● `color{brown}"DNA"` `color{brown}"as an acidic substance"`:

`star` DNA as an `color{Violet}"acidic substance"` present in nucleus was first identified by `color{Violet}"Friedrich Meischer"` in 1869.

`star` He named it as `color{Violet}"Nuclein"`

`star` However, due to `color{Violet}"technical limitation"` in isolating such a long polymer intact, the `color{Violet}"elucidation of structure"` of DNA remained `color{Violet}"elusive"` for a very long period of time.




● `color{brown}"Double Helix"``color{brown}" Model of DNA"`:

`star` It was only in `color{Violet}"1953"` that
- `color{Violet}"James Watson"` and `color{Violet}"Francis Crick"`, based on the `color{Violet}"X-ray"` `color{Violet}"diffraction data"`
- produced by `color{Violet}"Maurice Wilkins"` and `color{Violet}"Rosalind Franklin"`,
- proposed a very simple but famous `color{Violet}"Double Helix model"` for the structure of DNA.

`star` One of the hallmarks of their proposition was `color{Violet}"base pairing"` between the two strands of polynucleotide chains.





● `color{brown}"Chargaff’s Rule"`:
`star` The `color{Violet}"base pairing proposition"` was also based on the observation of `color{Violet}"Erwin Chargaff"`.

`star` It was that for a `color{Violet}"double stranded DNA"`,
- the `color{Violet}"ratios"` between `color{Violet}"Adenine and Thymine"` and `color{Violet}"Guanine and Cytosine"`
- are `color{Violet}"constant"` and equals `color{Violet}"one"`.

● The `color{Violet}"base pairing"` confers a very `color{Violet}"unique property"` to the polynucleotide chains.

● They are said to be `color{Violet}"complementary"` to each other, and therefore if the `color{Violet}"sequence of bases"` in one strand is known then the sequence in other strand can be predicted.

● Also, if each strand from a DNA (let us call it as a `color{Violet}"parental DNA"`) acts as a `color{Violet}"template"` for synthesis of a new strand, the two double stranded DNA (let us call them as `color{Violet}"daughter DNA"`) thus, produced would be `color{Violet}"identical"` to the parental DNA molecule.

● Because of this, the `color{Violet}"genetic implications"` of the structure of DNA became very clear.

● It is made of `color{Violet}"two polynucleotide chains"`, where the backbone is constituted by `color{Violet}"sugar-phosphate"`, and the `color{Violet}"bases project"` inside.

● The two chains have `color{Violet}"anti-parallel polarity"`. It means, if one chain has the polarity `color{Violet}"5'-3'"`, the other has `color{Violet}"3'-5'"`.

● The bases in two strands are paired through `color{Violet}"hydrogen bond"` (H-bonds) forming `color{Violet}"base pairs"` (bp).

`star` `color{Violet}"Adenine"` forms `color{Violet}"two hydrogen bonds"` with `color{Violet}"Thymine"` from opposite strand and vice-versa.
`star` Similarly, `color{Violet}"Guanine"` is bonded with `color{Violet}"Cytosine"` with `color{Violet}"three H-bonds"`.
`star` As a result, always a `color{Violet}"purine"` comes `color{Violet}"opposite"` to a `color{Violet}"pyrimidine"`.
`star` This generates approximately `color{Violet}"uniform distance"` between the two strands of the helix.

● The two chains are coiled in a `color{Violet}"right-handed fashion"`.

`star` The `color{Violet}"pitch"` of the helix is `color{Violet}"3.4 nm"` (a nanometre is one billionth of a metre, that is 10-9 m)
`star` There are `color{Violet}"roughly 10 bp"` in each turn.
`star` Consequently, the `color{Violet}"distance"` `color{Violet}"between a bp"` in a helix is approximately equal to `color{Violet}"0.34 nm"`.

● The `color{Violet}"plane"` of one base pair `color{Violet}"stacks"` over the other in double helix.

`star` This, in addition to `color{Violet}"H-bonds"`, confers `color{Violet}"stability"` of the helical structure