Is Chromosome A Chromatin: 5 Important Facts You Should Know

Chromatin and chromosome are related to each other. They are different levels of DNA arrangements.

All chromosomes have chromatin. Chromatin is a nucleoprotein complex that plays important role in the transfer of genetic material that is folded into compact chromosome. This article will provide 5 important insights regarding chromosomes and chromatin.

Is chromosome same as chromatin

Chromatin is the simplest form of DNA organization that undergo various structural modifications and form the compact chromosomes.

Basically, chromosomes and chromatin are the same things, both are the type of DNA organization. The only difference is chromatin is less condensed, extended DNA while chromosomes are condensed and compact DNA. Each unit of chromatin is called a nucleosome which contains the protein component histone and DNA. 

Under the electron microscope, nucleoprotein complex chromatin appears as beads on a string. During cell division, interphasic less condensed DNA is compacted into cylindrical, parallel metaphase chromosomes.

How does chromatin become a chromosome

There are series of events that takes place across the cell cycle which leads to the compaction of DNA and its associated proteins to form higher order of DNA packaging.

The important events that occurs during compaction starting with 11 nm nucleosome containing DNA wrapped around histone proteins to 30 nm solenoid and zigzag model and eventually 300 nm chromosome scaffold that are bounded by non-histone proteins which leads to metaphase chromosome.

The three major DNA packaging steps are as follows

• Nucleosome
• Solenoid and zigzag model
• Metaphase chromosome

Nucleosome : unit of chromatin

During the interphase and G0 phase of a cell, DNA remains as a nucleoprotein complex called chromatin. As the cell cycle proceeds to the M phase the chromatin gradually starts condensing and becomes the chromosomes. The condensation of chromatin requires a highly organized packaging system that folds and packs enormously long linear DNA along with specific proteins into a more compact structure. These characteristic proteins are called histone proteins. Nucleosomes form the basic level of chromatin organization.

Histones are highly basic proteins, rich in lysine and arginine residues, and form the structural support for the chromosome. H2A, H2B, H3, and H4 form the core histones while H1/H5 is present as the linker histones. Usually, histones exist as dimers. The nucleosome core contains dimmers of two H2A, H2B, and tetramer of H3 and H4. DNA is wrapped around these histone proteins. Together with linker histones, nucleosomes are called chromatosomes.

Interactions that occur between histone proteins and DNA

• Hydrogen bonds and salt bridges between the side chains of the positively charged amino acids and negatively charged phosphate groups of DNA backbone
• Hydrogen bonds present between DNA and the amide groups of amino acids
• Nonpolar interactions between histone proteins and deoxyribose sugar derivatives

Solenoid and zigzag model

After the compaction of DNA around the histones, the second level of chromatin organization is the compaction of 11 nm fiber into 30 nm diameter fibre. Two models are formed at this stage – the solenoid and zigzag model.

Solenoid model constitutes the 30 nm structure of chromatin fibre. In this structure, nucleosomes are further packed forming a helix. About six nucleosomes together form a single solenoid ring which are linked by 6 H1 linker histones. The diameter of solenoid ring is 300 A. Finally the solenoid rings are grouped together and form the scaffold. In solenoid model, adjacent nucleosomes can interact with each other.

In zigzag model, two strands comprising stacked nucleosome are folded in left- handed helix which implies interactions between alternate nucleosomes.

Metaphase chromosome

Further compaction of 30 nm requires scaffold proteins – the condensin proteins that form chromosome scaffold structure. The chromosome scaffold contains mainly non-histone proteins including condensing, type II topoisomerase, and kinesin.

Condensin

In eukaryotic cell, there are two types of condensins referred as condensin I and condensin II. Condensin proteins are also called structural maintenance of chromosome (SMC). These proteins belong to the family of chromosomal ATPase.

These proteins comprise of two domains- one nucleotide binding site (Walker- A domain) and another is catalytic domain (Walker-B domain) located in N- terminal and C- terminal respectively. These domains are connected by two -coiled domains separated by a hinge. Once the N- and C- terminal come together, it forms the ATP binding domain. In eukaryotes, SMC proteins contain cohesins and condensins

How are chromosomes and chromatin related

Chromosomes are composed of chromatin comprising of proteins and DNA.

Chromosomes and chromatin are various stages of DNA organization. Since chromosomes are metabolically inactive, chromatin helps in different gene regulation and expression activities. In chromatin, there are two distinct regions – euchromatin and heterochromatin.

Euchromatin regions are lightly stained less condensed part of chromatin that performs the metabolic activities. This region is transcriptionally active and contains protein-coding regions. Contrastingly, heterochromatin is the darkly stained region that remains inactive, and transcriptionally silent. Euchromatin is characterized by histone acetylation and methylation. heterochromatin can be of two types – constitutive and facultative.

Constitutive heterochromatin – remains condensed in G0 stage and contain highly repetitive sequences. It is found in pericentromeric and telomeric regions of the chromosomes. It is permanently silenced and transcriptionally inactive.

Facultative heterochromatin- These regions of euchromatin are converted in heterochromatin. These are functional at certain point of development. Example – Barr body.

Difference between chromatin and chromosome

There are several differences between chromatin and chromosome. The points are listed below.

Are chromatin part of chromosomes?

Chromosome that is transcriptionally active and plays important role in gene expression and recombination.

Ye,s chromatin is a tiny part of a chromosome. Chromatin is a DNA-protein complex that is present as units called nucleosomes. During metaphase, chromatin is folded into a compact structure and becomes a chromosome.  Thus, during cell division chromosomes are the prominent forms of chromatins.

Conclusion

Chromatin and chromosomes are different types of DNA arrangement. All chromosomes contain chromatin. Chromatin is the basic level of compaction of DNA. It contains DNA and associated histone proteins that provide structural support to the chromosomes. Along with these histone proteins there are other non- histone proteins, the scaffold proteins that helps in 300 nm compaction and leading to metaphase chromosome structure.

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Paurabi Das

I am a doctoral student of CSIR- CIMAP, Lucknow. I am devoted to the field of plant metabolomics and environmental science. I have completed my post-graduation from the University of Calcutta with expertise in Molecular Plant Biology and Nanotechnology. I am an ardent reader and incessantly developing concepts in every niche of biological sciences. I have published research articles in peer-reviewed journals of Elsevier and Springer. Apart from academic interests, I am also passionate about creative things such as photography and learning new languages.
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