Structure

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Structure of **Deoxyribonucleic acid** (DNA)
In 1953, James Watson and Francis Crick published a letter in a journal that was only one page in length, yet monumental in importance. Watson and Crick proposed that DNA is make of two chains of nucleotides held together by nitrogenous bases

__Double Helix__-shape of a DNA molecule formed when two twisted DNA strands are coiled into a springlike structure and held together by hydorgen bonds between the bbases.

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Structure of A and G
The 9 atoms that make up the fused rings (5 carbon, 4 nitrogen) are numbered 1-9. All ring atoms lie in the same plane. 

Pyrimidine Bases
Cytosine and thymine are pyrimidines. The 6 stoms (4 carbon, 2 nitrogen) are numbered 1-6. Like purines, all pyrimidine ring atoms lie in the same plane.

Deoxyribose Sugar
The deoxyribose sugar of the DNA backbone has 5 carbons and 3 oxygens. The carbon atoms are numbered 1', 2', 3', 4', and 5' to distinguish from the numbering of the atoms of the purine and pyrmidine rings. The hydroxyl groups on the 5'- and 3'- carbons link to the phosphate groups to form the DNA backbone. Deoxyribose lacks an hydroxyl group at the 2'-position when compared to ribose, the sugar component of RNA.

Nucleosides
A nucleoside is one of the four DNA bases covalently attached to the C1' position of a sugar. The sugar in deoxynucleosides is 2'-deoxyribose. The sugar in ribonucleosides is ribose. Nucleosides differ from nucleotides in that they lack phosphate groups. The four different nucleosides of DNA are deoxyadenosine (dA), deoxyguanosine (dG), deoxycytosine (dC), and (deoxy)thymidine (dT, or T).

Structure of dA
In dA and dG, there is an "N-glycoside" bond between the sugar C1' and N9 of the purine. 

Nucleotides
A nucleotide is a nucleoside with one or more phosphate groups covalently attached to the 3'- and/or 5'-hydroxyl group(s). 

DNA Backbone
The DNA backbone is a polymer with an alternating sugar-phosphate sequence. The deoxyribose sugars are joined at both the 3'-hydroxyl and 5'-hydroxyl groups to phosphate groups in ester links, also known as "phosphodiester" bonds.

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