A nucleotide has three components – a nitrogenous base, a pentose sugar deoxyribose , and a phosphate group. There are two types of nitrogenous bases – Purines (Adenine and Guanine), and Pyrimidines (Cytosine, and Thymine).
Nucleoside = nitrogenous base + pentose sugar (by a N- glycosidic bond ) eg. deoxyadenosine, deoxyguanosine, deoxycytidine and deoxythymidine.
Nucleotide = phospate + nucleoside ( by phospho ester bond at 5′ moiety of sugar)
Dinucleotide = nucleotide + nucleotide ( by phospho diester linkage )
More nucleotides can be joined in such a manner to form a polynucleotide chain which has a phosphate moiety at 5′ end of sugar while a free 3′ OH group at the other .
Sugar and phosphates comprise the backbone while the nitrogenous bases linked to sugar moiety project from the backbone double stranded DNA. The ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one.
The base pairing confers a very unique property to the polynucleotide chains. They are complementary to each other. Adenine forms two hydrogen bonds with Thymine from opposite strand & Guanine is bonded with Cytosine with three H-bonds. Always a purine comes opposite to a pyrimidine Each strand from a DNA acts as a template for synthesis of a new strand, the two double stranded DNA (daughter DNA) produced would be identical to the parental DNA molecule paired through hydrogen bond forming base pairs . Thus uniform distance is maintained between the two strands of the helix.The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 and there are roughly 10 bp in each turn. Therefore, distance between two base pairs in a helix is approximately equal to 0.34 nm
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