Describe the structure of DNA.

DNA (deoxyribonucleic acid) is a polymer made up of two nucleotide chains held together by hydrogen bonding between nucleotides in a double helix shape.
A nucleotide is made up of a deoxyribose sugar, a base group, and a phosphate group. Sugar and phosphate group in DNA is always constant; meaning the base determines what the nucleotide is. There are four bases in a DNA strand;adeninethymineguaninecytosineAdenine always pairs with thymine of the opposing DNA strand; and guanine always pairs with cytosine. This is called complementary base pairing. Hence, two DNA strands are called complementary strands.Bases of opposing nucleotides form hydrogen bonds; which hold the two strands together.
The reason DNA has its double helix shape is the two strands are parallel, but they run in opposite directions i.e. one strand is 3' to 5' where the opposing strand is in a 5' to 3' direction.