Molecular Biology - Structure of DNA and RNA

Nucleic acids, including $DNA$ and $RNA$, are polymers made of repeating units called nucleotides. Each nucleotide consists of a pentose sugar, a phosphate group ($PO_4^{3-}$), and a nitrogenous base.

The two types of pentose sugars are deoxyribose (found in $DNA$) and ribose (found in $RNA$). Ribose has a hydroxyl group ($-OH$) on the $2'$ carbon, whereas deoxyribose has a hydrogen atom ($-H$).

Nitrogenous bases are categorized into purines ($Adenine$ and $Guanine$, which have a double-ring structure) and pyrimidines ($Cytosine$, $Thymine$, and $Uracil$, which have a single-ring structure).

Nucleotides are linked by covalent bonds called phosphodiester bonds, which form between the phosphate group at the $5'$ carbon of one sugar and the hydroxyl group at the $3'$ carbon of the adjacent sugar.

$DNA$ is a double-stranded molecule forming a double helix. The two strands are antiparallel, meaning one runs in the $5' \rightarrow 3'$ direction while the other runs $3' \rightarrow 5'$.

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

Crick and Watson's model of $DNA$ structure was supported by Rosalind Franklin’s X-ray diffraction data, which suggested a helical shape with a consistent width.