Class 12 – : “PACKAGING OF DNA HELIX”– Molecular Basis of Inheritance” CBSE & NEET Notes

 

The average distance between the two adjacent base pairs of 0.34 nm (0.34 × 10–9 m or 3.4 Å). Length of DNA for a human diploid cell is 6.6 × 109 bp × 0.34 × 10–9 m/bp = 2.2 m. The length is far greater than the dimension of a typical nucleus (approximately 10–6 m).

The number of base pairs in Escherichia coli is 4.6 × 106. The total length is 1.36 mm. The long sized DNA accommodated in small area (about 1 mm in E. coli) only through packing or compaction. DNA is acidic due topresence of large number of phosphate group. Compaction occurs by folding acid attachment of DNA with basic proteins, polyamine in prokaryotes and histone in eukaryotes.

DNA packaging in Prokaryotes :

DNA is found in cytoplasm in supercoiled state. The coils are maintained by non histone basic protein like polyamines. This compact structure of DNA is called nucleoid or genophore.

DNA packaging in Eukaryotes :

It is carried out with the help of lysine and ariginine rich basic proteins called histone. The unit of compaction is nucleosome. There are five types of histone proteins H1 H2A, H2B, H3 andH4. Four of them occur in pairs to produce histone octamer (2 copies of each – H2A, H2B, H3 and H4), called nubody or core of nucleosome.Their positively charged ends are directed outside. The negatively charged DNAis wrapped around the positively charged histone octamer to form a structure called nucleosome. A typicalnucleosome contains 200 bp of DNA Helix. DNA present between two adjacent nucleosome is called linkerDNA. It is attached to H1 histone protein. Length of linker DNA varies from species to species. Nucleosomechain gives a beads on string appearance under electron microscope. The nucleosomes furthers coils to formsolenoid. It has diameter of 30 nm as found in chromatin. The beads on string structure in chromatin is packaged to form chromatin fibres that are further coiled and condensed at metaphase stage of cell division to formchromosomes. The packaging at higher level requires additional set of proteins (acidic) that collectively arereferred to as non-histone chromosomal (NHC) proteins.

 

Non-Histone chromosomal proteins are of three types :

(i) Structural NHC protein

(ii) Functional NHC protein e.g., DNA polymerase, RNA polymerase

(iii) Regulatory NHC protein

In a typical nucleus, some region of chromatin are loosely packed (and stains light) and are referred to as enchromatin. The chromatin that is more densely packed and stains dark is called as heterochromatin, specifically euchromatin is said to be transcriptionally active and heterochromatin is transcriptionally inactive.