Chemistry > Biomolecules > 8.0 Nucleic acids
Biomolecules
1.0 Carbohydrates
2.0 Monosaccharides
3.0 Disaccharides
4.0 Polysaccharides
5.0 Amino Acids
6.0 Proteins
7.0 Classification of Vitamins
8.0 Nucleic acids
8.1 Chemical Composition of Nucleic Acids
Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric acid and nitrogen containing heterocyclic compounds (called bases).
In DNA molecules, the sugar moiety is $\beta$-$D$-2-deoxyribose whereas in RNA molecule, it is $\beta$-$D$-ribose.
DNA contains four bases viz. adenine ($A$), guanine ($G$), cytosine ($C$) and thymine ($T$). RNA also contains four bases, the first three bases are same as in DNA but the fourth one is uracil ($U$).
Structure of Nucleic Acids
A unit formed by the attachment of a base to $1^?$ position of sugar is known as nucleoside. In nucleosides, the sugar carbons are numbered as $1^?$, $2^?$, $3^?$, etc. in order to distinguish these from the bases. When nucleoside is linked to phosphoric acid at $5^?$-position of sugar moiety, we get a nucleotide.
Nucleotides are joined together by phosphodiester linkage between $5^?$ and $3^?$ carbon atoms of the pentose sugar. The formation of a typical dinucleotide is shown in figure.
A simplified version of nucleic acid chain is
Double strand helix structure for DNA
Two nucleic acid chains are wound about each other and held together by hydrogen bonds between pairs of bases. The two strands are complementary to each other because the hydrogen bonds are formed between specific pairs of bases.
Adenine forms hydrogen bonds with thymine whereas cytosine forms hydrogen bonds with guanine. In secondary structure of RNA, helices are present which only single stranded are. Sometimes they fold back on themselves to form a double helix structure.
RNA molecules are of three types and they perform different functions. They are named as messenger RNA ($m$-RNA), ribosomal RNA ($r$-RNA) and transfer RNA (t-RNA).
