NIJ Training Course for Forensic DNA Officers of the Court: DNA Double Helix

DNA is made up of many different chemicals.  The main building blocks of the genetic code  are called bases, and these are supported by a  backbone made up of sugars and phosphate groups.

The sugar and phosphate backbone is made up  of the exact same components throughout the  length of a DNA strand but there are four  different kinds of bases found in DNA.  They are: Adenine, Cytosine, Guanine and Thymine,  more commonly referred to as A, C, G, and T.

Large numbers of these four bases  link together in a long string  connected along the backbone. The string forms  one side of a DNA strand and the pattern in  which the basis occurs determines the genetic  code carried on and relayed by the strand.

The other side of the DNA strand is  formed by a similar string of bases.  The order of these bases however  is not the same as the first set;  once strung together, the bases on the  second string need to be in such an  order as to be able to match up and bond to  the bases opposite them on the first string  and these bases bond according to a specific set  of rules. The matched bases are called base pairs:  A bonds only to T; C bonds only to G;  G bonds only to C; T bonds only to A.  The two sides of the DNA strand are bonded  together base to base like a long ladder  with the base pairs forming the rungs. This  ladder is then twisted like a rope into a  shape called a double helix this is the form  in which DNA strands are found in the cell.