Describe the structure of DNA

Deoxyribonucleic acid (DNA) is the hereditary material in almost all organisms and is responsible for carrying out genetic instructions in the cell. DNA is a polymer of nucleotides and each nucleotide has 3 essential components: 1) A phosphate group 2) A pentose sugar called deoxyribose 3) A nitrogenous base. The sugar and phosphate group are always the same for each nucleotide and are held together by a phosphodiester bond. The nitrogenous base is attached to the sugar and can either be Adenine, Thymine, Cytosine or Guanine which is shortened to A, T, C and G. When the nucleotides join together to form a DNA stand the order of these bases make up the genetic code. 

DNA is not just a single strand, and two strands come together to from the famous double helix. These two strands are held together by hydrogen bonds between the nitrogenous bases of each strand. Hydrogen bonds however cannot be made between any combination of the 4 bases. Adenine (A) always bonds with Thymine (T), and Cytosine (C) always bonds wwith Guanine (G), these are called complimentary bases. Because of this specific base pairing, the second DNA strand is always an exact compliments of the coding strand (ie T instead of A and G instead of C, and vice versa). This purpose of the second strand is to make DNA easy to replicate, as the second strand acts as a template for new DNA to be synthesized.