The Structure of DNA
DNA has a structure that looks like a twisted ladder. That "twisted ladder" is called the double helix, which is made up of two sugar-phosphate backbones as the "rails" of the "laddder", and four different nitrogen bases bonded by hydrogen bonds as the "rungs". The four nitrogen bases are: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). Adenine and Thymine are complementary pairs, which means that they pair together as one "rung" in the double helix. Cytosine and Guanine are aslo complementary pairs. Th complementary pairs are joined together by the hydrogen bonds.
(Information from class notes)
(Information from class notes)
Nitrogen Bases
Adenine and Guanine are both nitrogen bases called purines, which is a two-carbon ring. Thymine and Cytosine are what's know as pyrimidines, which has only one carbon ring. Purines and pyrimidines are the reason why Adenine and Thymine are paired together and why Cytosine and Guanine are paired together; if you put two pyrimidines together then the "rungs" will be too narrow since they only have one carbon atom. if you put two purines together then the "rungs" will be too wide because of the two carbon atoms.
(Information from class notes and http://pangzho.blogspot.com/ )
(Information from class notes and http://pangzho.blogspot.com/ )