Unity and diversity - Nucleic acids

Nucleic acids are polymers called polynucleotides, composed of monomers known as nucleotides. Each nucleotide consists of a pentose sugar, a phosphate group, and a nitrogenous base.

The pentose sugar is either ribose ($C_{5}H_{10}O_{5}$) in RNA or deoxyribose ($C_{5}H_{10}O_{4}$) in DNA. The carbon atoms of the sugar are numbered $1'$ to $5'$.

Nucleotides are linked by covalent phosphodiester bonds formed via condensation reactions between the $5'$ phosphate group of one nucleotide and the $3'$ hydroxyl ($-OH$) group of another.

DNA consists of two antiparallel strands, meaning one strand runs in the $5' \rightarrow 3'$ direction while the other runs $3' \rightarrow 5'$.

Nitrogenous bases are categorized into Purines (Adenine $A$ and Guanine $G$), which have a double-ring structure, and Pyrimidines (Cytosine $C$, Thymine $T$, and Uracil $U$), which have a single-ring structure.

Complementary base pairing occurs via hydrogen bonds: Adenine pairs with Thymine ($A=T$) using $2$ hydrogen bonds, and Guanine pairs with Cytosine ($G \equiv C$) using $3$ hydrogen bonds. In RNA, Uracil ($U$) replaces Thymine.

The 'Unity' of nucleic acids refers to the universal nature of the genetic code across all living organisms, while 'Diversity' arises from the unique sequences of nitrogenous bases that encode different proteins.