● `color{Violet}"Deoxyribonucleic acid"` (DNA) and `color{Violet}"ribonucleic acid"` (RNA) are the two types of nucleic acids found in living systems.

● `color{brown}"Role of DNA"`:
`star` DNA acts as the `color{Violet}"genetic material"` in most of the organisms.

● `color{brown}"Role of RNA"`:

`star` `color{Violet}"RNA"` though it also acts as a genetic material in some `color{Violet}"viruses"`, mostly functions as a `color{Violet}"messenger"`.

`star` RNA has additional roles as well. It functions as `color{Violet}"adapter"`, `color{Violet}"structural"`, and in some cases as a `color{Violet}"catalytic molecule"`.

● DNA is a `color{Violet}"long polymer"` of `color{Violet}"deoxyribonucleotides"`.

● The length of DNA is usually defined as `color{Violet}"number of nucleotides"` (or a pair of nucleotide referred to as `color{Violet}"base pairs"`) present in it.

● This also is the `color{Violet}"characteristic"` of an organism.

● For example,
`star` A bacteriophage known as `color{Violet}"φ 174"` has 5386 nucleotides

`star` `color{Violet}"Bacteriophage lambda"` has 48502 base pairs (bp)

`star` `color{Violet}"Escherichia coli"` has 4.6 × 106 bp

`star` Haploid content of `color{Violet}"human DNA"` is 3.3 × 109 bp.

● A nucleotide has three components –
`star` A `color{Violet}"nitrogenous base"`

`star` A `color{Violet}"pentose sugar"` (ribose in case of RNA, and deoxyribose for DNA)

`star` A `color{Violet}"phosphate group"`.



● There are two types of nitrogenous bases – `color{Violet}"Purines"` (Adenine and Guanine), and `color{Violet}"Pyrimidines"` (Cytosine, Uracil and Thymine).

● `color{Violet}"Cytosine"` is common for both `color{Violet}"DNA and RNA"` and `color{Violet}"Thymine"` is present in `color{Violet}"DNA"`.

● `color{Violet}"Uracil"` is present in `color{Violet}"RNA"` at the place of Thymine.

● A nitrogenous base is linked to the pentose sugar through a `color{Violet}"N-glycosidic linkage"` to form a `color{Violet}"nucleoside"`

● E.g.: `color{Violet}"adenosine"` or `color{brown}"deoxyadenosine"`, `color{Violet}"guanosine"` or `color{brown}"deoxyguanosine"`, `color{Violet}"cytidine"` or `color{brown}"deoxycytidine"` and `color{Violet}"uridine"` or `color{brown}"deoxythymidine"`.



.

● When a `color{Violet}"phosphate group"` is linked to `color{Violet}"5'-OH"` of a nucleoside through `color{Violet}"phosphoester linkage"`, a corresponding `color{Violet}"nucleotide"` (or deoxynucleotide depending upon the type of sugar present) is formed.

● Two nucleotides are linked through `color{Violet}"3'-5' phosphodiester linkage"` to form a `color{Violet}"dinucleotide"`.

● More nucleotides can be joined in such a manner to form a `color{Violet}"polynucleotide chain"`.

● A polymer thus formed has at one end a `color{Violet}"free phosphate"` moiety at `color{Violet}"5'-end"` of ribose sugar, which is referred to as `color{Violet}"5’-end"` of polynucleotide chain.

● Similarly, at the other end of the polymer the ribose has a `color{Violet}"free 3'-OH"` group which is referred to as `color{Violet}"3' -end"` of the polynucleotide chain.

● The `color{Violet}"backbone"` in a polynucleotide chain is formed due to `color{Violet}"sugar and phosphates"`.

● The `color{Violet}"nitrogenous bases"` linked to sugar moiety `color{Violet}"project"` from the backbone.

● In RNA, every nucleotide residue has an `color{Violet}"additional –OH"` group present at `color{Violet}"2' -position"` in the ribose.

● Also, in RNA the `color{Violet}"uracil"` is found at the place of thymine (`color{Violet}"5-methyl uracil"`, another chemical name for thymine).