Nucleic acids are the building blocks of living organisms. They work together to help cells to replicate and build proteins and can be found within nucleus and cytoplasm of the cells. These are macromolecules that store genetic information and enable protein synthesis.
Nucleic acids are macromolecules made out of units called nucleotides. They have two naturally occurring varieties: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA).
Nucleotides are made of three parts:
- Pentose sugar (five-carbon sugar).
- Nitrogenous bases.
- Phosphate group [An ion of phosphoric acid (PO43-)].
Pentose sugar: It is a monosaccharide with 5 carbon atoms. The sugar molecule has a central position in the nucleotide, with the base attached to one of its carbons and phosphate group (or groups) attached to another.
The five-carbon sugar in DNA is called deoxyribose, while in RNA, the sugar is ribose. These two are very similar in structure, however second carbon of ribose bears a hydroxyl group, while equivalent carbon of deoxyribose has a hydrogen instead.
Nitrogenous bases: These are organic molecules made up of nitrogen-containing ring structures. There are five nitrogenous bases present in nucleic acids: Adenine (A), Guanine (G) Cytosine (C), Thymine (T) and Uracil (U). Nitrogen bases are of two types:
Purine is a heterocyclic aromatic organic compound that consists of a pyrimidine ring fused to an imidazole ring. It consists of a six-membered and a five-membered nitrogen-containing ring. It is double ringed structure. Some common purine includes:
- Guanine = 2-amino-6-oxy purine
- Adenine = 6-amino purine
- Xanthine = 2,6-dioxy purine
- Hypoxanthine = 6-oxy purine
Hypoxanthine and xanthine are not incorporated into nucleic acids but are important intermediates in synthesis and degradation of the purine nucleotides. Adenine and Guanine are present in DNA and RNA.
Pyrimidines are heterocyclic aromatic compounds having six-membered ring composed of two nitrogen atoms and four carbon atoms, with hydrogen atoms attached to each carbon. It is single ringed structure. Some common examples of pyrimidine includes:
- Orotic acid = 2,4-dioxy-6-carboxy pyrimidine
- Thymine = 2,4-dioxy-5-methyl pyrimidine
- Uracil = 2,4-dioxy pyrimidine
- Cytosine = 2-oxy-4-amino pyrimidine
Cytosine is found in both DNA and RNA. Uracil is found only in RNA. Thymine is normally found in DNA. Some tRNAs have some thymine as well as uracil.
Phosphate group: It is a molecule containing one atom of phosphorus covalently bound to four oxygen residues, two of which may be expressed as a hydroxyl group. They are relatively reactive molecules that readily form phophoester bonds by the interaction with hydroxyl groups. It is found in genetic material DNA and RNA, and also in molecules such as adenosine triphosphate (ATP) that provide energy to cells.
Nucleotides are linked together to form polynucleotide chains. They are joined to one another by covalent bonds between the phosphate of one and the sugar of another. These linkages are called phosphodiester linkages. Phosphodiester linkages form the sugar-phosphate backbone of both DNA and RNA. They are the activated precursors of nucleic acids.
Nucleotides are joined together by 3′-5′ phosphodiester bonds to form polynucleotides. Polymerization of ribonucleotides produce an RNA while polymerization of deoxyribonucleotides leads to DNA.
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