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A DNA (abbreviation for deoxyribonucleic acid) molecule is made up of two parallel spiral-like strands which form a double helix. In between the strands are four different types of nucleobases: adenine, thymine , cytosine   and guanine  . The basic unit of DNA is known as nucleotide and it is consists of a 5-carbon sugar (deoxyribose), a nitrogen containing base attached to the sugar and a phosphate group. The sugar found in DNA is known as deoxyribose, and the “deoxy-” prefix suggests that deoxyribose has one less oxygen atom. There are two types of nitrogenous bases: pyrimidine bases and purines. There are three pyrimidine bases and each of them is made up of a six-member ring, containing nitrogen and carbon atoms. Thymine and Cytosine are pyrimidines. The purines consist of two heterocyclic rings, made up of carbon and nitrogen. Adenine and Guanine are purines. The phosphate in DNA is bounded to the sugar by ester bond, forming a nucleotide. There are three phosphates in DNA since two more phosphate groups are lost during the synthesis of DNA. Nucleotides can join together by a condensation reaction between the phosphate group of one nucleotide and the hydroxyl group on carbon 3 of the sugar of the other nucleotide. Phosphodiester bonds link the nucleotides together. Bases do not take part in the polymerization so there is a sugar-phosphate backbone with the bases extending off it. Two polynucleotide stands alongside each other, causing the DNA to be double-stranded. They are also anti-parallel and run in opposite directions. They are wound round each other, forming a double helix. The strands are also joined by hydrogen bonds between the bases, therefore forming base pairs. Base pairs are specific and there is a sequence of bases along each strand. 