Discovering DNA:
Go to the following web page: http://www.dnai.org/index.htm.
Click on timeline for the history of DNA
Briefly browse the time line by moving the cursor back and forth of the screen. Page down and click on 19201949 at the bottom of the web page.
Browse the time line until you get to1944.
Click on the picture of Oswald Avery to read his bio and answer the following questions:
Where did Avery get his education? College of physicians and surgeons
Click on the question mark above the picture of Avery, and answer the following questions by clicking
through the pop up tutorial:
What strain of bacteria did Avery use? strains of Pneumococcus bacteria
What findings did Griffith find? Pneumococcus grows on the body of the host. It can also be grown on solid
and liquid cultures .
What are the main differences between S and R strains of the bacteria Griffith found?
The S colonies have a smooth surface, and the R colonies look rough
Summarize Giffith’s experiment to test the strains of bacteria. Griffen found that mice injected with the S strain
developed pneumonia and died within days. Mic injected with R strain do not get pneumonia. He was also able
to isolate live S strain from blood of infected mice
Why did Avery decide to start testing using a test tube? He wanted to find out the principle .
How did Avery find that the “principle” of Griffith’s experiments were in the lysate?
He used lysate for transmission assays. Heat-killed S lysate could change R to S.
What were Avery and his partners the first to do? to isolate nucleic acids from pneumococcus .
How did they destroy the RNA in the test tubes? RNaze enzyme .
What did Avery and his partners conclude? DNA is the transforming principle .
Close out the window.
Click on the link to the early fifties.
Scroll the timeline to Hershey and Chase. Click on Hershey’s picture to read his bio and answer the following
question:
Where did Hershey receive his education? A University School of Medicine in the Department of Bacteriology.
Close the window. Click on the blender above Chase.
What are bacteriophages? They are viruses that specifically attack and infect bacteria.
Describe the structure of a bacteriophage. It looks like a spider .
What is the principal difference between DNA and proteins? DNA is high in phosphorus atoms but has no
sulfur, protein is the same but switched around.
What did Hershey and Chase mix their solution with? by mixing the culture in a waring blender.
What did Hershey and Chase find when they looked at 32P samples after the mix?
32p always pelleted with the bacteria. New phage made by these infected bacteria contained radioactive 32P.
Phage DNA was used inside the bacteria to make new phage particles
What did Hershey and Chase conclude from their experiments? The phage DNA alone carries the instructions
needed to replicate phages inside the bacteria. So, DNA is genetic material
Close the window.
Scroll back through the timeline to Erwin Chargraff, and click on the image above his picture. Click through the
tutorial and answer the following questions:
What did Chargaff isolate? isolated DNA from different organisms and measured the levels of each of the four
nitrogenous bases.
What did he find about Adenine and Thymine? The amount of Adenine is very close to Thymine.
Guanine and Cytosine? There is just as much guanine as there is cytosine.
In DNA, there are 4 base pairs. A, T, G, C. A goes with T and G with C.
Close the window.
Scroll to Rosalind Franklin. Click on the image in the bottom right hand corner. Click through the tutorial and
answer the following questions:
When using x rays, they produce a diffraction of an image. What did Franklin compare this diffraction to? The
scattered x rays then interfere with each other and produce spots of different intensities and these can be
recorded on Photographic film..
From these diffractions, what was Franklin able to calculate? the basic dimensions of the DNA molecule.
What was this information used to solve? It solved the 3-dimensional structure of DNA.
Close the window. Click on the link to the late 50’s. Scroll the timeline to Francis Crick. Click on the image
above Crick. Click through the tutorial and answer the following questions:
What did Watson and Crick solve? three dimensional structure .
What puzzle did they solve together? How DNA can direct the synthesis of proteins that are made exclusively
in the cell's cytoplasm .
What did Crick propose? He proposed the Central Dogma where information flows from DNA to protein via a
carrier molecule.
What is one difference between RNA and DNA? RNA is a nucleic acid found mostly in the cell's cytoplasm.
RNA uses the same nitrogenous bases except that DNA uses the nucleotide base thymine, whereas RNA uses
uracil.
How is RNA usually stranded? allows for the binding of anticodons during translation.
What was the other problem that Crick discovered after proposing the Central Dogma?
How did amino acids interact with the carrier RNA, Was his problem