Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

A molecule of Nitrogen base joins the molecule of pentose sugar. It results into the formation of nucleosides. Similarly when phosphoric acid joins the nucleoside it forms nucleotide. i.e. a nucleotide has three components :
1. Pentose sugar (ribose in RNA and deoxyribose in DNA)
2. A phosphate group
3. Nitrogen base
1. Pentose sugar : Depending on the type of pentose sugar, nucleic acid are of two types :
Nucleic acid with ribose sugar is ribonucleic acid (RNA)
Nucleic acid with deoxyribose is deoxyribonucleic acid (DNA) 


The main difference between the two sugars lies at second carbon atom which has -OH group in ribose sugar and hydrogen atom in deoxyribose sugar. Sugar acts as a link to join the nitrogen base and phosphate group.
2. Phosphate or phosphoric acid : This is the ortho-phosphoric acid that connects the two successive nucleotides. This bond is called as phospho-ester bond which joins the nucleoside at 3' carbon atom of sugar to that of 5' carbon atom of successive nucleosides
3. Nitrogen base :
Pyrimidine : These include thymine, cytosine and uracil of these DNA has thymine and cytosine while RNA has cytosine and uracil.
Purine : They are Adenine and Guanine in DNA and RNA both.
One important feature of the structure of double helical DNA is that DNA is made of two polynucleotide chains which has a backbone of sugar phosphate with nitrogen bases projected inside. The two chains exhibit anti-parallel polarity which means if the one chain has the polarity in 5' to 3' end direction then other has 3' to 5' end.