Question 38 of 122 | SWAYAM Cell Biology

• Genomes in a polymeric complex called chromatin. The fundamental unit of the chromatin polymer is the nucleosome, which repeats every 160 to 240 bp across the genome.
• Each nucleosome contains a nucleosome core, composed of an octameric complex of the core histone proteins, which forms a spool to wrap 145 to 147 bp of DNA. The nucleosome core is connected to the adjacent nucleosome core through a segment of linker DNA, which often associates with the linker histone protein (H1 or H5). The nucleosome core with ${\displaystyle \sim 165}$ bp of DNA together with the linker histone is called the chromatosome. The chromatosome and the additional linker DNA constitutes a nucleosome.
• Despite these technical definitions, the nucleosome core particle is often colloquially referred to as the nucleosome. The composition of the nucleosome had long since been realized, the 1997 ${\displaystyle 2.8\AA }$ crystal structure of the nucleosome core particle (NCP) solved by Luger et al. afforded the first atomic depiction of this fundamental genomic unit.
• The structure showed 146 bp of the human alpha-satellite sequence wrapped 1.65 times around an octameric scaffold of Xenopus laevis histone proteins in a left-handed super helix. A single base pair is centred on the nucleosome dyad. : The complexity of the NCP surface is furthered by the histone N-terminal tails that protrude from the nucleosome surface either outside (H4 and H2A) or between (H3 and H2B) the DNA gyres making statement 1 incorrect.

Alternatively

• The nucleosome core particle is important for the packing of DNA molecule in the chromosomes. Eukaryotic chromosomes generally there are two class of proteins they are histones and non-histones. The information stored in DNA is organized, replicated and read with the help of a those DNA binding proteins. The histones are structural proteins and the control or functional proteins are called as non-histone proteins.
• The nucleosome core particle which consists of 14 turns of B- Form of DNA around an octamer of histone proteins. This nucleosomal proteins are eight molecules of 4 types of small proteins. They are responsible for coiling of DNA into nucleosome.
• Those molecule represented as two molecules of each H2A, H2B, H3 and H4 proteins. This histones are the responsible for the basic level of chromosome organization. Nucleosome are the fundamental packing unit of chromatin and the chromatin are beads on a string appearance.
• Under scanning of electron microscope most of the chromatin is in the form of fiber with diameter of 30 nm when during the interphase broke. The string is DNA and each bead which consist of nucleosome core particle and a smaller spacer or linker DNA.
  • Core particle: which is consist of octamer of histones having 2 copies of each H2A, H2B, H3, H4. it՚s about 11 nm in diameter and 6 nm in height. A strand of DNA having 146 bp tightly wrapped around this core forming two circles.
  • Spacer DNA: small segment of DNA having around 40 bp H1 Histone is associated.
• Chromatin isolated directly from interphase phase and nucleus appearance as 30 nm thick
• This fig: shows the lengths of chromatin that has been unpacked after isolation to see the nucleosome.

• A nucleosome which contain eight histone molecule. When core particle released from chromatin by the digestion of liker DNA with a nuclease enzyme that break DNA.
• After the dissociation of the nucleosome into protein core and DNA, the length were determined. The length is 146 bp to wrap 1.65 times around the histone core.