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 : 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.
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.
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.
|This is lower order of DNA organization||This is the higher order of DNA organization|
|This phase appears during interphase||This phase appears during metaphase|
|It is composed of nucleosomes||It is composed of chromatin fibres|
|These are thin, long uncoiled DNA fibers||Thick, compact, ribbon like structures|
|Present singly, unpaired DNA fibers||Exist in pairs|
|Condensed upto 50 times||Condensed upto 10,000 times|
|Allows various functions like DNA replication, gene expression and recombination||Do not show any metabolic activity|
|Under elctron microscope It appear as a bead and string structure||It appears as characteristic four arms structure|
|It has two conformation – euchromatin and heterochromatin||It is usually heterochromatic present in various shapes like metacentric, submetacentric, acrocentric and telocentric|
|The genetic material is packed inside the nucleus and helps in gene expression||This ensures proper arrangement of genetic material for equal separation among the cells.|
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.
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.