Download Chapter 16: The Double Helix

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
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How?
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
Purines
Pyrimidines
Adenine
Thymine
Guanine
Cytosine
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
happening

they added enzymes that degrade proteins and RNAase, but the transformation still
happened (showing RNA nor protein is the genetic material)
BACTERIOPHAGE EXP/s
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
MORE PROOF THAT GENES ARE ON DNA
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)
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Left to multiply
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Put in blender to separate intra- and extra-cellular material and examined for isotopes
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Found: S34 remained outside but P35 was inside cells
PROVED: GENETIC MATERIAL IS DNA (NOT protein)!!!

(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
(GMOs)

Possible because ALL LIVING THINGS USE THE SAME GENETIC CODE !!!
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
one
5’ →
3’
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