquator der Zelle drängen. Mikrotubuli-plus-Ende-gerichtete Motorproteine, wie Chromokinesine und Kinesin-7, erzeugen ebenfalls polare Ausstoßkräfte, indem sie Chromosomen zum Äquator der Zelle treiben.

Die gleichzeitige, aber ungleiche Wirkung von polwärts und polaren Ausstoßkräften verursacht die Oszillation der Chromosomen während der Prometaphase; jedoch erleben die biorientierten Schwesterchromatiden während der Metaphase gleich große, aber entgegengesetzte Kräfte. Dies erzeugt genügend Spannung, um den Spindel-Assemblierungs-Checkpoint-Pfad zu deaktivieren und ermöglicht es den Zellen, in die Anaphase überzugehen. In der Anaphase wirken polwärts gerichtete Kräfte auf die Schwesterchromatiden, was zu ihrer erfolgreichen Segregation in die Tochterzellen führt.

Zusätzlich zu den oben genannten Kräften werden Chromosomen auch kohäsiven und auflösenden Kräften ausgesetzt. Die kohäsive Kraft, die von Cohesin ausgeübt wird, hält die Schwesterchromatiden bis zum Ende der Metaphase zusammen. Andererseits ermöglicht die durch Kondensine erzeugte auflösende Kraft, dass Chromosomen deutlich stabförmige Strukturen bilden, was ihre ordnungsgemäße Trennung während der Anaphase unterstützt.

Kräfte, die auf Chromosomen wirken

Während der Mitose treten Chromosomenbewegungen durch das Zusammenspiel mehrerer Kräfte des Piconewton-Niveaus auf. In der Prometaphase helfen diese Kräfte in der Chromosomen- ...

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chromosome movement during mitosis

Chromosome movement during mitosis depends on several small forces that act at different times and positions in the cell. In prometaphase, these forces help chromosomes move to the equatorial plane and line up at the metaphase plate. Because the forces change with location and stage, chromosome behavior also changes as mitosis continues.

Microtubules and motor proteins create two main kinds of force on chromosomes: poleward force and anti-poleward force, also called polar-ejection force. Poleward force comes from kinetochore microtubule depolymerization, which pulls a chromosome toward the spindle pole. Polar-ejection force comes from kinetochore-microtubule polymerization, which pushes the chromosome toward the cell’s equator.

Motor proteins also help drive chromosomes toward the middle of the cell. Plus-end directed proteins such as chromokinesins and kinesin-7 generate polar-ejection forces. During prometaphase, the unequal balance between poleward and polar-ejection forces makes chromosomes oscillate. In metaphase, the sister chromatids are bioriented, so they experience equal and opposing forces.

That balanced tension is important because it silences the spindle assembly checkpoint pathway. Once this checkpoint is off, the cell can move into anaphase. In anaphase, poleward forces act on sister chromatids and separate them into the daughter cells.

Chromosomes are also controlled by cohesive and resolving forces. Cohesin provides the cohesive force that keeps sister chromatids together until the end of metaphase. Condensins generate the resolving force, which helps chromosomes form distinct rod-shaped structures and supports their proper separation during anaphase.

Chromosome Movement During Mitosis

mitosis and cytokinesis

Mitosis and Cytokinesis Stages

Mitosis and cytokinesis are the two main steps of cell division. Mitosis is the process that divides the nucleus, while cytokinesis splits the rest of the cell. During mitosis, the cell’s genetic material is separated into two sets. This helps form two new nuclei with the same genetic information. Cytokinesis then completes division by separating the cytoplasm, which creates two daughter cells. These stages work together to make cell division possible in growing and repairing tissues.

duplication of chromatin structure

Chromatin Rebuilding During DNA Replication

Chromatin rebuilding during DNA replication helps cells copy DNA and pass on chromosome structure to daughter cells. During cell division, the genome must be disrupted and then reassembled across the whole chromosome. This process allows chromatin to be inherited, rebuilt, and maintained in new cells. The basic unit of chromatin is the nucleosome. A nucleosome contains DNA wrapped around an octamer of histone proteins, with short linker DNA between neighboring nucleosomes. The histone proteins...

