Name: how condensins shape mitotic chromosomes
Uploaded: 2021-04-07T12:12:01.000Z
Duration: 2 min 15 s
Channel: JoVE

جزء أساسي من الحياة هو قدرة الخلية على تكرار جينومها وانقسامها ؛ تحدث هذه العمليات على مرحلتين رئيسيتين من دورة الخلية.
أولا ، خلال المرحلة S ، يتم تكرار الحمض النووي للكروموسومات. خلال المرحلة M ، يتم فصل الكروموسومات المكررة وتوزيعها على خليتين ابنتيتين متطابقتين وراثيا.
بعد المرحلة S ، يكون الحمض النووي للكروماتيدات الشقيقة طويلا جدا ومتشابكا. قد يؤدي فصل الكروماتيدات الشقيقة في هذه الحالة إلى انقطاع الكروموسوم والفصل غير السليم وحتى موت الخلايا.
لتجنب هذه الأزمة المحتملة ، تكرس الخلية قدرا كبيرا من الطاقة أثناء الانقسام المبكر لإعادة تنظيم الكروماتيدات الشقيقة تدريجيا إلى هياكل أقصر تنفصل بسهولة أكبر.
تعتمد إعادة التنظيم هذه على المكثف ، وهو مركب بروتيني يشارك في تكثيف الكروموسومات.
يتكون المكثف من خمس وحدات فرعية. في حقيقيات النوى ، ترتبط الوحدتان الفرعيتان الرئيسيتان ، SMC2 و SMC4 ، في مجالات رأس ATPase الخاصة بهما بواسطة الوحدات الفرعية الثلاث الأخرى: واحدة كلايسين ووحدتان فرعيتان لتكرار الحرارة.
تستخدم المكثفات الطاقة الناتجة عن التحلل المائي ATP لتعزيز عمليتين رئيسيتين تسهلان فصل الكروماتيد الشقيق: تكثيف الكروموسوم ودقة الكروماتيد الشقيقة.
أثناء تكثيف الكروموسومات ، تصبح الكروماتيدات معبأة بإحكام. أثناء حل الكروماتيد الشقيق ، تصبح الكروماتيدات الشقيقة المكثفة هياكل مميزة عندما تكون الحمض النووي الشقيقة غير مرتبطة ، أو منفصلة ، عن طريق الإزالة الجزئية للتماسك.
بحلول الوقت الذي تصل فيه الخلية إلى الطور الطوري ، تكون الكروماتيدات الشقيقة متصلة بشكل فضفاض فقط على طول الذراعين ولكنها لا تزال مرتبطة بإحكام عند السنتروميرات. 
يمكن أن يغير المكثف لف الحمض النووي ويحفز تكثيف الكروموسوم ودقة الكروماتيد الشقيق عن طريق تكوين هياكل حلقية تحيط بحلقات من الحمض النووي.

<ol>
	<li>باتي وبول ودانيال دبليو جيرليش. 2019. "ميكانيكا الكروموسوم الانقسامي: كيف تفصل الخلايا جينومها". الاتجاهات في بيولوجيا الخلية 29 (9): 717-26. https://doi.org/10.1016/j.tcb.2019.05.007. </li>
	<li>سكيبينز ، روبرت ف. 2019. "المكثفات والتماسك - أحد هذه الأشياء ليس مثل الآخر!" مجلة علوم الخلية 132 (3). https://doi.org/10.1242/jcs.220491. </li>
</ol>

المكثفات

Condensins

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المكثفات عبارة عن مجمعات بروتينية كبيرة تستخدم أدينوسين ثلاثيُّ فوسفات (ATP) لتغذية تجميع الكروموسومات أثناء الانقسام. إنها تحول الكتلة المتشابكة عديمة الشكل من الحمض النووي بعد الطور البيني إلى كروموسومات فردية عن طريق ضغط الحمض النووي الصبغي وتنظيمه وفصله.

تحتوي الخلايا النباتية والحيوانية على نوعين من مجمعات المكثفات - المكثف الأول والمكثف الثاني. يحتوي كلا المجمعين على خمس وحدات فرعية: وحدتان فرعيتان SMC (الصيانة الهيكلية للكروموسومات)، ووحدة فرعية كليسين، ووحدتان فرعيتان متكررتان HEAT.

