Organization of chromosomes

Eukaryotic chromosomes are usually composed of nucleosomes, subunits of a DNA-protein complex termed chromatin. The way eukaryotes condense and arrange their chromatin helps regulate gene expression while allowing a large quantity of DNA to fit in a small space. The DNA that controls cellular functions is organized into chromosomes, which contain most genetic material. Prokaryotes usually are haploid, with only a single circular chromosome in the nucleoid. DNA is arranged into numerous linear chromosomes present in the nucleus of diploid eukaryotes.

Eukaryotic organization of chromosomes

The human chromosome with the most base pairs is 0.3 billion base pairs long. It would be a chromosome longer than 10 cm in length if it is in normal confirmation. A single, extraordinarily long molecule of DNA makes up each eukaryotic chromosome. Huge amounts of packing and folding are necessary to fit all of this DNA into the nucleus. The chromosomes are arranged in an extended thread like pattern called chromosomes

Chromatin structure

The stainable material in a cell nucleus is chromatin, consisting of DNA and proteins. The phrase is used to describe the structure and function of eukaryotic chromosomes. In all eukaryotes, the underlying structure of chromatin is nearly the same. 

Histones and nonhistones are two types of proteins found in chromatin linked to DNA. Both proteins are vital in determining the eukaryotic chromosome’s physical structure. In chromatin, histones are the most prevalent proteins. They’re tiny, basic proteins having a net positive charge that makes binding to negatively charged DNA easier. H1, H2A, H3 H2B,and H4 are the five primary histones found in eukaryotic nuclear DNA.

Even amongst distantly related species, the amino acid sequences of histones H2A, H3, H2B,  and H4 are substantially conserved evolutionarily. The evolutionary conservation of these sequences indicates that histones play the same basic role in arranging DNA in all eukaryotes’ chromosomes. 

Levels of organization of chromosomes

The packing of DNA is aided by histone and nonhistone proteins. Both proteins belong to the same type of protein, although they belong to different subclasses. In the packing of DNA, there are several phases.

The first level of organisation of eukaryotic chromosome

The formation of nucleosomes by the interaction of DNA with histone is the initial stage of packing. The nucleosome resembles beads on a DNA strand. It is referred to as “beads on a thread.” The nucleosome is a cylinder with a diameter of 11 nanometers. It has an octamer of histones (H2A, H2B, H3, and H4)2 wrapped up to 146 bp by 1.67 times of core left-handed DNA. On the same side of the nucleosome, linker histone (H1) is found in the exit and entry sites of DNA. The size of the linker DNA varies from 8 to 114 base pairs and is located between two core nucleosomes. Thus, each nucleosome has a total length of DNA of 200 bp, but this can vary between 154 and 260 bp.

Assembly of a histone octamer

H3 and H4 first form heterodimers, and then two heterodimers combine to make a tetramer or dimer of the heterodimer. This structure has a horseshoe shape and binds to DNA. H2A and H2B then form a heterodimer but not a tetramer. The creation of the histone octamer occurs when this dimer attaches to both sides of the tetramer or flanks it. The tail is the exposed N terminal extension on each histone core. This tail is easily obtained from the whole nucleosome and is also known as a large alteration site.

The second level of organization of chromosomes

The development of the solenoid structure is the second stage of packing. By coiling the nucleosome in the helical array-like structure, 40 folds of DNA are compacted, forming a 30 nm diameter fiber. This solenoid has a central cavity that linker DNA stabilizes. One helical turn of linker DNA contains six nucleosomes. A 30 nm fiber was created by coiling two rows of nucleosomes into a solenoid. Two-start helix conformation refers to the two rows of nucleosome coiling. Histone tails, particularly the N terminal of the tail, help sustain internucleosomal connections in 30 nm fibers, and this is made possible by the presence of H1 histone. It occurs in high-ionic-strength environments.

The third level of chromosome organisation

The formation of chromatin fibers with widths of 60-300 nm, known as “chromonema fibers,” completes the compact structure. With the formation of a metaphase chromosome with a diameter of 700 nm, the interphase chromosome compacts even more. The overall compaction of this mitotic chromosome is 10,000 fold, with euchromatin being generated by a 1000 fold compact structure. Heterochromatin is made up of a ten-thousand-fold compact structure.

The development of the metaphase chromosome is in progress. Nonhistone protein is the first protein to build a protein scaffold, which is then joined by additional proteins such as matrix protein, HMG, topoisomerase II, structural maintenance chromosome (SMC), and non-SMC protein. The inner nuclear membrane contains lamins. Lanines support the atomic structure. In prophase, the nuclear membrane disappears, allowing lamins (A1, B1, B2, C) to become free. After that, CDk-1 phosphorylates A1 and C, and cyclin B and A1 form a tight connection. CDk-1 and cyclin B both phosphorylate this compact association matrix protein. 

Conclusion

The condensation hierarchy of eukaryotic chromosomes is discussed in relation to their arrangement in mitosis and interphase. The most likely structure appears to be an alternating coiling and loop creation induced by histones and nonhistone proteins, respectively.