With a distance of 0-34 nm between two base pairs, the 6-4 x 109 bp long DNA of humans has a length of 2-2 metres while 4-6 x 10° bp long DNA of Escherichia coli would reach a length of 1.36 mm.
Context
- DNA Packaging In Prokaryotes
- DNA Packaging In Eukaryotes
They are packed in a space which is one thousand to one hundred thousand times smaller. Packing is different in prokaryotes and eukaryotes.
1. DNA Packaging In Prokaryotes
In prokaryotes like Escherichia coli, a well developed nucleus is absent. However, DNA does not appear in unfolded scattered form. It is folded with the help of DNA to form supercoiled complex of numerous loops.
The complex is called nucleoid or prochromosome. In nucleoid the coiling is stabilised with the help of non- histone polyamines and some positively charged ions that get attached to negatively charged DNA (due to phosphate groups).
2. DNA Packaging In Eukaryotes
In Eukaryotes, DNA is stabilised with the help of set of positively charged basic protein called Histone. Histone are rich in basic amino acids, lysine and arginine. Therefore, they carry positive charges on their side chains. Histone and DNA are organized to form nucleosomes.
Nucleosome. Nucleosome is submicroscopic sub-unit of chromatin which is formed by wrapping of DNA over a core of histone proteins. The term was coined by Oudet et al (1975). Nucleosome is oblate structure with a length of 10 nm (100 Å) and a thickness of 5-5.7 nm (50- 57 A). Its core is called nu-body. The latter is formed of 4 pairs of histone molecules-H,A, HB, H, and H4. The hydrophobic regions of all the eight histone molecules are towards the centre of octamer or nu-body. Their charged regions are towards the surface. The charge is positive. They attract the negatively charged phosphate containing backbones of DNA double strand. DNA of about 200 bp makes 1-75 turns over the octamer to form a nucleosome.
Two adjacent nucleosomes are connected by a short segment of unbound DNA called linker DNA. A fifth type of histone called H is attached over the linker DNA. Nucleosome-DNA association appears as a beaded string or beads on string under the electron microscope. It is the active part or unfolded part of chromatin which is 10 nm in thickness. The normal thickness of chromatin fibre is 30 nm.
It is formed by solenoid coiling of nucleosome string in which 6-7 nucleosomes occur in each turn of the coil. It is called euchromatin. Euchromatin is loosely packed. It gets lightly stained. In some regions chromatin is densely packed, thicker and darkly stained. The same is called heterochromatin. Euchromatin is transcriptionally active while heterochromatin is inactive. Chromatin undergoes further packaging during formation of chromosomes. A chromatid has a thickness of about 700 nm. It is achieved by condensation of solenoid coils over non-histone chromosomal proteins. Previously it was believed to be achieved by super solenoid coiling first from 30-200 nm and then 200-700 nm.
Two adjacent nucleosomes are connected by a short segment of unbound DNA called linker DNA. A fifth type of histone called H is attached over the linker DNA. Nucleosome-DNA association appears as a beaded string or beads on string under the electron microscope. It is the active part or unfolded part of chromatin which is 10 nm in thickness. The normal thickness of chromatin fibre is 30 nm.
It is formed by solenoid coiling of nucleosome string in which 6-7 nucleosomes occur in each turn of the coil. It is called euchromatin. Euchromatin is loosely packed. It gets lightly stained. In some regions chromatin is densely packed, thicker and darkly stained. The same is called heterochromatin. Euchromatin is transcriptionally active while heterochromatin is inactive. Chromatin undergoes further packaging during formation of chromosomes. A chromatid has a thickness of about 700 nm. It is achieved by condensation of solenoid coils over non-histone chromosomal proteins. Previously it was believed to be achieved by super solenoid coiling first from 30-200 nm and then 200-700 nm.
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