الوحدات الفرعية الأساسية لكل من المكثف I والمكثف II هي SMC2 وSMC4. تغير بروتينات SMC ترتيب الحمض النووي بطريقة تعتمد على ATP. الوحدات الفرعية الثلاثة الأخرى – الوحدات غير SMC أو الوحدات الفرعية المساعدة – تختلف بين المجمعين.

أظهرت الدراسات التي يتم فيها استنفاد مكثف الفقاريات أدوارًا مميزة للمكثفات I و II في تكوين الكروموسوم الانقسامي. تؤدي إزالة Condensin II إلى كروموسومات أطول وأكثر مرونة، وتشابك كروموسوم، وجسر كروماتين ضخم أثناء الطور الانفصالي، وتقصير جذري في الطور التمهيدي. في المقابل، تؤدي إزالة المكثف I إلى كروموسومات أقصر وأوسع وتعطيل الطور الانفصالي الذي يكون أقل حدة ولكنه لا يزال يؤدي إلى فشل التحريك الخلوي.

أحد التفسيرات الشائعة لكيفية ضغط المكثفات على الكروموسومات هو نموذج البثق الحلقي. يفترض هذا النموذج أن جزيء المكثف يمكن أن يرتبط بموقعين قريبين من الحمض النووي ويحركهما في اتجاهين متعاكسين، مما يؤدي إلى إنشاء حلقة DNA متنامية. قد تتفاعل المكثفات أيضًا مع بعضها البعض لتكوين وحدات متعددة تالربط أجزاء بعيدة من الكروماتين.

تم الربط طفرات الكوندينسين بعدة أنواع من السرطان. على سبيل المثال، أصيبت الفئران التي لديها طفرة مغلوطة في الجين الخاص بوحدة فرعية من المكثفات II بسرطان الغدد الليمفاوية في الخلايا التائية. في حين أن الآليات التي من خلالها تؤثر المكثفات على بنية الكروموسومات لا تزال قيد التوضيح، فإن مجمعات البروتين هذه تعد جزءًا لا يتجزأ من دورة الخلية وبقاء الخلية.

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المكثفات عبارة عن مجمعات بروتينية كبيرة تستخدم ATP لتغذية تجميع الكروموسومات أثناء الانقسام. أنها تحول متشابكة, كتلة عديمة الشكل من ...

Education

JoVE Core

Molecular Biology

البيولوجيا الجزيئية

Unit 2

الوحدة الثانية

Cell Division

انقسام الخلية

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

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 subunit, and two are HEAT-repeat subunits. The core SMC subunits in both complexes are SMC2 and SMC4, which change DNA arrangement in an ATP-dependent way. The other three subunits are non-SMC, or auxiliary, subunits, and they differ between condensin I and condensin II.

Studies in vertebrate cells show that condensin I and condensin II have distinct roles in mitotic chromosome formation. When condensin II is removed, chromosomes become longer and more flexible. They can also become tangled, form bulky chromatin bridges during anaphase, and move into a much shorter prophase. When condensin I is removed, chromosomes become shorter and wider. The anaphase defect is less severe, but cytokinesis still fails.

One common explanation for chromosome compaction is the loop extrusion model. In this model, a condensin molecule binds two nearby DNA sites and slides them in opposite directions. This creates a growing DNA loop. Condensins may also work together as multimers that connect distant parts of chromatin. Condensin mutations have been linked to several cancers, including T cell lymphomas in mice with a missense mutation in a condensin II subunit gene. Researchers are still working out exactly how condensins control chromosome structure, but these protein complexes are important for the cell cycle and cell survival.

How Condensins Shape Mitotic 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

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 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 condensins compact and organize DNA during mitosis, how condensin I and II differ, and why these protein complexes matter for cell division and cancer.

How Condensins Shape Mitotic Chromosomes | JoVE K12

molecular biology

JoVE Core Molecular Biology showcases the molecular processes of the cell through clear and concise animated video lessons. Additionally, in scientist-in-action videos, these concepts' application is demonstrated through real experiments in modern laboratories.

How Condensins Shape Mitotic Chromosomes

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