Purines

• There are two purines: adenine and guanine. They differ from pyrimidines in that they consist of two carbon-nitrogen chemical "rings," whereas pyrimidines have only one ring. Thus, they are larger and heavier than pyrimidines. Adenine and guanine are found in both DNA and RNA.
  
  

Pyrimidines

• There are three pyrimidines: thymine, cytosine and uracil. Pyrimidines are made of only one carbon-nitrogen ring, making them smaller than purines. Cytosine is found in both DNA and RNA, while thymine is only found in DNA and uracil is only found in RNA.
  
  

Base Pairs

• Pyrimidines and purines form base pairs to create the double-stranded structure of DNA. Adenine pairs with thymine and guanine pairs with cytosine. When an RNA molecule bonds to a DNA strand, the adenine on DNA pairs with the uracil on RNA.
  
  

Genes

• Each three-base string, called a triplet codon, codes for an amino acid, meaning that enzymes "read" the codons and make portions of proteins based on the individual codes. Codons in turn are arranged into genes, which make up a DNA strand. DNA strands are wound into chromosomes, which are housed in the nucleus of eukaryotic cells.