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Bio102: Introduction to Cell Biology and Genetics
The Double Helix
Key Terms:
nucleotide
5’ end
3’ end
antiparallel
base
base-pairing
complementary
phosphate
deoxyribose
phosphodiester bond
purine
pyrimidine
helix
sugar-phosphate backbone
bacteriophage
transformation
proteinase
nuclease
Key Questions:
 Why did many researchers originally believe that genes were probably made of protein?
 As you know, all amino acids contain an NH2 group...why couldn't Hershey and Chase have
used radioactive nitrogen instead of radioactive sulfur to track the bacteriophage protein?
 Why is it important to the overall structure of DNA that a pyrimidine always pairs with a purine?
 A dideoxy nucleotide looks just like a regular nucleotide, except that it has no -OH group on its
3' carbon. What would happen if DNA polymerase inserted a dideoxy nucleotide into a DNA
chain?
Lecture Outline:
How do we define a gene?
Griffith Experiment
R and S Streptococcus pneumoniae
dead S cells can transform R cells to make them virulent
Avery, McCarty, MacLeod Experiment
treated dead S cells with RNase, DNase or Proteinase
DNase treatment blocked the ability to transform the R cells
therefore, DNA must be the genetic material
Hershey and Chase Experiment
radioactively labeled protein or DNA in a bacteriophage
only the radioactive DNA got into the host cell. Therefore, DNA must be the genetic material
Watson and Crick (and Franklin and Wilkins) determine DNA structure. Implies function
nucleotides are composed of a base (G, A, T, or C) and a deoxyribose sugar
A and G are purines; C and T are pyrimidines
polymer of nucleotides joined by sugar-phosphate linkages
distince 5’ and 3’ ends, based on the deoxyribose sugar
base-pairing (G to C and A to T) holds two strands together in an antiparallel arrangement
noncovalent hydrogen bonding
in three dimensions, the strands wrap around each other in a double helix