cohesins

Cohesins in Meiosis and Genetic Disorders

Cohesin protein complexes act like molecular glue that keeps sister chromatids together. They are important in both mitosis and meiosis. In mitosis, cohesin complexes are removed from chromosomes before anaphase begins. Meiosis has two rounds of chromosome separation. Meiosis I separates homologous chromosomes, while Meiosis II separates sister chromatids. This process produces four daughter cells. The Meiosis I cohesin complex is made of four subunits: Smc1, Smc3, Rec8, and Scc3. Rec8...

condensins

How Condensins Shape Mitotic Chromosomes

Condensins shape chromosomes during mitosis, the stage when a cell divides. These large protein complexes use ATP, the cell’s energy molecule, to help turn loose post-interphase DNA into compact, separate chromosomes. They organize and segregate chromosomal DNA so it can be passed on correctly. Plant and animal cells contain two condensin complexes: condensin I and condensin II. Each complex has five subunits. Two are SMC, or Structural Maintenance of Chromosomes, subunits. One is a kleisin...

the mitotic spindle

How the Mitotic Spindle Separates Chromosomes

The mitotic spindle is a microtubule-based structure that helps separate chromosomes during cell division. It forms in eukaryotic cells and moves sister chromatids to opposite ends of the parent cell. This leaves each new cell nucleus with the correct genetic material. A bipolar spindle has two poles, and that shape is important for chromosome segregation. In many animal cells, two centrosomes are present when mitosis begins. Each centrosome helps organize a circular set of microtubules at one...

centrosome duplication

Centrosome Duplication in the Cell Cycle

Centrosome duplication is a key part of the animal cell cycle. The centrosome is the main microtubule organizing center, or MTOC, in animal cells. It contains two cylindrical centrioles at its core. Each centriole has nine sets of three microtubules held together by proteins. The two centrioles sit at right angles and are surrounded by the pericentriolar matrix, or PCM, a shapeless protein cloud. Duplication starts early in the cell cycle so each daughter cell can receive a centrosome after...

microtubule instability

Microtubule Growth and Shrinkage

Microtubules are hollow, cylindrical filaments in the cell. They are about 25 nm wide and can range from 200 nm to 25 μm in length. Their main building blocks are GTP-bound tubulin subunits that form αβ-heterodimers, which then assemble into protofilaments. These protofilaments join by lateral bonds to create stable microtubule cylinders. Microtubules are highly dynamic because they repeatedly assemble and break down. They can even grow on one end while shrinking on the other at the same time.

spindle assembly

How the Mitotic Spindle Forms

The mitotic spindle forms through centrosome-mediated, chromatin-mediated, and microtubule-mediated pathways. These pathways often work at the same time. Together, they build a strong spindle apparatus for cell division. In most cells, centrosomes are the main sites where microtubules begin to grow. During the G2 to prophase transition, centrosomes mature and make more microtubules. The growing microtubules extend from both centrosomes, and their plus-ends search for chromosomes by attaching...

attachment of sister chromatids

Kinetochore-Microtubule Binding in Mitosis

Kinetochore-microtubule binding helps chromosomes line up and separate during mitosis. After the nuclear envelope breaks down, condensed chromosomes are exposed to the bipolar microtubules of the mitotic spindle. The kinetochore is a large, disc-shaped protein complex at the centromere region of each sister chromatid, and it serves as the microtubule binding site. Usually, the plus-end of a single microtubule inserts into the kinetochore. In some cases, a kinetochore first makes lateral...

forces acting on chromosomes

Chromosome movement during mitosis depends on several small forces that act at different times and positions in the cell. In prometaphase, these forces help chromosomes move to the equatorial plane and line up at the metaphase plate. Because the forces change with location and stage, chromosome behavior also changes as mitosis continues. Microtubules and motor proteins create two main kinds of force on chromosomes: poleward force and anti-poleward force, also called polar-ejection force.

