Name: anaphase start and chromatid release
Uploaded: 2021-04-07T12:12:01.000Z
Duration: 2 min 17 s
Channel: JoVE

K12 Standards

Next Generation Science Standards

High School

Life Science

from-molecules-to-organisms-structures-and-processes

From Molecules to Organisms: Structures and Processes

HS-LS1-4

Separation of Sister Chromatids

Am Ende der Metaphase richten sich die bi-orientierten Chromosomen an der Metaphasenplatte aus. In dieser Phase halten die Kohäsin-Protein-Ringkomplexe die Schwesterchromatiden im Zentromerbereich zusammen und verhindern, dass sie auseinandergezogen werden.
Die Progression von Metaphase zu Anaphase wird durch die Cyclin-Cdk-induzierte Phosphorylierung eines Ubiquitin-Ligase-Enzyms mit mehreren Untereinheiten ausgelöst – des Anaphase-fördernden Komplexes, der auch als Zyklosom oder APC/C bekannt ist. Phosphoryliertes APC/C bindet an ein Protein, Cdc20, und bildet einen aktiven Komplex.
Der aktive APC/C-Komplex erkennt ein hemmendes Protein namens Securin, das an ein Proteaseenzym namens Separase gebunden ist. Vor dieser Erkennung hemmt Securin die Aktivität von Separase.
Der aktive APC/C-Komplex markiert Securin mit dem Protein Ubiquitin und zielt auf dessen proteasomalen Abbau ab. Durch die Zerstörung von Securin wird Separese freigesetzt.
Der Cyclin-Cdk-Komplex reguliert auch die ungebundene Separaseaktivität durch inhibitorische Phosphorylierung negativ.
Der aktive APC/C-Komplex bewirkt die Ubiquitinierung von Cyclinen und zielt auf deren proteasomalen Abbau ab. Durch die Zerstörung von Cyclin wird die enzymatische Aktivität von Cyclin-abhängigen Kinasen oder Cdks beseitigt.
Die Inaktivierung von Cdks ermöglicht es Phosphatase-Enzymen, Separase zu dephosphorylieren und verhindert die Cdk-vermittelte inhibitorische Rephosphorylierung von Separase. Die Dephosphorylierung ermöglicht es dem Separase-Enzym, den Cohesin-Ring-Komplex zu spalten, der die Schwesterchromatiden zusammenhält.
Die Kohäsinspaltung führt zu einem absoluten Verlust der Schwesterchromatiden-Kohäsion, was den Übergang von der Metaphase zur Anaphase markiert. Der Verlust der Kohäsion ermöglicht es den Schwesterchromatiden, sich zu trennen und sich zu entgegengesetzten Polen der Spindel zu bewegen.

<ol>
	<li>Claire Acquaviva und Jonathon Pines. 20016. "Der Anaphase-fördernde Komplex/Zyklosom: APC/C". Zeitschrift für Zellwissenschaft. 119: 2401-2404. [<a href="https://jcs.biologists.org/content/119/12/2401#sec-2">Quelltext</a>]</li>
	<li>Frank Uhlmann, Friedrich Lottspeich und Kim Nasmyth. 1999. "Die Schwester-Chromatiden-Trennung zu Beginn der Anaphase wird durch die Spaltung der Kohäsin-Untereinheit Scc1 gefördert." Nature 400 (6739). Nature Publishing Group: 37–42. [<a href="https://mcb.berkeley.edu/courses/mcb230/WEB/PAPERS/SCC1.pdf">Quelltext</a>]</li>
	<li>Farr, Katie A. und Orna Cohen-Fix. "Der Übergang von der Metaphase zur Anaphase: ein Fall von produktiver Zerstörung." Europäische Zeitschrift für Biochemie 263, Nr. 1 (1999): 14-19.</li>
	<li>Fang, Guowei, Hongtao Yu und W. Kirschner. "Kontrolle mitotischer Übergänge durch den anaphasenfördernden Komplex." Philosophische Transaktionen der Royal Society of London. Reihe B: Biological Sciences 354, Nr. 1389 (1999): 1583-1590.</li>
</ol>

trennung der schwesterchromatiden

Beim Übergang von der Prophase zur Metaphase verringert sich die Kohäsion entlang der chromosomalen Arme, was zur Auflösung der Schwesterchromatiden führt. Es verbleiben jedoch residuelle Kohäsinkontakte, die die Schwesterchromatiden zusammenhalten, bis der Übergang von der Metaphase zur Anaphase erfolgt. Diese residuale Verbindung verhindert eine vorzeitige Trennung der Schwesterchromatiden und blockiert damit das Risiko einer Aneuploidie in den Tochterzellen.

Mit dem Beginn der Anaphase wird Separase, ein proteolytisches Enzym, aktiviert. Die aktivierte Separase spaltet die Scc1-Untereinheit der residualen Kohäsinringe, was zu einem totalen Verlust der Kohäsion führt. Die Schwesterchromatiden trennen sich in Abwesenheit einer Kraft, die sie zusammenhält. Das Fehlen von Kohäsion erlaubt es den polwärts gerichteten Kräften, die entlang der Mikrotubuli ziehen, die getrennten Chromatiden zu den Spindelpolen zu ziehen.

In der frühen Mitose verhindert der Spindel-Assemblierungs-Checkpoint (SAC) die Ubiquitinierung von Proteinen durch den anaphasefördernden Komplex oder Cyclosom (APC/C), wie Securin und M-Phase-Cycline, deren Abbau für die Anaphase notwendig ist. Erst wenn jedes Chromosom korrekt auf dem mitotischen Spindel ausgerichtet ist, wird der SAC deaktiviert, um die Phosphorylierung und Aktivierung der regulatorischen Untereinheiten des APC/C – CDC20 und CDH1 – zu ermöglichen.

