Name: how the mitotic spindle separates chromosomes
Uploaded: 2021-04-07T12:12:01.000Z
Duration: 2 min 27 s
Channel: JoVE

Il fuso mitotico separa i cromatidi fratelli e li sposta sui lati opposti della cellula durante l'anafase della mitosi.
Le strutture fondamentali del fuso mitotico sono cilindri cavi chiamati microtubuli. Due serie di microtubuli sono disposte alle estremità opposte, o poli, del fuso mitotico.
Ogni microtubulo ha un'estremità negativa e un'estremità positiva. Le estremità negative dei microtubuli si incontrano al centro dei poli del fuso. Le estremità positive si estendono verso l'esterno dai poli.
Esistono diversi tipi di microtubuli con posizioni e ruoli distinti nel fuso mitotico.
I microtubuli cinetocore legano i cromosomi al polo del fuso attaccandosi alle loro estremità positive ai cinetocori. I cinetocori sono grandi complessi proteici assemblati al centromero del cromatide, una sequenza di DNA specializzata che collega i cromatidi fratelli.
La disposizione dei microtubuli interpolari ricorda un paio di mani giunte. L'estremità positiva di un microtubulo interpolare si sovrappone all'estremità positiva di un altro microtubulo interpolare che si estende dal polo opposto. Le proteine motorie si associano ai microtubuli interpolari per dirigere l'assemblaggio del fuso.
I microtubuli astrali ancorano il fuso nella cellula. Questi microtubuli assomigliano collettivamente a uno starburst, con ogni estremità positiva che si proietta verso l'esterno dal polo del fuso alla corteccia cellulare.
Nella maggior parte delle cellule animali, i microtubuli sono organizzati attorno a un organello chiamato centrosoma. Un centrosoma è presente in ogni polo del fuso.
Un centrosoma è costituito da due centrioli circondati da una massa informe di proteine chiamata matrice pericentriolare. Il centrosoma produce, organizza e ancora i microtubuli nella cellula.
Due famiglie di proteine motrici sono parte integrante della costruzione e del funzionamento del fuso mitotico: le proteine correlate alla chinesina e la dineina. Tipicamente, le proteine correlate alla chinesina si muovono verso le estremità positive dei microtubuli e la dineina si sposta verso le estremità negative.
La polarità intrinseca del fuso mitotico e dei suoi microtubuli facilita la segregazione mitotica dei cromosomi, preparando la cellula alla divisione.

<ol>
	<li>Heald, Rebecca e Alexey Khodjakov. 2015. "Trent'anni di ricerca e cattura: la complessa semplicità dell'assemblaggio del fuso mitotico". Il giornale di biologia cellulare 211 (6): 1103–11. <a href="https://doi.org/10.1083/jcb.201510015">https://doi.org/10.1083/jcb.201510015</a>.</li>
	<li>Petry, Sabine. 2016. "Meccanismi di assemblaggio del fuso mitotico". Rassegna annuale di biochimica 85 (1): 659–83. <a href="https://doi.org/10.1146/annurev-biochem-060815-014528">https://doi.org/10.1146/annurev-biochem-060815-014528</a>.</li>
</ol>

il fuso mitotico

The Mitotic Spindle

Il fuso mitotico — o apparato del fuso— è una struttura citoscheletrica eucariotica costituita da lunghe fibre proteiche chiamate microtubuli. Formato durante la divisione cellulare, il fuso separa i cromatidi fratelli e li sposta alle estremità opposte di una cellula parentale, dove i cromosomi ora individuali vengono distribuiti ai due nuclei delle cellule figlie.

La configurazione bipolare del fuso mitotico facilita la segregazione cromosomica, preparando la cellula alla divisione. Un meccanismo che garantisce la formazione del fuso mitotico bipolare si basa sui centrosomi.

Alcune cellule, come gli ovociti dei vertebrati e le cellule vegetali superiori, mancano di centrosomi; tuttavia, la maggior parte delle cellule animali ha due centrosomi quando entrano nella mitosi. Ciascun centrosoma è associato a una disposizione circolare di microtubuli sulle estremità opposte — o poli— del fuso mitotico. In altre parole, i centrosomi nucleano i microtubuli.

Le proteine motore — in particolare le chinesine e la dineina— operano tipicamente in corrispondenza o in prossimità delle estremità dei microtubuli e facilitano la formazione del fuso mitotico bipolare e la separazione dei cromatidi fratelli.

Ad esempio, i motori kinesin-5 nella zona centrale del fuso si attaccano e si allontanano da due microtubuli che si estendono dai poli opposti del fuso; questo processo promuove la bipolarità e l'allungamento del fuso allontanando i poli del fuso l'uno dall'altro.

Si ritiene che l'attività della chinesina-5 sia controbilanciata dalla chinesina-14. I motori Kinesin-14 tirano i microtubuli che si estendono dai poli opposti, unendo efficacemente i poli. L'attività coordinata di questi motori consente il corretto montaggio del fuso.

Kinesin-4 e kinesin-10 sono cromochinesine, kinesine che possono associarsi ai cromosomi mitotici. Kinesin-4 e kinesin-10 si associano ai bracci cromosomici, allontanando i cromosomi e il polo del fuso.

Le dineine organizzano i microtubuli in varie parti della cellula. Ad esempio, collegano i microtubuli astrali al citoscheletro di actina, allontanando i poli del fuso l'uno dall'altro.

La struttura, l'organizzazione e i componenti del fuso mitotico consentono ai cromatidi fratelli di separarsi, preparando la cellula per la corretta divisione.

Il fuso mitotico

Il fuso mitotico, o apparato del fuso, è una struttura citoscheletrica eucariotica costituita da lunghe fibre proteiche chiamate microtubuli. Formato...

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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 pole, which means centrosomes nucleate microtubules. Some cells, including vertebrate oocytes and higher plant cells, do not have centrosomes.

Motor proteins help build and position the spindle. Kinesins and dynein work at or near microtubule ends and support bipolar spindle formation as well as sister chromatid separation. Kinesin-5 motors at the spindle midzone slide two microtubules apart, which pushes the poles away from each other and helps the spindle elongate.

Kinesin-14 acts in the opposite way. It pulls on microtubules that extend from opposite poles and brings the poles closer together. This balance between kinesin-5 and kinesin-14 helps the spindle assemble correctly. Kinesin-4 and kinesin-10 are chromokinesins, which means they can bind to mitotic chromosomes. They attach to chromosome arms and help push the chromosomes and spindle poles apart.

Dyneins also take part in spindle organization. They link astral microtubules to the actin cytoskeleton and help move the spindle poles away from each other. Together, the spindle structure, its poles, and its motor proteins allow sister chromatids to separate and prepare the cell for proper division.

How the Mitotic Spindle Separates Chromosomes

how-the-mitotic-spindle-separates-chromosomes

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

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 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 the mitotic spindle uses microtubules, centrosomes, kinesins, and dynein to separate sister chromatids during cell division.

How the Mitotic Spindle Separates Chromosomes | JoVE K12

molecular biology

Le video animazioni di JoVE Core Molecular Biology, organizzate in video lezioni, illustrano i processi molecolari cellulari in maniera chiara e concisa. Inoltre, nei video di "scienziati in azione", i concetti scientifici vengono applicati e dimostrati da ricercatori attraverso esperimenti in laboratorio.

How the Mitotic Spindle Separates Chromosomes

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