Introduction to Deoxyribose Nucleic Acid (DNA)
College Biology
Mr. Lee
Room 320
Objectives
 Explain the principle function of DNA
 Define the term complementary base pairing
 Describe the structure of DNA
Watson and Crick
 Crick was a British physicist and Watson was an American biologist
 They made a model of DNA as a double helix, where two strands were wound around
each other
Structure of DNA
 DNA is a long molecule shaped like a double helix made up of units called
nucleotides
 Each nucleotide is made up of three basic components:
  A five-carbon sugar called deoxyribose
  A phosphate group
  A nitrogenous base
 The backbone of a DNA chain is formed by
  sugar and phosphate groups of each nucleotide
  The two stands of DNA are joined at the bases by hydrogen bonds
Four Nitrogenous Bases of DNA
  Adenine (A)
  Thymine (T)
  Cytosine (C)
  Guanine (G)
 Adenine pairs with Thymine
 Guanine pairs with Cytosine
Purines (two rings)
  Adenine
  Guanine
Pyrimidines (one ring)
  Thymine
  Cytosine
Chargaff’s Rules
 Erwin Chargaff was an American biochemist
 He observed that the percentages of guanine and cytosine were almost equal in any
DNA sample
 Chargaff’s rule is A = T and G = C
  If A = 23%, then T = 23% (Total 46%)
  Then
  G = 27% and C = 27% (total 54%)