Biology
Discovery of DNA
Spring 2016
Objective: To learn about the structure and function of DNA and the people who discovered it, and how our
understanding of DNA evolved and progressed.
Standard : 2.7 DNA Structure and Technology
Area of Interaction: Human Ingenuity
Criterion A: One world.
Your Task: To learn about DNA and the people who discovered it, then understand its molecular basis and
importance.
DNA
DNA is found in the nucleus, has two fundamental functions:
1)
2)
Your body has many copies of your DNA
Have you ever watched the show CSI? They’re always using DNA to figure out why the innocent person
was murdered, and what bad guy did it. The reason they can use DNA to tie people to certain crimes is
because every cell in your body has a copy of your genetic material in the form of chromosomes made of
DNA.
Frederick Griffith’s experiment (1928) How is genetic information passed on?
-was looking for a pneumonia vaccine
-found that bacteria can exchange genetic information.
-Used smooth and rough strain DNA
-Smooth strain= (polysaccharide capsule) killed mice.
-Rough strain=(no capsule) did not kill mice
-Rough strain inherited characteristics/genetic info from heat-killed smooth strain and killed mice.
R Bacteria __________________________________.
S bacteria __________________________________.
But when_____________________________________________________________________
______________________________________________________________________________.
S->R Genetic material from S entered R and killed mouse.
Transformation:
Avery (1944)
Demonstrated the DNA is the material that is responsible for transformation. Supported
Griffith.
What was learned ___________________________________________________________.
Chargaff (1949)
Observes that all organisms tested have equal amount of:
adenine and thymine (A=T)
guanine and cytosine (G=C)
Layed foundation for understanding based pairing.
Adenine=_________
Guanine=_________.
What was learned _____________________________________.
Wilkins and Rosalind Franklin (1952)
Took x-ray photos of DNA, showed DNA to be a tight-coiled helix
What was learned _____________________________________________________________.
Alfred Hershey and Martha Chase (1952) “Is genetic material found in DNA or proteins?”
Verified Avery’s work: Cell genetic material is found in DNA molecule. But how did they do it?
-Used T2 bacteriophage. Virus that infects a bacteria (AKA phage)
and radioactive isotopes.(easy to follow because unstable and give off energy that can be observed and
measured to track where it is)
In phage Phosphorous marker placed in DNA
Sulfur placed in protein molecules
-Grew a new batch of T2 and allowed phages to infect Escherischia coli separately.
-Each culture was buzzed and centrifuged.
-Only the radioactive phosphate was found in the cells of Escherischia coli. Sulfur/protein was not
Only the DNA entered the cells and reprogrammed the genetics. Genetic material must be in DNA.
What was learned ________________________________________________________.
Summarize this experiment in your own words:
Watson and Crick (1953) DNA Structure (Double helix ) So, what does DNA look like?
-Wore bowties and determined how genetic material is stored in DNA molecule.
-Deciphered the molecular structure of DNA: The alpha helix.
-Used knowledge of covalent and hydrogen bonding.
-Used Chargaff’s findings to support base pairing (hydrogen bonding) A-T, G-C.
-Used Wilkin’s and Franklin’s prediction of the coiled helix.
-DNA alpha helix looks like a twisted ladder (rungs and rails)
-Designed a 3D model of the DNA double helix that accounts for both Chargaff and W/F
findings
DNA-
Structure of DNA:
DNA is made of nucleotides with three main parts:
1.
2.
3.
Sugar: 5-Carbon deoxyribose
Phosphate group: Alternates with sugar to make side rails of ladder.
Nitrogen bases: 4 different ones (adenine, thymine, guanine, cytosine)
Classified into two types:
Purines (two rings):
Adenine
Guanine
Pyrimidines (one ring):
Thymine
Cytosine
These three structures bond to form the alpha helix, or twisted ladder model.
Base paired by hydrogen bonds (A-T, G-C). Make up rungs of latter.
Complementary bases: adenine always bonds with thymine
Guanine always with cytosine
base pairing.
Extension:
Please explain how Watson and Crick used the information discovered be previous scientists to create the
alpha helix model of DNA. Please be sure to include the discoveries of Griffith, Chargaff, Hershey and
Chase, and Franklin and Wilkins in your description. Then, explain if their methods were completely
honorable.