Nucleic Acids: Composition, Structure, and Function
Composition of Nucleic Acids
Nucleic acids are essential biological macromolecules responsible for storing, transmitting, and expressing genetic information in all living organisms. They include DNA and RNA, both of which are composed of nucleotides. Nucleotides consist of a nitrogenous base, a five-carbon sugar (pentose), and a phosphoric acid molecule.
Nitrogenous Bases
Nitrogenous bases are heterocyclic compounds containing carbon and nitrogen, with a flat structure. There are two types:
- Purines: Adenine (A) and guanine (G), found in both DNA and RNA.
- Pyrimidines: Cytosine (C), thymine (T) in DNA, and uracil (U) in RNA.
Pentoses
RNA contains ribose, while DNA contains deoxyribose as the pentose sugar.
Phosphoric Acid
Phosphoric acid is found in nucleotides as a phosphate ion.
Nucleosides
Nucleosides are formed by an N-glycosidic bond between the C1 carbon of a pentose and a nitrogenous base, with the loss of a water molecule. They are named by adding ‘-osine’ for purine bases (e.g., adenosine) or ‘-idine’ for pyrimidine bases (e.g., cytidine). Deoxyribose-containing nucleosides add the prefix ‘deoxy-‘ (e.g., deoxyadenosine).
Nucleotides
Nucleotides are phosphate esters of nucleosides, formed by joining a nucleoside with a phosphoric acid molecule. The ester bond occurs between the hydroxyl group on the pentose’s C5 carbon and phosphoric acid. They are named by adding ‘5’-phosphate’ to the nucleoside name. Free nucleotides play roles in metabolism and its regulation, acting as enzyme activators and energy carriers.
Adenine Nucleotides
- ADP and ATP: These nucleotides have high-energy phosphate bonds, storing and releasing energy. ATP is the primary energy currency of cells.
- Cyclic AMP (cAMP): Formed from ATP, cAMP acts as a second messenger, transmitting and amplifying hormonal signals within cells.
Nucleotide Coenzymes
Coenzymes are non-protein organic molecules involved in enzyme-catalyzed reactions, often acting as electron carriers.
Flavin Nucleotides
These coenzymes, containing flavin, are involved in oxidation-reduction reactions catalyzed by dehydrogenases.
Pyridine Nucleotides
NAD and NADP are coenzymes of dehydrogenases, participating in various metabolic processes like cellular respiration.
Coenzyme A (CoA)
CoA carries acyl groups in enzymatic reactions, playing a crucial role in cell metabolism.
DNA
DNA (deoxyribonucleic acid) is a macromolecule composed of carbon, hydrogen, oxygen, nitrogen, and phosphorus. It is a linear polymer of deoxyribonucleotides (adenine, guanine, cytosine, and thymine) and has multiple structural levels.
Primary Structure
The primary structure is the sequence of nucleotides linked by phosphodiester bonds between the C5 phosphate of one nucleotide and the C3 hydroxyl of the next. DNA strands have a 5′ phosphate end and a 3′ hydroxyl end.
Secondary Structure
DNA’s secondary structure features:
- A long, stiff, unfolded molecule.
- Repeating structural details.
- Equal purine and pyrimidine content in a given species.
The Watson-Crick model describes DNA as a double helix:
- Two polynucleotide chains wound around an imaginary axis, with nitrogenous bases inside.
- The helix is right-handed (dextrorotatory) and plectonemic (requires unwinding to separate).
- Nucleotide pairs are spaced 0.34 nm apart, with 10 pairs per helix turn.
- The chains are antiparallel (5′ to 3′ orientation is opposite) and complementary.