DNA differs from RNA by owning sugar deoxyribose and the nucleotides adenine, cytosine, guanine and thymine.
In RNA, sugar is the ribose and the nucleotides are adenine, cytosine, guanine and uracilla (uracil replaces thymine).
DNA: A “Twisted Ladder”
From experiments by several researchers and using the results of the complex X-ray diffraction technique, Watson and Crick concluded that in DNA, complementary strands are helical, suggesting the idea of a twisted ladder.
In this stairway, the handrails are formed by phosphates and deoxyribose, while the steps consist of nitrogen base pairs.
The carbon atoms of ribose and deoxyribose molecules are numbered as shown below. Note that sugar carbons are numbered with a line (') to distinguish them from other nucleotide carbons.
In each strand of DNA, the "handrail" is formed by bonds between sugar molecules and phosphate radicals. Note that the phosphate radical binds to the 3 'carbon of one sugar and the 5' carbon of the following.
The two nucleotide strands of DNA are joined together by bonds called hydrogen bridges, which form between the nitrogenous bases of each tape.
Base pairing occurs precisely: a purine base binds to a pyrimidine adenine (A) of one chain paired with thymine (T) of the other and guanine (G) paired with cytosine (C).