DNA is a polymer made up of nucleotide chains. Each nucleotide is made up of a sugar, a phosphate group and a base.
All nucleotides have the same sugar and phosphate structure, but they differ in their base. Each nucleotide can have one of four bases, which are:
Around 3 billion pairs of these bases are found in human DNA.
The two strands of DNA are chemically linked by hydrogen bonds between pairs of bases. They are always paired in a certain way, which is known as complementary base pairing:
These nucleotides join together to form long strands by connecting the sugar of one nucleotide and the phosphate of another nucleotide. A DNA molecule consists of two strands that form a twisted double helix. The backbone of each strand is the alternating sugar and phosphate groups.
Each cell stores its genetic information in the sequence of DNA bases, which makes up the genetic code. This sequence determines the genotype.
The arrangement of the bases is critical. It functions as a code that instructs cells to produce specific types of proteins. The variations in these proteins distinguish different living organisms, from plants like cacti to animals like gerbils, and even between two individual humans.