the direction of helical axis. Electron cloud interactions ( ∏ – ∏ ) between the bases in the helical stacks contribute to the stability of the double helix. • The phosphodiester linkages gives an inherent polarity to the DNA helix. They form strong covalent bonds, gives the strength and stability to the polynucleotide chain (Figure .
). A T A T A T A T A T A T A T A T A G C G C G C G C G C A T G C G C A T A T A T A T A T A T A T A T A G C G C G C G C G C A T G C G C Blue bands - Two sugar-phosphate chains Major Groove Minor Groove ’ ’ ’ ’ The two chains run in opposite direction Complementary base parings: A=T and C G ’ ’ ’ ’ o .4A o .4A o 20A Figure . : Structure of DNA • Plectonemic coiling - the two strands of the DNA are wrapped around each other in a helix, making it impossible to simply move them apart without breaking the entire structure. Whereas in paranemic coiling the two strands simply lie alongside one another, making them easier to pull apart.
• Based on the helix and the distance between each turns, the DNA is of three forms – A DNA, B DNA and Z DNA (Figure . ). Figure . : Forms of DNA .
. Ribonucleic Acid (RNA) Ribonucleic acid ( RNA ) is a polymeric molecule essential in various biological roles in - - . . PM coding, decoding, regulation and expression of genes.
RNA is single stranded and is unstable when compared to DNA. . . Types of RNA • mRNA (messenger RNA): Single stranded, carries a copy of instructions for assembling amino acids into proteins.
It is very unstable and comprises % of total RNA polymer. Prokaryotic mRNA (Polycistronic) carry coding sequences for many polypeptides. Eukaryotic mRNA (Monocistronic) contains information for only one polypeptide. • tRNA (transfer RNA): Translates the code from mRNA and