DNA, RNA, Nucleotides, and Nitrogenous Bases

• Purine and pyrimidine bases and their derivatives, nucleosides, and nucleotides are the building blocks of nucleic acids.
• Nucleic acids have 2 main classifications: deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA).
• Nucleotides are the building blocks of nucleic acids.
• Nucleotides are composed of a nitrogenous base, a pentose sugar, and a phosphate group.

Nitrogenous Bases

• Nitrogenous bases are heterocyclic ring compounds that contain carbon and nitrogen.
• Nitrogenous bases are classified based on their abundance within cells.

Major Bases

• Major bases are present in large amounts inside cells.
• Purines: adenine and guanine, are present in both DNA and RNA.
• Pyrimidines: cytosine is present in both DNA and RNA, thymine is present in DNA only, and uracil is present in RNA only.

Minor Bases

• Minor bases are present in trace amounts and are always methylated, for example, 5-methyl cytosine and N7-methylguanine.
• Minor bases are involved in oligonucleotide recognition, regulating the half-life of RNAs and protecting host DNA from digestion by DNAses.
• Bases such as xanthine, hypoxanthine, and uric acid are present in a free state, while methylated xanthine appears as caffeine in coffee and theophylline in tea.
• Theobromine appears in coca.

Nucleosides

• Nucleosides consist of a base and a sugar (ribose or deoxyribose).
• Sugars bind bases via a covalent, β-N-glycosidic bond that links C₁ of sugar with N-9 of a purine, or N-1 of a pyrimidine.
• The numbering of the sugar atoms uses a prime symbol to distinguish sugar atoms from those of the base.

Nucleotides

• Nucleotides consist of a base, a sugar, and a phosphate group.
• Phosphate is attached to the C-5 position of the sugar.

Tautomerism

• Purines and pyrimidines exist in two isoforms: keto (or lactam) or enol (lactim) form, and amino (-NH2) or imino (=NH) form.
• Tautomerism is the change from one isoform to the other.
• Keto and amino forms are common at physiological pH.

Syn and Anti-Conformers

• Nucleotides exist in stable non-interconvertible syn and anti conforms.
• The β-N-glycosidic bond is a very firm bond with restricted rotation.
• Interconversion between conformers requires rupture and reformation of the glycosidic bond.
• The anti-conformer predominates.

Nucleotide Functions

• Both purine and pyrimidine nucleotides are structural components of nucleic acids (DNA and RNA).
• Purine nucleotides are key components of high-energy storage molecules like ATP and GTP.
• Intracellular signaling molecules such as cAMP and cGMP, act as secondary messengers for many hormones.
• Purines enter the structure of hydrogen carrier coenzymes, such as NAD, NADP, FAD, and FMN.
• Purines serve as acid carrier coenzymes, such as CoASH.
• Purines are part of the active methyl donor S-adenosyl methionine (SAM).
• Purines are part of the active sulfate donor 3'-phosphoadenosine-5'-phosphosulfate (PAPS).
• ADP levels regulate the rate of oxidative phosphorylation in mitochondria.
• Pyrimidine nucleotides are components of high-energy intermediates e.g. uridine diphosphate glucose (UDP-glucose) used in glycogenesis and the uronic acid pathway.
• Pyrimidines are components of high energy stores like CTP and UTP.

Explore the fundamental building blocks of nucleic acids: purines, pyrimidines, nucleosides, and nucleotides. Learn about DNA and RNA classifications, nucleotide composition, and the roles of major (adenine, guanine, cytosine, thymine, uracil) and minor nitrogenous bases.