DNA stands for deoxyribonucleic acid. Strands of DNA are long polymers built of millions of nucleotides that are linked together. Individually, nucleotides are quite simple, consisting of three distinct parts:  One of four nitrogen bases  Deoxyribose (a five-carbon sugar)  A phosphate group  The image below shows a simplified representation of a nucleotide. The P represents the phosphate molecule, the S represents the sugar (deoxyribose), and B represents one of the four nitrogen bases. The structure of the phosphate group is shown below. 1. Nitrogen Bases The four nitrogen bases are: Adenine  Guanine  Cytosine  Thymine  Nucleotides are named after which of the four nitrogen bases it has. So the names of the four DNA nucleotides are adenine, guanine, cytosine, and thymine. These will be referred to as A, G, C, and T respectively.  (Note: Throughout this site, the words nucleotide and base are often used to represent the same thing - a nucleotide). Adenine and guanine are classified as purines since they are double-ringed molecules. Cytosine and thymine and pyrimidenes due to the fact that they are single-ringed molecules. Structural diagrams of the four bases are shown in the table below.

Base	Adenine (A)	Guanine (G)	Thymine (T)	Cytosine (C)
Purine/ Pyrimidene	Purine	Purine	Pyrimidene	Pyrimidene
Chemical  Structure *
Simplified  Representation

* C = Carbon, N = Nitrogen, O = Oxygen.
A single line between atoms is a single bond.
A double line between atoms is a double bond.

A pyrine binds with a pyrimidene in DNA to form a basepair. Adenine and thymine bind together to form the A-T basepair. Likewise, guanine and cytosine come together to form the G-C basepair. The bases are joined together by weak hydrogen bonds, and it is this hydrogen bonding that produces DNA's familiar double helix shape. An image illustrating the how two bases pair with hydrogen bonding is shown below (The blue lines are the hydrogen bonds.)  2. Deoxyribose Deoxyribose is a five carbon sugar, and to fully understand many of the concepts that are presented later on, one must know the structure of deoxyribose. A visual representation of the sugar and how it relates to the other two components of a nucleotide is shown below in figure 1.  The carbons of deoxyribose sugar are numbered sequentially from right to left. The first carbon is 1' (read as one prime), the second is 2' (two prime), and so on. The nitrogenous base attaches to the 1' carbon, and the phosphate group attaches to the 5' carbon. The nucleotide below is covalently bonded to the 3' carbon. This allows for a long strand to be built. An example of a single strand of DNA is shown below.  Instead of always seeing a huge molecular diagram of a DNA strand, what one often sees in a string of letters, such as "ATCTTAG". This string represents the what bases are in a certain side of a strand of DNA. The above string (ATCTTAG) represents the string "adenine-thymine-cytosine-thymine-thymine-adenine-guanine."  DNA has two strands. Whatever nucleotides are in one strand, they rigidly fix the sequence of nucleotides in the other strand due to the way base pairing occurs (A with T, G with C). The two strands are complementary. They aren't identical, but fit together just right.  In addition, it must be noted that the two strands are antiparallel. That means that they run in opposite directions. One strand goes in a 5' to 3' direction while the other goes in a 3' to 5' direction. By convention, the strand which goes in the 5' to 3' direction is placed on the left in 2-dimensional drawing. Figure 2 gives a visual example of this concept as well as showing how the strands are complementary.  In this next image, the double-helix shape of DNA is shown. The two strands are clearly visible, one being coloured blue, and the other red.