separation of sister chromatids

Anaphase Start and Chromatid Release

Anaphase starts when sister chromatids are released from one another and pulled toward opposite spindle poles. Before that happens, cohesion along the chromosomal arms is reduced from prophase to metaphase, which helps the sister chromatids resolve. A residual cohesin link still holds them together until the metaphase-to-anaphase transition. This leftover connection helps prevent early separation and reduces the risk of aneuploidy, or the wrong number of chromosomes, in daughter cells. At the...

the spindle assembly checkpoint

How Cells Safeguard Chromosome Separation

The spindle assembly checkpoint helps cells safeguard chromosome separation during anaphase. It acts as a molecular surveillance system. The checkpoint checks whether the steps needed for chromosome segregation are complete before the cell moves on. Many proteins work together to control this checkpoint. If mutations affect these proteins, a cell may enter anaphase too early. Chromosomes can then separate before the needed steps are finished. When this happens, the daughter cells may receive...

anaphase a and b

Chromatid Movement by Microtubule Forces

Chromatid movement during anaphase depends on microtubule forces at the spindle. Kinetochore microtubules grow from the spindle poles, and their plus ends attach to kinetochores on sister chromatids. The kinetochore protein complex Ndc80 makes low-affinity links with the microtubule plus end.

the contractile ring

How Cells Build the Contractile Ring

Contractile rings are built from microfilaments and help separate daughter cells during cytokinesis. This ring forms as the cell cycle moves forward, but the exact timing and coordination with other cell cycle events are still not fully understood. RhoA, a small GTPase in the Ras superfamily, controls contractile ring assembly and function. The inactive form of RhoA is bound to GDP. A guanine nucleotide exchange factor, or Rho-GEF, is located in the cortex region, where the next cell division...

determining the plane of cell division

How Cells Set the Division Plane

How cells set the division plane during cell division affects daughter cell size and, in many cases, cell fate. The plane of division is perpendicular to the plane of chromosome segregation. Different organisms use different cues to place it, based on their shape and needs. In animal cells, the cleavage furrow forms along the division plane from the cell cortex, the area just below the plasma membrane. The mitotic spindle is the main guide for this position. If the spindle axis changes, the...

the phragmoplast

How Plant Cells Build a New Wall

Plant cells build a new wall during cytokinesis with help from the phragmoplast. This plant structure forms from the central spindle and related cell parts as the cell finishes dividing. In animal cells, the comparable structure is the midbody, but in plants the phragmoplast expands outward to guide new wall formation. The mature phragmoplast has a donut-like shape. It has an outer leading region, a middle transition zone, and an inner lagging region. New microtubules are assembled...

distribution of cytoplasmic content

How Organelles Are Split During Cytokinesis

Cytokinesis splits a cell’s chromosomes and organelles into two daughter cells. Before cell division, organelles grow and divide. They cannot be made from scratch, so each daughter cell must inherit at least one copy to survive. Many details of this organelle sorting process are still not fully known. The cytoplasm contains organelles, salts, proteins, and water. Small organelles such as peroxisomes and lysosomes can be shared by simple diffusion. Because cells contain many copies of these...

anaphase promoting complex

APC Activates Chromosome Separation

The anaphase-promoting complex, or APC, helps control cell cycle progression by destroying specific proteins at the right time. These proteins are tagged with ubiquitin by ubiquitin ligases and then broken down by the proteasome. The SCF (Skp1/Cullin/F-box) complex and APC are two major ubiquitin ligases involved in this process. SCF stays active throughout the cell cycle, but APC turns on during the metaphase-to-anaphase transition. APC becomes active when Cdc20 or Cdh1 binds to it. These...

key terms and concepts

Learn how poleward and polar-ejection forces move chromosomes during mitosis, align sister chromatids at metaphase, and support separation in anaphase.

Chromosome Movement During Mitosis | JoVE K12

molecular biology

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Chromosome Movement During Mitosis

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