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Ch. 12 Notes: DNA Structure
Chromosomes are made of DNA and proteins in approximately equal amounts
originally it was believed that proteins carried the information because they are more complex
o 20 types of amino acids exist, while DNA only has 4 types
o Believed to be like the alphabet and could be arranged in a variety of ways.
DNA Trail
Genetic material had to meet 4 criteria:
1) It must carry the genetic information from parent to daughter cell and from generation to
generation and carry a great deal of information
2) It must contain information for producing a copy of itself
3) It must be chemically stable
4) It must be capable of mutation (change)
Frederick Griffith (1928) – bacteriologist
looking for vaccine for S. pneumoniae (pneumonia)
Two forms:
virulent (disease-causing) – polysaccharide capsule
nonvirulent (harmless) – no capsule
Tested to see if heat–killed (harmless) virulent could be used as vaccine (See Fig. 14-3)
o Injected mouse with live virulent  mouse dies
o Injected mouse with live non-virulent  mouse lives
o Injected mouse with heat-killed virulent  mouse lives
o Injected mouse with live nonvirulent and heat-killed mixture  mouse dies
(both harmless)
o Blood sample removed showed live virulent
o Transformation = when a cell incorporates DNA from surroundings
o Transforming Factor = the “something” that causes the conversion (was DNA)
Miescher (1869) isolated DNA
eventually named DeoxyRibonucleic Acid
Feulgen – DNA has high affinity for fuchsin stain
showed DNA exists in ALL CELLS, in their nuclei
Levene (1920’s) – showed DNA could be broken down into units made of a 5-C sugar, a phosphate,
and one of four N-bases = nucleotide
Avery, et al. showed took up where Griffith left off. They showed that if:
the DNA from the virulent strain was added to nonvirulent bacteria, it became virulent
they added DNAase (an enzyme that digests DNA), it prevents the transformation from
they added enzymes that degrade proteins and RNAase, but the transformation still
happened (showing RNA nor protein is the genetic material)
Delbruick and Luria (1940)
Bacteriophage – viruses that attack bacteria (“bacteria eaters”)
ex. E-coli phages: T1 – T7 life cycle
o Upon infection- disappear except for a few fragments
o After 10 minutes – virus detectable in large numbers
Alfred D. Hershey and Martha Chase (1952)
Two samples: one where DNA was traced with P35 (which is radioactive)
other where protein was traced with S34 (also radioactive)
Left to multiply
Put in blender to separate intra- and extra-cellular material and examined for isotopes
Found: S34 remained outside but P35 was inside cells
(We now know phages inject their DNA into the cell and leave protein coats outside)
Transformation of Organisms Today
Knowledge of DNA and genetics has led to our ability to take select genes of one organism
and put them into another, where they are expressed = Genetically Modified Organisms
Watson – Crick Model
James Watson and Francis Crick (1950s)
Did not do much research themselves, but based their theory on Known Data:
o DNA molecules known to be very large – very long and thin, and made of nucleotides
containing N-bases (A, G, T, C)
o Linus Pauling (1950) had shown that a protein’s chains of amino acids are often
arranged in the shape of a helix and are held in their form by H-bonds between
successive turns. He suggested that DNA might be the same.
o X-ray diffraction photos of DNA (Maurice Wilkins and Rosalind Franklin) showed
patterns that reflected the turns of a giant helix
o Chargaff’s data showed that the amount of T = A and C = G.
Building the Model
THE BIG BREAKTHROUGH: postulated that DNA is a DOUBLE helix (like twisted ladder)
“Rungs” = N-bases
o 2 per rung – held together by H-bonds
o each base bonded to sugar – phosphate unit (to complete the nucleotide)
o each base pair was a purine – pyrimidine combination
Nucleotides along one side can be arranged in any order
o Allowed for a possibility of great variation
Each strand has direction
o each phosphate group is attached to one sugar at 5’ position and to the next sugar at
the 3’ position
o There is a 5’ end and a 3’ end to each strand
Strands are antiparallel = run in opposite directions
5’ →
other 3’ ← 5’ (opposite direction)
Because of structures
A - T (A = T)
form 2 H bonds
C - G (C ≡ G)
form 3 H bonds
These “complementary bases” are what allow DNA to replicate itself so precisely