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Related Concept Videos

Cohesins02:20

Cohesins

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Cohesin protein complexes are a molecular glue that holds two sister chromatids together. They play an important role both in mitosis and meiosis. In mitosis, all cohesin complexes present on the chromosomes are removed before the start of the anaphase stage.
Cohesin complexes in Meiotic Division
Meiosis involves two distinct rounds of chromosomal segregation and cell divisions— Meiosis I followed by Meiosis II – producing four daughter cells. Meiosis I includes the separation of...
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Condensins02:15

Condensins

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Condensins are large protein complexes that use ATP to fuel the assembly of chromosomes during mitosis. They transform the tangled, shapeless mass of post-interphase DNA into individualized chromosomes by compacting, organizing, and segregating chromosomal DNA.
The plant and animal cells contain two types of condensin complexes—condensin I and condensin II. Both complexes have five subunits: two SMC (Structural Maintenance of Chromosomes) subunits, a kleisin subunit, and two HEAT-repeat...
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Nucleosome Remodeling02:54

Nucleosome Remodeling

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Nucleosomes are the basic units of chromatin compaction. Each nucleosome consists of the DNA bound tightly around a histone core, which makes the DNA inaccessible to DNA binding proteins such as DNA polymerase and RNA polymerase. Hence, the fundamental problem is to ensure access to DNA when appropriate, despite the compact and protective chromatin structure.
Nucleosome remodeling complex
Eukaryotic cells have specialized enzymes called ATP-dependent nucleosome remodeling enzymes. These enzymes...
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Forces Acting on Chromosomes02:11

Forces Acting on Chromosomes

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During mitosis, chromosome movements occur through the interplay of multiple piconewton level forces. In prometaphase, these forces help in chromosome assembly or congression at the equatorial plane, eventually leading to their alignment at the metaphase plate. The forces acting on the chromosomes are space and time-dependent; therefore, they vary with the position of the chromosomes as the cell progresses through mitosis. 
Microtubules and motor proteins exert two types of forces on...
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Attachment of Sister Chromatids02:57

Attachment of Sister Chromatids

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As cells progress into mitosis, the nuclear envelope breaks down, and the condensed chromosomes are exposed to the array of bipolar microtubules of the mitotic spindle. The kinetochore, a large, disc-shaped protein complex, is present at the centromere region of the sister chromatids and acts as a binding site for the microtubules.  Usually, the plus-end of a single microtubule is embedded within the kinetochore. However, some kinetochores first establish lateral contact with the side-wall...
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Separation of Sister Chromatids02:17

Separation of Sister Chromatids

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At the transition from prophase to metaphase, there is a reduction in cohesion along the chromosomal arms, resulting in the resolution of sister chromatids. However, residual cohesin connections remain to hold the sister chromatids together until the transition from metaphase to anaphase. The residual connection prevents any premature separation of sister chromatids, blocking the risks of aneuploidy within the daughter cells.
At the onset of anaphase, separase, a proteolytic enzyme, is...
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Updated: Jun 6, 2025

Author Spotlight: Investigating the Motion Dynamics of the Eukaryotic Replisome Components at the Single-Molecule Level
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SMC-mediated chromosome organization: Does loop extrusion explain it all?

Tatsuya Hirano1, Kazuhisa Kinoshita1

  • 1Chromosome Dynamics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.

Current Opinion in Cell Biology
|November 27, 2024
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Summary

Loop extrusion is a key model for chromosome organization by structural maintenance of chromosomes (SMC) proteins. This review explores alternative SMC mechanisms beyond loop extrusion, clarifying unresolved questions in chromosome biology.

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Area of Science:

  • Molecular Biology
  • Genetics
  • Cell Biology

Background:

  • Structural maintenance of chromosomes (SMC) protein complexes, including condensin and cohesin, are crucial for chromosome organization.
  • Loop extrusion is a prominent proposed mechanism for SMC-mediated chromosome organization.
  • The precise physiological relevance and operation of loop extrusion remain under investigation.

Purpose of the Study:

  • To review proposed non-loop extrusion mechanisms of chromosome organization.
  • To clarify unresolved questions regarding SMC protein complex functions.
  • To provide a comprehensive understanding of how SMC complexes operate in the cell.

Main Methods:

  • Literature review of proposed non-loop extrusion models.
  • Analysis of existing evidence for alternative SMC-mediated mechanisms.
  • Synthesis of current knowledge and identification of research gaps.

Main Results:

  • Several non-loop extrusion mechanisms for chromosome organization have been proposed in scientific literature.
  • The exact conditions and extent to which loop extrusion functions in vivo are not fully established.
  • Alternative or additional roles for SMC complexes beyond simple loop extrusion are plausible.

Conclusions:

  • Understanding the diverse functions of SMC complexes requires considering mechanisms beyond loop extrusion.
  • Further research is needed to elucidate the complete repertoire of SMC-mediated chromosome organization strategies.
  • Clarifying these mechanisms is essential for a comprehensive view of genome architecture and function.