DNA Nucleotides

If your chemistry is a little rusty, you might want to start with the BASICS.  This will rez a red box, which you can click on to get information about atoms, small molecules, and more complex molecules.  The BASICS module will take you through some of the information that is summarized below.  

DNA is composed of complex subunits called NUCLEOTIDES.   A nucleotide itself contains three subunits:   a base (purine or pyrimidine), a five-carbon sugar, and a phosphate group.   

The subunits are arranged so that both the phosphate (P) and the base (B)  are connected to the sugar molecule (S) like this:  

P - S - B
     
There are four bases in DNA.  Two of the bases have a core structure consisting of two interlocking rings; these are called purines.  The core structure of the other two bases consists of a single ring:  these are  called pyrimidines.  
  
The bases adenine and guanine are purines.  The bases thymine and cytosine are pyrimidines.  You can see models of the four bases by clicking again on the base ball and selecting which base you want to see.  Compare the purines with the pyrimidines.  In the models, each atom is represented by a different color:

        Red = oxygen (O)
        White = hydrogen (H)
        Black = carbon (C)
        Blue = nitrogen (N)
        
The "long names" for the purines and pyrimidines identify small groups of atoms that "decorate" the purine and pyrimidine cores.  Different bases have different "decorations."  These groups are:

        Amino = NH2
        Keto = O (attached by a double bond)
        Methyl = CH3

The numbers specify the position of the core atom to which these groups are attached, NOT the number of functional groups.  For example, 6-amino purine means one amino group attached to the #6 atom in the purine ring.  

DNA is a polynucleotide, a polymer formed by connecting adjacent nucleotides together.  Nucleotides are connected by linking the sugar of one nucleotide to the phosphate of an adjacent nucleotide:, for example:  

P - S   + P - S   +  P - S  +  P - S    ===>  P - S - P - S - P - S - P - S   
    A         C          T         G              A       C       T       G


The phosphates and sugars then alternate to form a chain from which the bases dangle like charms on a bracelet.  Each base is attached to a sugar molecule.  Nucleotides containing the four different bases can be connected in any order, so what makes on DNA different from another is the order in which the bases occur.  The strand has an unambiguous direction:  one end has a free phosphate group.  This is called the 5' end (after the #5 carbon on the sugar it is connected to).  The other end of the chain is called the 3' end. 

Thanks to Timothy Trenchmouth for providing the base models seen here.