biology Biology is the scientific study of life and living organisms. It is a broad natural science that encompasses a wide range of fields and unifying principles that explain the structure, function, growth, History of life, origin, evolution, and ...

, the chromosome scaffold is the backbone that supports the structure of the

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

s. It is composed of a group of non-histone

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...

that are essential in the structure and maintenance of

eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

chromosomes throughout the

cell cycle The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell (biology), cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA (DNA re ...

. These scaffold proteins are responsible for the

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...

chromatin Chromatin is a complex of DNA and protein found in eukaryote, eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important r ...

during

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

Origin

In the late 1970s, Ulrich K. Laemmli and colleagues discovered a backbone structure in

chromosomes A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most importa ...

after they depleted the

histone In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei and in most Archaeal phyla. They act as spools around which DNA winds to create structural units called nucleosomes ...

proteins. This backbone was localized along the chromosome axis, and was termed the ‘chromosome scaffold’.

Proteins of the scaffold

eukaryotic organism The eukaryotes ( ) constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes. They constitute a major group of li ...

s, the

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

of each

cell Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...

is organized into separated

, which are composed of

, a mixture of DNA and many different groups of

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s. Among them, the

structural protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...

s (that are not

histones In biology, histones are highly Base (chemistry), basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei and in most Archaea, Archaeal Phylum, phyla. They act as spools around which DNA winds to create st ...

) bind the chromatin fiber around themselves forming a long, continuous axis or backbone that gives the chromosomes their shape. For this reason they are known as the ‘scaffold’ of chromosomes. Three protein groups have been identified as the main components of the scaffold: DNA topoisomerase IIα,

condensin Condensins are large protein complexes that play a central role in chromosome condensation and segregation during mitosis and meiosis (Figure 1). Their subunits were originally identified as major components of mitotic chromosomes assembled in ' ...

s, and the

KIF4A Kinesin family member 4A is a protein that in humans is encoded by the ''KIF4A'' gene. Function Kinesins, such as KIF4A, are microtubule-based motor proteins that generate directional movement along microtubules. They are involved in many cru ...

kinesin. When these proteins are removed, the chromosome shape does not appear and the chromatin fibers spread out.

Topoisomerase IIα

The

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

DNA topoisomerase IIα prominently appears along the chromosome axis as part of the scaffold. In mitosis, it is concentrated at the

centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fiber ...

s and the axis along the chromosome arms. It is thought that the protein has a role in untangling the DNA as the loops become more concentrated along the axis during the condensation of the chromosomes. The removal of this protein causes a dramatic loss of the chromosome structure in mitosis, and the cell cycle comes to a stop.

SMC family proteins

Condensin Condensins are large protein complexes that play a central role in chromosome condensation and segregation during mitosis and meiosis (Figure 1). Their subunits were originally identified as major components of mitotic chromosomes assembled in ' ...

complexes, formed from the union of

SMC2 Structural maintenance of chromosomes protein 2 (SMC-2), also known as chromosome-associated protein E (CAP-E), is a protein that in humans is encoded by the ''SMC2'' gene. SMC2 is part of the SMC protein family and is a core subunit of condens ...

and

SMC4 Structural maintenance of chromosomes protein 4 (SMC-4) also known as chromosome-associated polypeptide C (CAP-C) or XCAP-C homolog is a protein that in humans is encoded by the ''SMC4'' gene. SMC-4 is a core subunit of condensin I and II, large ...

(among other proteins), are responsible for the condensation of chromosomes. Condensin I regulates the timing of chromosome condensation and is essential for changing the chromatin organization at the beginning of mitosis, from

TADs Text Adventure Development System (TADS) is a prototype-based domain-specific programming language and set of standard libraries for creating interactive fiction (IF) games. History The original TADS 1 was released by High Energy Software ...

to an array of loops around the chromosome axis. Condensin II drives the compaction of the chromosome loops along the axis. In particular, SMC2 (present in condensin I and II) is detected in the interior of the chromosome as part of the scaffold. When SMC2 is inhibited, the structure of the mitotic chromosome suffers grave defects.

KIF4

, a chromo

kinesin A kinesin is a protein complex belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule (MT) filaments and are powered by the hydrolysis of adenosine triphosphate (ATP) (thus kinesins are ATPases, a typ ...

, is implicated in the shaping of chromosomes during

. It binds to condensin I through the CAP-G subunit. It is known that KIF4A regulates the behavior of condensin I, because in absence of KIF4A the chromosome axis does not become enriched with condensin I.

References

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