● The proposition of a `color{Violet}"double helix structure"` for DNA and its `color{Violet}"simplicity"` in explaining the genetic implication became revolutionary.

● Very soon, `color{Violet}"Francis Crick"` proposed the `color{BROWN}"Central dogma"` `color{brown}"in molecular biology"`, which states that the genetic information flows from DNA-RNA-Protein.

● In some `color{Violet}"viruses"` the flow of information is in `color{Violet}"reverse direction"`, that is, from RNA to DNA.

● Taken the `color{Violet}"distance"` between two `color{Violet}"consecutive base pairs"` as `color{Violet}"0.34 nm"` (0.34×`10^(–9)` m),
- if the `color{Violet}"length of DNA"` double helix in a `color{Violet}"typical mammalian cell"` is calculated

- simply by `color{Violet}"multiplying"` the total number of bp with distance between two consecutive bp,

- that is, `color{Violet}(6.6 × 10^9 bp × 0.34 × 10^(-9)m//bp)`,

- it comes out to be approximately `color{Violet}"2.2 metres"`.

● A length that is `color{Violet}"far greater"` than the dimension of a typical `color{Violet}"nucleus"` (approximately `10^(–6)` m).

● In `color{Violet}"prokaryotes"`, such as, `color{Violet}"E. coli"`, though they do not have a defined nucleus, the `color{Violet}"DNA is not scattered"` throughout the cell.

● DNA (being `color{Violet}"negatively charged"`) is held with some proteins (that have `color{Violet}"positive charges"`) in a region termed as `color{Violet}"nucleoid"`.

● The DNA in nucleoid is organised in `color{Violet}"large loops"` held by `color{Violet}"proteins"`.

● In `color{Violet}"eukaryotes"`, this organisation is much more `color{Violet}"complex"`.

● There is a set of `color{Violet}"positively charged"`, `color{Violet}"basic proteins"` called `color{Violet}"histones"`.

● A protein `color{Violet}"acquires charge"` depending upon the abundance of `color{Violet}"amino acids"` residues with `color{Violet}"charged side chains"`.

● Histones are rich in the basic amino acid residues `color{Violet}"lysines and arginines"`.

● Both the amino acid residues carry `color{Violet}"positive charges"` in their side chains.

● Histones are organised to form a `color{Violet}"unit of eight"` molecules called as `color{Violet}"histone octamer"`.

● The negatively charged `color{Violet}"DNA"` is `color{Violet}"wrapped"` around the positively charged `color{Violet}"histone octamer"` to form a structure called `color{Violet}"nucleosome"`.

● A typical nucleosome contains 200 bp of DNA helix.

● Nucleosomes constitute the `color{Violet}"repeating unit"` of a structure in nucleus called `color{Violet}"chromatin"`, thread-like stained (`color{Violet}"coloured"`) bodies seen in nucleus.

● The nucleosomes in chromatin are seen as `color{Violet}"beads-on-string"` structure when viewed under `color{Violet}"electron microscope"` (EM) .



● The beads-on-string structure in chromatin is `color{Violet}"packaged"` to form `color{Violet}"chromatin fibers"` that are further coiled and condensed at `color{Violet}"metaphase stage"` of cell division to form `color{Violet}"chromosomes"`.

● The packaging of chromatin at `color{Violet}"higher level"` requires additional set of proteins that collectively are referred to as `color{Violet}"Non-histone Chromosomal"` (`color{Violet}"NHC"`) `color{Violet}"proteins"`.

● In a typical nucleus, some region of chromatin are `color{Violet}"loosely packed"` (and stains `color{Violet}"light"`) and are referred to as `color{Violet}"euchromatin"`.

● The chromatin that is more `color{Violet}"densely packed"` and stains `color{Violet}"dark"` are called as `color{Violet}"Heterochromatin"`.

● Euchromatin is said to be `color{Violet}"transcriptionally active chromatin"`, whereas heterochromatin is `color{Violet}"inactive"`.