Answer in Brief
Short Answer Question:
Write a short note on DNA packaging in the eukaryotic cell.
- The organization of DNA is much more complex in eukaryotes.
- Histone proteins are rich in lysine and arginine residues which are basic in nature and are positively charged.
- These histones organize themselves to make a unit of 8 molecules known as histone octamer.
- The negatively charged helical DNA is wrapped around the positively charged histone octamer, forming a structure known as a nucleosome.
- The nucleosome core is made up of two molecules of each of four types of histone proteins viz. H2A, H2B, H3 and H4. H1 protein binds the DNA thread where it enters (arrives) and leaves the nucleosome.
- One nucleosome approximately contains 200 base pair long DNA helix wound around it.
- About 146 base pair long segment of DNA remains present in each nucleosome.
- Nucleosomes are the repeating units of chromatin, which are threadlike, stained (coloured) bodies present in nucleus. These look like ‘beads-on-string’, when observed under an electron microscope.
- DNA helix of 200 bp wraps around the histone octamer by 1¾ turns.
- Six such nucleosomes get coiled and then form solenoid that looks like coiled telephone wire.
- The chromatin is packed to form a solenoid structure of 30 nm diameter (300Å) and further supercoiling tends to form a looped structure called chromatin fiber, which further coils and condenses at the metaphase stage to form the chromosomes.
- The packaging of chromatin at higher levels, needs an additional set of proteins that are called Non-Histone Chromosomal proteins (NHC).
Concept: DNA Packaging
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