DNA

 Deoxyribonucleic acid is the hereditary material in organisms as it carries genetic instructions. It is found in the chromosomes in the nucleus of cells. DNA is a long molecule in the form of a double helix. (two long thin strands twisted around each other. ).

The DNA molecule is made up of subunits called nucleotides. Each nucleotide is made up of three chemical groups. Group one is a sugar molecule called deoxyribose sugar,  group two is a nitrogenous base and group three is a phosphate part. 

There are then four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). This bases have complementary pairing. A always combines with T and C with G. Between this complementary base pairs there are weak hydrogen bonds. Between A and T there are two hydrogen bonds and there are three between G and C. Adenine and Guanine are called purines (large bases) while cytosine and thymine are called pyrimidines (small bases).

DNA is a polynucleotide because it is made up of many nucleotides joined together.