CDC20 wird durch Cdk1/Cyclin B phosphoryliert, um aktives APC/C-CDC20 zu bilden. Aktives APC/C-CDC20 katalysiert dann den Abbau von Securin, Cyclin A und Cyclin B, um den Übergang zur Anaphase zu fördern. Da die Cdk-Aktivität cyclinabhängig ist, führt der Abbau von Cyclin B zum Verlust der Cdk1-Aktivität. Der Verlust des Cdk1/Cyclin B-Komplexes deaktiviert APC/C-CDC20, aber aktiviert eine andere regulatorische Untereinheit, APC/C-CDH1, die das Ende der Metaphase-Anaphase-Transition signalisiert. Das aktive APC/C-CDH1 erleichtert den Austritt aus der Mitose und stabilisiert die anschließende G1-Phase, indem es die vorzeitige Anhäufung von mitotischen Cyclinen verhindert.

Der Wechsel in den Aktivitäten von APC/C-CDC20 und APC/C-CDH1 hat zwei bedeutende Konsequenzen. Erstens lösen diese regulatorischen Untereinheiten überlappende, aber unterschiedliche Substratspezifitäten aus und fördern dadurch einen geordneten Zellzyklusübergang. Zweitens werden CDC20 und CDH1 durch unterschiedliche Mechanismen reguliert. Wenn APC/C-CDC20 aktiv ist, erfährt CDH1 eine hemmende Phosphorylierung durch Cyclin B/Cdk1, was dessen Bindung an APC/C verhindert. Im Gegensatz dazu wird die Aktivität von CDC20 durch einen mitotischen Kontrollkomplex – einen multiproteinaktivierten Komplex (BUBR1, BUB3, CDC20 und MAD2) – gehemmt, der vom SAC aktiviert wird.

Trennung der Schwesterchromatiden

Beim Übergang von der Prophase zur Metaphase kommt es zu einer Verringerung der Kohäsion entlang der Chromosomenarme, was zur Auflösung der Schwester ...

Education

Lehre

JoVE Core

Molecular Biology

Molekularbiologie

Unit 2

Einheit 2

Cell Division

Zellteilung

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

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 onset of anaphase, separase, a protein-cutting enzyme, becomes active. It cleaves the Scc1 subunit of the remaining cohesin rings. Once that cohesion is lost, the sister chromatids separate without needing a force to pull them apart first. Then the poleward forces along the microtubules can draw the separated chromatids toward the spindle poles.

The spindle assembly checkpoint, or SAC, controls this timing in early mitosis. It blocks the anaphase-promoting complex or cyclosome, called APC/C, from ubiquitinating proteins such as securin and M-phase cyclins until each chromosome is correctly aligned on the mitotic spindle. After alignment, the SAC is turned off. This allows the APC/C regulatory subunits CDC20 and CDH1 to be phosphorylated and activated in sequence.

Phosphorylated CDC20, together with Cdk1/cyclin B, forms active APC/C-CDC20. This complex promotes anaphase by triggering the degradation of securin, cyclin A, and cyclin B. Because Cdk activity depends on cyclins, cyclin B breakdown lowers Cdk1 activity. That loss inactivates APC/C-CDC20 and activates APC/C-CDH1, which signals that the metaphase-to-anaphase transition is complete.

APC/C-CDH1 helps cells exit mitosis and stabilize the next G1 phase by stopping mitotic cyclins from building up too early. The switch between APC/C-CDC20 and APC/C-CDH1 has two important effects. It gives these regulators overlapping but different substrate targets, which helps keep the cell cycle in order. It also reflects different control systems. When APC/C-CDC20 is active, CDH1 is held back by inhibitory phosphorylation from cyclin B/Cdk1. In contrast, CDC20 is blocked by the mitotic checkpoint complex, which includes BUBR1, BUB3, CDC20, and MAD2 and is activated by the SAC.

Anaphase Start and Chromatid Release

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

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 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 the spindle checkpoint, separase, and APC/C control sister chromatid release at anaphase and help prevent chromosome errors in mitosis.

Anaphase Start and Chromatid Release | JoVE K12

molecular biology

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New York State Science Learning Standards

Inheritance and Variation of Traits

HS-LS1-4.

Characteristics of Life

Biology is a natural science that studies life and living organisms, including their structure, function, development, interactions, evolution, ...

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The Nucleus

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Globular and Fibrous Proteins

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Intrinsically Disordered Proteins

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Protein Complex Assembly

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Protein Complexes with Interchangeable Parts

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Construct an explanation based on evidence for how the structure of DNA determines the structure of proteins, which carry out the essential functions of life through systems of specialized cells. [Assessment Boundary: Assessment does not include identification of specific cell or tissue types, whole body systems, specific protein structures and functions, or the biochemistry of protein synthesis.]

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The Parasympathetic Nervous System

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Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms. [Clarification Statement: Emphasis is on functions at the organism system level such as nutrient uptake, water delivery, and organism movement in response to neural stimuli. An example of an interacting system could be an artery depending on the proper function of elastic tissue and smooth muscle to regulate and deliver the proper amount of blood within the circulatory system.] [Assessment Boundary: Assessment does not include interactions and functions at the molecular or chemical reaction level.]

HS-LS1-2

Anaphase Start and Chromatid Release

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