Biomolecules - Nucleic Acids: DNA and RNA

Nucleic acids are long-chain polymers of nucleotides, hence they are also called polynucleotides. They are responsible for the storage and transfer of genetic information.

A nucleotide consists of three chemical components: a pentose sugar, a nitrogenous base, and a phosphate group (derived from $H_3PO_4$).

In $DNA$, the sugar component is $\beta\text{-D-2-deoxyribose}$ ($C_5H_{10}O_4$), whereas in $RNA$, it is $\beta\text{-D-ribose}$ ($C_5H_{10}O_5$).

Nitrogenous bases are categorized into Purines: Adenine ($A$) and Guanine ($G$); and Pyrimidines: Cytosine ($C$), Thymine ($T$, only in $DNA$), and Uracil ($U$, only in $RNA$).

A nucleoside is formed by the linkage of a nitrogenous base to the $1'$ position of the pentose sugar. A nucleotide is formed when the $5'\text{-OH}$ group of a nucleoside is esterified with phosphoric acid.

Nucleotides are joined by phosphodiester linkages between the $5'$ carbon of one sugar and the $3'$ carbon of the next sugar.

The Double Helix structure of $DNA$ (Watson and Crick model) consists of two polynucleotide chains held together by hydrogen bonds between complementary base pairs: $A=T$ (two hydrogen bonds) and $G \equiv C$ (three hydrogen bonds).

Chargaff's Rule states that in a double-stranded $DNA$, the ratio of Adenine to Thymine and Guanine to Cytosine is constant and equals one, i.e., $[A] + [G] = [T] + [C]$.